Download - Protecting BitLocker From Pre-Boot Attacks
CountermeasuresProtecting BitLocker-encrypted devices from attacks
January 2014
Table of contents
3 Attacks3 Bootkit and rootkit attacks5 Brute-force sign-in attacks5 Direct memory access attacks7 Hyberfil.sysattacks8 Memory remanence attacks
10 Countermeasures10 Protection before startup14 Protection during pre-boot: pre-boot authentication16 Protection during startup17 Protection after startup: DMA attack protection
18 Choosing the right countermeasures
21 Summary
1PROTECTING BITLOCKER-ENCRYPTED DEVICES FROM ATTACKS
CountermeasuresProtecting BitLocker-encrypted devices from attacks
Full-volumeencryptionusingBitLockerDriveEncryptionisvitalforprotectingdataandsystemintegrityondevicesrunningtheWindows8.1,Windows8,orWindows7operatingsystem.ItisequallyimportanttoprotecttheBitLockerencryptionkey.OnWindows7devices,sufficientlyprotectingthatkeyoftenrequiredpre-bootauthentication,whichmanyusersfindinconvenientandcomplicatesdevicemanagement.
MicrosofthasmadeimprovementsinWindows8.1andworkedcloselywithhardwaremanufacturerstodeliverWindows8.1andWindows8devicesthatarefundamentallyresistanttoknownattacksagainsttheBitLockerencryptionkey.Asaresult,many organizations can now meet their security requirementswithoutusingpre-bootauthentication,reducingcomplexityandinconvenience.
Thispaperprovidesdetailedinformationthatwillhelpyou understand the circumstances under which the use of pre-boot authentication is recommended and when it canbesafelyomittedfromadevice’sconfiguration.
NOTE
Forthelatestinformation,pleaseseehttp://aka.ms/bitlockerinfo.
2PROTECTING BITLOCKER-ENCRYPTED DEVICES FROM ATTACKS
BitLockerusesencryptiontoprotectthedataonyourdrive,butBitLockersecurityisonlyeffectivewhentheencryptionkeyisprotected.Manyusershavereliedonpre-bootauthenticationtoprotecttheoperatingsystem’sintegrity,diskencryptionsolution(e.g.,encryptionkeys),andthePC’sdatafromofflineattacks.Withpre-bootauthentication,usersmustprovidesomeformofcredentialbeforeunlockingencryptedvolumesandstartingWindows.Typically,theyauthenticatethemselvesusingaPINoraUSBflashdriveasakey.
Pre-bootauthenticationprovidesexcellentstartupsecurity,butitinconveniencesusersandincreasesITmanagementcosts.EverytimethePCisunattended,thedevicemustbesettohibernate(i.e.,shutdownandpoweredoff);whenthecomputerrestarts,usersmustauthenticatebeforetheencryptedvolumesareunlocked.ThisrequirementincreasesrestarttimesandpreventsusersfromaccessingremotePCsuntiltheycanphysicallyaccessthecomputertoauthenticate,makingpre-bootauthenticationunacceptableinthemodernITworld,whereusersexpecttheirdevicestoturnoninstantlyandITrequiresPCstobeconstantlyconnectedtothenetwork.
IfuserslosetheirUSBkeyorforgettheirPIN,theycan’taccesstheirPCwithoutarecoverykey.Withaproperlyconfiguredinfrastructure,theorganization’ssupportwillbeabletoprovidetherecoverykey,butdoingsoincreasessupportcosts,andusersmightlosehoursofproductiveworktime.
Windows8andnewdevicesdesignedforWindows8changeeverything.TheUnifiedExtensibleFirmwareInterface(UEFI)SecureBoot and Windows Trusted Boot startup process ensures operating systemintegrity,allowingWindowstostartautomaticallywhileminimizingtheriskofmaliciousstartuptoolsandrootkits.Inaddition,manymodernmobiledevicesarefundamentallyphysicallyresistanttosophisticatedattacksagainstthecomputer’smemory,andnowWindowsauthenticatestheuserbeforemakingdevicesthatmayrepresentathreattothedeviceandencryptionkeysavailableforuse.
ThesectionsthatfollowhelpyouunderstandwhichPCsstillneed pre-boot authentication and which can meet your security requirementswithouttheinconvenienceofit.
TheUnifiedExtensibleFirmwareInterface(UEFI)SecureBootandWindows Trusted Boot startup process ensures operating system integrity,allowingWindows to start automaticallywhileminimizing the risk of maliciousstartuptoolsandrootkits.
3PROTECTING BITLOCKER-ENCRYPTED DEVICES FROM ATTACKS
AttacksThenextfewsectionsdescribeeachtypeofattackthatcouldbeusedtocompromiseavolumeencryptionkey,whetherforBitLockeroranon-Microsoftencryptionsolution.Afteranattackerhascompromisedavolumeencryptionkey,theattackercanreaddatafromyoursystemdriveoreveninstallmalwarewhileWindowsisoffline.Eachsectionbeginswithagraphicaloverviewoftheattack’sstrengthsandweaknessesaswellassuggestedmitigationsforWindows8–andWindows7–certifieddevices.
Bootkit and rootkit attacks
Rootkitsareasophisticatedanddangeroustypeofmalwarethatrunsinkernelmode,usingthesameprivilegesastheoperatingsystem.Becauserootkitshavethesameorpossiblyevenmorerightsthantheoperatingsystem,theycancompletelyhidethemselvesfromWindowsandevenanantimalwaresolution.Often,rootkitsarepartofanentiresuiteofmalwarethatcanbypasslocallogins,recordpasswords,transferprivatefiles,andcapturecryptographykeys.
Differenttypesofbootkitsandrootkitsloadatdifferentsoftwarelevels:
• Kernellevel Rootkitsrunningatthekernellevelhavethehighestprivilegeintheoperatingsystem.Theymaybeabletoinjectmaliciouscodeorreplaceportionsofthecoreoperatingsystem,includingboththekernelanddevicedrivers.
• Applicationlevel Theserootkitsareaimedtoreplaceapplicationbinarieswithmaliciouscode,suchasaTrojan,andcanevenmodifythebehaviorofexistingapplications.
• Librarylevel Thepurposeoflibrary-levelrootkitsistohook,patch,orreplacesystemcallswithmaliciouscodethatcanhidethemalware’spresence.
• Hypervisorlevel Hypervisorrootkitstargetthebootsequence.Theirprimarypurposeistomodifythebootsequencetoloadthemselvesasahypervisor.
• Firmwarelevel TheserootkitsoverwritethePC’sBIOSfirmware,givingthemalwarelow-levelaccessandpotentiallytheabilitytoinstallorhidemalware,evenifit’scleanedorremovedfromtheharddisk.
Regardlessoftheoperatingsystemorencryptionmethod,rootkitshaveaccesstoconfidentialdataonceinstalled.Application-levelrootkitscanreadanyfilestheusercanaccess,bypassingvolume-levelencryption.Kernel-,library-,hypervisor-,andfirmware-levelrootkitshavedirectaccesstosystemfilesonencryptedvolumesandcanalsoretrieveanencryptionkeyfrommemory.
4PROTECTING BITLOCKER-ENCRYPTED DEVICES FROM ATTACKS
Windows7offerssubstantialprotectionfrombootkitsandrootkits,butitispossibletobypassoperatingsystemsecuritywhenanattackerhasphysicalaccesstothedeviceandcaninstallthemalwaretothedevicewhileWindowsisoffline.Forexample,anattackermightbootaPCfromaUSBflashdrivecontainingmalwarethatstartsbeforeWindows.ThemalwarecanreplacesystemfilesorthePC’sfirmwareorsimplystartWindowsunderitscontrol.
TosufficientlyprotectaPCfrombootandrootkits,devicesmustusepre-bootauthenticationorUEFI-basedSecureBoot,ortheencryptionsolutionmustusethedevice’sTrustedPlatformModule(TPM)asameansofmonitoringtheintegrityoftheend-to-endbootprocess.Pre-bootauthenticationisavailableforanydevice,regardlessofthehardware,butbecauseitisinconvenienttousers,itshouldbeusedonlytomitigatethreatsthatareapplicabletothedevice.UEFI-basedSecureBootisrequiredforallWindows8.1–andWindows8–certifieddevices.Onthosedevices,youdonotneedtousepre-bootauthenticationtoprotectagainstbootandrootkitattacks.
AlthoughpasswordprotectionoftheUEFIconfigurationisimportantforprotectingadevice’sconfigurationandpreventinganattackerfromdisablingUEFI’sSecureBootfeature,useofaTPManditsPlatformConfigurationRegister(PCR)measurements(PCR7)toensurethatthesystem’sbootloader(whetheraWindowsornon-Microsoftencryptionsolution)istamperfreeandthefirstcodetostartonthedeviceiscritical.Anencryptionsolutionthatdoesn’tuseadevice’sTPMtoprotectitscomponentsfromtamperingmaybeunabletoprotectitselffrombootkit-levelinfectionsthatcouldlogauser’spasswordoracquireencryptionkeys.Forthisreason,whenBitLockerisconfiguredonWindows8–andWindows7–certifieddevicesthatincludeaTPM,theTPManditsPCRsarealwaysusedtosecureandconfirmtheintegrityofthepre–operatingsystemenvironmentbeforemakingencryptedvolumesaccessible.
AnychangestotheUEFIconfigurationinvalidatesthePCR7andrequiretheusertoentertheBitLockerrecoverykey.Becauseofthisfeature,it’snotcriticaltopassword-protectyourUEFIconfiguration.IfanattackersuccessfullyturnsoffSecureBootorotherwisechangestheUEFIconfiguration,theywillneedtoentertheBitLockerrecovery
UEFI-basedSecureBootisrequiredforallWindows8.1–andWindows8–certifieddevices.Onthosedevices,youdonotneed to use pre-boot authentication to protect against boot androotkitattacks.
5PROTECTING BITLOCKER-ENCRYPTED DEVICES FROM ATTACKS
key,butUEFIpasswordprotectionisabestpracticeandisstillrequiredforsystemsnotusingaTPM(suchasnon-Microsoftalternatives).
Brute-force sign-in attacks
Attackerscanfindanypasswordifyouallowthemtoguessenoughtimes.Theprocessoftryingmillionsofdifferentpasswordsuntilyoufindtherightoneisknownasabrute-force sign-in attack. Intheory,anattackercouldobtainanypasswordbyusingthismethod.
Threeopportunitiesforbrute-forceattacksexist:
• Against the pre-boot authenticator Anattackercouldattackthedevicedirectlybyattemptingtoguesstheuser’sBitLockerPINoranequivalentauthenticator.TheTPMmitigatesthisapproachbyinvokinganantihammeringlockoutcapabilitythatrequirestheusertowaituntilthelockoutperiodendsorentertheBitLockerrecoverykey.
• Against the recovery key Anattackercouldattempttoguessthe48-digitBitLockerrecoverykey.Evenwithoutalockoutperiod,thekeyislongenoughtomakebrute-forceattacksimpractical.Specifically,theBitLockerrecoverykeyhas128bitsofentropy;thus,theaveragebrute-forceattackwouldsucceedafter18,446,744,073,709,551,616guesses.Ifanattackercouldguess1millionpasswordspersecond,theaveragebrute-forceattackwouldrequiremorethan580,000yearstobesuccessful.
• Against the operating system sign-in authenticator An attacker can attempt to guess a validusernameandpassword.Windowsimplementsadelaybetweenpasswordguesses,slowingdownbrute-forceattacks.Inaddition,allrecentversionsofWindowsallowadministratorstorequirecomplexpasswordsandpasswordlockouts.Similarly,administratorscanuseMicrosoftExchangeActiveSyncpolicyorGroupPolicytoconfigureWindows8.1andWindows8toautomaticallyrestartandrequiretheusertoentertheBitLocker48-digitrecoverykeyafteraspecifiednumberofinvalidpasswordattempts.Whenthesesettingsareenabledandusersfollowbestpracticesforcomplexpasswords,brute-forceattacksagainsttheoperatingsystemsign-inareimpractical.
Ingeneral,brute-forcesign-inattacksarenotpracticalagainstWindowswhenadministratorsenforcecomplexpasswordsandaccountlockouts.
Direct memory access attacks
Directmemoryaccess(DMA)allowscertaintypesofhardwaredevicestocommunicatedirectlywithadevice’ssystemmemory.Forexample,ifyouuseThunderbolttoconnectanotherdeviceto
6PROTECTING BITLOCKER-ENCRYPTED DEVICES FROM ATTACKS
yourcomputer,theseconddeviceautomaticallyhasReadandWriteaccesstothetargetcomputer’smemory.
Unfortunately,DMAportsdon’tuseauthenticationandaccesscontroltoprotectthecontentsofthecomputer’smemory.WhereasWindowspreventssystemcomponentsandappsfromreadingandwritingtoprotectedpartsofmemory,adevicecanuseDMAtoreadanylocationinmemory,includingthelocationofanyencryptionkeys.
DMAattacksarerelativelyeasytoexecuteandrequirelittletechnicalskills.AnyonecandownloadatoolfromtheInternet,suchasthosemade by Passware,ElcomSoft,andothers,andthenuseaDMAattacktoreadconfidentialdatafromaPC’smemory.Becauseencryptionsolutionsstoretheirencryptionkeysinmemory,theycanbeaccessedbyaDMAattack.
ToperformaDMAattack,attackerstypicallyconnectasecondPCthatisrunningamemory-scanningtool(e.g.,Passware,ElcomSoft) totheFireWireorThunderboltportofthetargetcomputer.Whenconnected,thesoftwarescansthesystemmemoryofthetargetandlocatestheencryptionkey.Onceacquired,thekeycanbeusedtodecryptthedriveandreadormodifyitscontents.
Amuchmoreefficientformofthisattackexistsintheory:AnattackercraftsacustomFireWireorThunderboltdevicethathastheDMAattacklogicprogrammedonit.Now,theattackersimplyneedstophysicallyconnectthedevice.Iftheattackerdoesnothavephysicalaccess,theycoulddisguiseitasafreeUSBflashdriveanddistributeittoemployeesofatargetorganization.Whenconnected,theattackingdevicecoulduseaDMAattacktoscanthePC’smemoryfortheencryptionkey.Itcouldthentransmitthekey(oranydatainthePC’smemory)usingthePC’sInternetconnectionoritsownwirelessconnection.Thistypeofattackwouldrequireanextremelyhighlevelofsophistication,becauseitrequiresthattheattackercreateacustomdevice(devicesofthesetypesarenotreadilyavailableinthemarketplaceatthistime).
Themostcommon,legitimateuseforDMAportsisdeveloperdebugging,ataskthatsomedevelopersneedtoperformandonethatfewconsumerswilleverperform.BecauseUSB;DisplayPort;andother,moresecureporttypessatisfyconsumers,mostnewmobilePCsdonotincludeDMAports.Microsoft’sviewisthatbecauseof
NOTE
NotallporttypesarevulnerabletoDMAattacks.USBinparticulardoesnotallowDMA,butdevicesthathaveanyofthefollowingporttypesarevulnerable:
• FireWire
• Thunderbolt
• ExpressCard
• PCMCIA
• PCI
• PCI-X
• PCIExpress
7PROTECTING BITLOCKER-ENCRYPTED DEVICES FROM ATTACKS
theinherentsecurityrisksofDMAports,theydonotbelongonmobiledevices,andMicrosofthasprohibitedtheirinclusiononanyInstantGo-certifieddevices.InstantGodevicesoffermobilephone–likepowermanagementandinstant-oncapabilities;atthetimeofwriting,theyareprimarilyfoundinWindowstablets.In2014,MicrosoftexpectstoseeInstantGotrickledownintomoremobiledevicetypes,suchasconvertiblesandtraditionallaptops.
DMA-basedexpansionslotsareanotheravenueofattack,buttheseslotsgenerallyappearonlyondesktopPCsthataredesignedforexpansion.OrganizationscanusephysicalsecuritytopreventoutsideattacksagainsttheirdesktopPCs.Inaddition,aDMAattackontheexpansionslotwouldrequireacustomdevice;asaresult,anattackerwouldmostlikelyinsertaninterfacewithatraditionalDMAport(forexample,FireWire)intotheslottoattackthePC.
NewtoWindows8.1isacapabilitybywhichWindowswon’tenablenewlyattachedDMAdevicesuntiltheoperatingsystemstartsandausersignsin.EverytimethePCswitchestosuspend,hibernation,orsleepmode,WindowswaitsfortheusertosigninbeforegrantingnewdevicesDMAaccess.ThisdelayhelpspreventDMAattackswhenanauthorizeduserisn’tpresent.ThisnewWindows8.1behaviorsuccessfullymitigatestheDMAattackvectorandeliminatestheneedforpre-bootauthenticationinmostscenarios.AnotheroptionisforadministratorstoconfigurepolicysettingstodisableFireWireandotherdevicetypesthathaveDMA;manyPCsallowthosedevicestobedisabledbyusingfirmwaresettings.Althoughtheneedforpre-bootauthenticationcanbeeliminatedatthedevicelevelorthroughWindowsconfiguration,theBitLockerpre-bootauthenticationfeatureisstillavailablewhenneeded.Whenused,itsuccessfullymitigatesalltypesofDMAportandexpansionslotattacksonanytypeofdevice.
Hyberfil.sys attacks
Thehyberfil.sysfileistheWindowshibernationfile.ItcontainsasnapshotofsystemmemorythatisgeneratedwhenadevicegoesintohibernationandincludestheencryptionkeyforBitLockerandotherencryptiontechnologies.Attackershaveclaimedthattheyhavesuccessfullyextractedencryptionkeysfromthehyberfil.sysfile.
Windows8.1waitsfor the user to sign in before granting new devicesDMAaccess.ThisnewbehaviorsuccessfullymitigatestheDMAattackvector.
8PROTECTING BITLOCKER-ENCRYPTED DEVICES FROM ATTACKS
LiketheDMAportattackdiscussedintheprevioussection,toolsareavailablethatcanscanthehyberfile.sysfileandlocatetheencryptionkey,includingatoolmadebyPassware.MicrosoftdoesnotconsiderWindowstobevulnerabletothistypeofattack,becauseWindowsstoresthehyberfil.sysfilewithintheencryptedsystemvolume.Asaresult,thefilewouldbeaccessibleonlyiftheattackerhadbothphysicalandsign-inaccesstothePC.Whenanattackerhassign-inaccesstothePC,therearefewreasonsfortheattackertodecryptthedrive,becausetheywouldalreadyhavefullaccesstothedatawithinit.
Inpractice,theonlyreasonanattackonhyberfil.syswouldgrantanattackeradditionalaccessisifanadministratorhadchangedthedefaultWindowsconfigurationandstoredthehyberfil.sysfileonanunencrypteddrive.Bydefault,bothWindows8andWindows7aredesignedtobesecureagainstthistypeofattack.
Memory remanence attacks
A memory remanence attackisaside-channelattackthatreadstheencryptionkeyfrommemoryafterrestartingaPC.AlthoughaPC’smemoryisoftenconsideredtobeclearedwhenthePCisrestarted,memorychipsdon’timmediatelylosetheirmemorywhenyoudisconnectpower.Therefore,anattackerwhohasphysicalaccesstothePC’smemorymightbeabletoreaddatadirectlyfromthememory—includingtheencryptionkey.
Whenperformingthistypeofcoldbootattack,theattackeraccessesthePC’sphysicalmemoryandrecoverstheencryptionkeywithinafewsecondsorminutesofdisconnectingpower.Thistypeof attack was demonstrated by researchers at PrincetonUniversity.Withtheencryptionkey,theattackerwouldbeabletodecryptthedriveandaccessitsfiles.
Toacquirethekeys,attackersfollowthisprocess:
1. FreezethePC’smemory.Forexample,anattackercanfreezethememoryto−50°Cbysprayingitwithaerosolairdusterspray.
2.RestartthePC.
3. InsteadofrestartingWindows,boottoanotheroperatingsystem.Typically,thisisdonebyconnectingabootableflashdriveorloadingabootableDVD.
4.Thebootablemedialoadsthememoryremanenceattacktools,whichtheattackerusestoscanthesystemmemoryandlocatetheencryptionkeys.
5.Theattackerusestheencryptionkeystoaccessthedrive’sdata.
9PROTECTING BITLOCKER-ENCRYPTED DEVICES FROM ATTACKS
Iftheattackerisunabletobootthedevicetoanotheroperatingsystem(forexample,ifbootableflashdriveshavebeendisabledorUEFISecureBootisenabled),theattackercanattempttophysicallyremovethefrozenmemoryfromthedeviceandattachittoadifferent,possiblyidenticaldevice.Fortunately,thisprocesshasprovenextremelyunreliable,asevidencedbytheDefenceResearchandDevelopmentCanada(DRDC)Valcartiergroup’sanalysis(see“Anin-depthanalysisofthecoldbootattack”athttp://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA545078).Onanincreasingportionofmoderndevices,thistypeofattackisnotevenpossible,becausememoryissoldereddirectlytothemotherboard.
AlthoughPrinceton’sresearchprovedthatthistypeofattackwaspossibleondevicesthathaveremovablememory,devicehardwarehaschangedsincetheresearchwaspublishedin2008:
• Windows8–certifieddevicesincludeUEFI-basedSecureBoot,whichpreventsthemalicioustoolsthatthePrincetonattackdependsonfromrunningonthetargetdevice.
• Windows8andWindows7systemswithBIOSorUEFIcanbelockeddownwithapassword,andbootingtoaUSBdrivecanbeprevented.
• IfbootingtoUSBisrequiredonthedevice,itcanbelimitedtostartingtrustedoperatingsystemsonWindows8–certifieddevices(UEFI-basedSecureBoot).
• Thedischargeratesofmemoryarehighlyvariableamongdevices,andmanydeviceshavememorythatiscompletelyimmunetomemoryremanenceattacks.
• Increaseddensityofmemorydiminishestheirremanencepropertiesandreducesthelikelihoodthattheattackcanbesuccessfullyexecuted,evenwhenmemoryisphysicallyremovedandplacedinanidenticalsystemwherethesystem’sconfigurationmayenablebootingtothemalicioustools.
Becauseofthesefactors,thistypeofattackisrarelypossibleonmoderndevices.Evenincaseswheretheriskfactorsexistonlegacydevices,attackerswillfindtheattackunreliable.Fordetailedinformationaboutthepracticalusesforforensicmemoryacquisitionandthefactorsthatmakeacomputervulnerableorresistanttomemoryremanenceattacks,read“Anin-depthanalysisofthecoldbootattack”athttp://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA545078.
TheBitLockerpre-bootauthenticationfeaturecansuccessfullymitigatememoryremanenceattacksonmostdevices,butyoucanalsomitigatesuchattacksbyprotectingthesystemUEFIorBIOSandpreventthePCfrombootingfromexternalmedia(suchasaUSBflashdriveorDVD).Thelatteroptionisoftenabetterchoice,becauseitprovidessufficientprotectionwithoutinconveniencinguserswithpre-bootauthentication.
10PROTECTING BITLOCKER-ENCRYPTED DEVICES FROM ATTACKS
CountermeasuresBitLockerwasintroducedintheWindowsVistaoperatingsystemaspartofastrategicapproachtosecuringmobiledatathroughencryptiontechnology.Dataonalostorstolencomputerisvulnerabletounauthorizedaccess,eitherbyrunningasoftwareattacktoolagainstitorbytransferringthecomputer’sharddisktoadifferentcomputer.Today,BitLockerhelpsmitigateunauthorizeddataaccessonlostorstolencomputersbeforetheoperatingsystemisstartedby:
• Encrypting the hard drives on your computer Forexample,youcanturnonBitLockerforyouroperatingsystemdrive(thedriveonwhichWindowsisinstalled),afixeddatadrive(suchasadifferentvolumeonthesystemdriveoraseparateinternalharddrive),oraremovabledatadrive(suchasaUSBflashdrive).TurningonBitLockerforyouroperatingsystemdriveencryptsallsystemfilesontheoperatingsystemdrive,includingtheswapfilesandhibernationfiles.
• Ensuring the integrity of early boot components and boot configuration data OnWindows7–certifieddevicesthathaveaTPMversion2.0or1.2,BitLockerusestheenhancedsecuritycapabilitiesoftheTPMtohelpensurethatyourdataisaccessibleonlyifthecomputer’sbootcomponentsappearunalteredandtheencrypteddiskislocatedintheoriginalcomputer.OnWindows8–certifieddevices,acombinationofUEFIandTPMhelpsensureintegrity.
ThesectionsthatfollowprovidemoredetailedinformationaboutthedifferenttechnologiesthatWindowsusestoprotectagainstattacksontheBitLockerencryptionkeyinfourdifferentbootphases:beforestartup,duringpre-boot,duringstartup,andfinallyafterstartup.
Protection before startup
BeforeWindowsstarts,youmustrelyonsecurityfeaturesimplementedaspartofthedevicehardware,includingTPMandUEFISecureBoot.Fortunately,manymoderncomputersfeatureTPM,andallWindows8.1–andWindows8–certifieddevicessupportallofthesefeatures.
Trusted Platform Module
Softwarealoneisn’tsufficienttoprotectasystem.Afteranattackerhascompromisedsoftware,thesoftwaremightbeunabletodetectthecompromise.Therefore,asinglesuccessfulsoftwarecompromiseresultsinanuntrustedsystemthatmightneverbedetected.Hardware,however,ismuchmoredifficulttomodify.
11PROTECTING BITLOCKER-ENCRYPTED DEVICES FROM ATTACKS
A TPMisamicrochipdesignedtoprovidebasicsecurity-relatedfunctions,primarilyinvolvingencryptionkeys.TheTPMisusuallyinstalledonthemotherboardofacomputerand communicates with the rest of the system through a hardwarebus.Physically,TPMsaredesignedtobetamper-proof.Ifanattackertriestophysicallyretrievedatadirectlyfromthechip,they’llprobablydestroythechipintheprocess.
BybindingtheBitLockerencryptionkeywiththeTPMandproperlyconfiguringthedevice,it’snearlyimpossibleforanattackertogainaccesstotheBitLocker-encrypteddatawithoutobtaininganauthorizeduser’scredentials.Therefore,computerswithaTPMcanprovideahighlevelofprotectionagainstattacksthatattempttodirectlyretrievetheBitLockerencryptionkey.
OndevicesrunningWindows8,thecombinationofaTPMandUEFISecureBootprovidessufficientdeviceintegrity–relatedsecurity.OndevicesrunningWindows8orWindows7withoutUEFI-basedSecureBoot,theTPMwillbeusedtoprotectthesystem’sboot-relatedcomponents.
UEFI and Secure Boot
Nooperatingsystemcanprotectadevicewhentheoperatingsystemisoffline.Forthatreason,Microsoftworkedcloselywithhardwarevendorstorequirefirmware-levelprotectionagainstbootandrootkitsthatmightcompromiseanencryptionsolution’sencryptionkeysinallWindows8–certifieddevices.
TheUEFIisaprogrammablebootenvironmentintroducedasareplacementforBIOS,whichhasforthemostpartremainedunchangedforthepast30years.LikeBIOS,PCsstartUEFIbeforeanyothersoftware;itinitializesdevices,andUEFIthenstartstheoperatingsystem’sbootloader.Aspartofitsintroductionintothepre–operatingsystemenvironment,UEFIservesanumberofpurposes,butoneofthekeybenefitsistoprotectnewerdevicesagainstasophisticatedtypeofmalwarecalledabootkit through the useofitsSecureBootfeature.
RecentimplementationsofUEFI(startingwithversion2.3.1,whichisinallWindows8–certifieddevices)canverifythedigitalsignatures
By binding the BitLockerencryptionkey with the TPM and properlyconfiguringthedevice,it’snearlyimpossibleforanattacker to gain access totheBitLocker-encrypted data without obtaining anauthorizeduser’scredentials.
12PROTECTING BITLOCKER-ENCRYPTED DEVICES FROM ATTACKS
ofthedevice’sfirmwarebeforerunningit.BecauseonlythePC’shardwaremanufacturerhasaccesstothedigitalcertificaterequiredtocreateavalidfirmwaresignature,UEFIcanpreventfirmware-basedbootkits.Thus,UEFIisthefirstlinkinthechainoftrust.
TheUEFI-basedSecureBootfeatureisthefoundationofplatformandfirmwaresecurityandwascreatedtoenhancesecurityinthepre-bootenvironmentregardlessofdevicearchitecture.Usingsignaturestovalidatetheintegrityoffirmwareimagesbeforetheyareallowedtoexecute,SecureBoothelpsreducetheriskofbootloaderattacks.ThepurposeofSecureBootistoblockuntrustedfirmwareandbootloaders(signedorunsigned)frombeingabletostartonthesystem.
WiththelegacyBIOSbootprocess,thepre–operatingsystemenvironmentisvulnerabletoattacksbyredirectingbootloaderhandofftopossiblemaliciousloaders.Theseloaderscouldremainundetectedtooperatingsystemandantimalwaresoftware.Thediagram in Figure 1contraststheBIOSandUEFIstartupprocesses.
WithSecureBootenabled,UEFI,incoordinationwiththeTPM,canexaminethebootloaderanddeterminewhetherit’strustworthy.Todeterminewhetherthebootloaderistrustworthy,UEFIexaminesthebootloader’sdigitalsignature.Usingthedigitalsignature,theUEFI:
• Verifiesthatthebootloaderhasn’tbeenmodifiedsinceitwassigned
• Verifiesthatthebootloaderwassignedusingatrustedcertificate(inthecaseofWindows8,Microsoft’scertificate)
Ifthebootloaderpassesthesetwotests,theUEFIknowsthatthebootloaderisn’tabootkitandstarts
FigurE 1 The BIOS and UEFI startup processesBIOS
UEFI VerifiedOS loader
Any OS loader(including malware)
OS Start
OS Start
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it.Atthispoint,Windows8.1’sTrustedBootfeaturetakesover,andtheWindows8bootloader,usingthesamecryptographictechnologiesthatUEFIusedtoverifythebootloader,thenverifiesthattheWindowssystemfileshaven’tbeenchanged.
AllWindows8–certifieddevicesmustmeetseveralrequirementsrelatedtoUEFI-basedSecureBoot:
• TheymusthaveSecureBootenabledbydefault.
• TheymusttrustMicrosoft’scertificate(andthusanybootloaderMicrosofthassigned).
• TheymustallowtheusertoconfigureSecureBoottotrustothersignedbootloaders.
• ExceptforWindowsRTdevices,theymustallowtheusertocompletelydisableSecureBoot.
Theserequirementshelpprotectyoufromrootkitswhileallowingyoutorunanyoperatingsystemyouwant.Youhavethreeoptionsforrunningnon-Microsoftoperatingsystems:
• Use an operating system with a certified bootloader BecauseallCertifiedforWindows8PCsmusttrustMicrosoft’scertificate,Microsoftoffersaservicetoanalyzeandsignnon-MicrosoftbootloaderssothattheycanbetrustedbyallCertifiedforWindows8PCs.TheLinuxcommunityisusingthisprocesstoenableLinuxtotakeadvantageofUEFISecureBootonWindows-certifieddevices.
• Configure UEFI to trust your custom bootloader AllCertifiedforWindows8PCsallowyoutotrustasigned,noncertifiedbootloaderthatyouspecifyintheUEFIdatabase,allowingyoutorunanyoperatingsystem,includinghomemadeoperatingsystems.
• Turn off Secure Boot AllCertifiedforWindows8PCsallowyoutoturnoffSecureBootsoyoucanrunanysoftware.Thisdoesnothelpprotectyoufrombootkits,however.
Topreventmalwarefromabusingtheseoptions,theuserhastomanuallyconfiguretheUEFIfirmwaretotrustanoncertifiedbootloaderortoturnoffSecureBoot.SoftwarecannotchangetheSecureBootsettings.
Anydevicethatdoesn’trequireSecureBootorasimilarbootloader-verificationtechnology,regardlessofthearchitectureoroperatingsystem,isvulnerabletobootkits,whichcanbeusedtocompromisetheencryptionsolution.Bydefault,allWindows8–certifieddeviceshaveUEFI-basedSecureBootenabled.
UEFIissecurebydesign,butit’scriticaltoprotecttheSecureBootconfigurationbyusingpasswordprotection.Inaddition,althoughseveralwell-publicizedattacksagainstUEFIhaveoccurred,
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theywereexploitingfaultyUEFIimplementations.ThoseattacksareineffectivewhenUEFIisimplementedproperly.
FormoreinformationaboutSecureBoot,referto“SecuringtheWindows8BootProcess”athttp://technet.microsoft.com/en-US/windows/dn168167.aspx.
Protection during pre-boot: pre-boot authentication
Pre-bootauthenticationwithfull-diskencryptionproducts(includingBitLocker)isaprocessthatrequiresausertoauthenticatepriortomakingthecontentsofthesystemdriveaccessible.InthecaseofBitLocker,BitLockerencryptstheentiredrive,includingallsystemfiles.BitLockeraccessesandstorestheencryptionkeyinmemoryonlyafterauserprovidesaspecificPINorUSBstartupkey.
IfWindowscan’taccesstheencryptionkey,thedevicecan’treadoreditthefilesonthesystemdrive.EvenifanattackertakesthediskoutofthePCorstealstheentirePC,theywon’tbeabletoreadoreditthefileswithouttheencryptionkey.Theonlyoptionforbypassingpre-bootauthenticationisenteringthehighlycomplex,48-digitrecoverykey.
TheBitLockerpre-bootauthenticationcapabilityisnotspecificallydesignedtopreventtheoperatingsystemfromstarting:That’smerelyasideeffectofhowBitLockerprotectsdataconfidentialityandsystemintegrity.Pre-bootauthenticationisdesignedtopreventtheencryptionkeyfrombeingloadedtosystemmemoryondevicesthatarevulnerabletocertaintypesofcoldbootattacks.Manymoderndevicespreventanattackerfromeasilyremovingthememory,andMicrosoftexpectsthosedevicestobecomeevenmorecommoninthefuture.
OncomputerswithacompatibleTPM,operatingsystemdrivesthatareBitLocker-protectedcanbeunlockedinfourways:
• TPM-only UsingTPM-onlyvalidationdoesnotrequireanyinteractionwiththeusertodecryptandprovideaccesstothedrive.IftheTPMvalidationsucceeds,theuserlogonexperienceisthesameasastandardlogon.IftheTPMismissingorchangedoriftheTPMdetectschangestocriticaloperatingsystemstartupfiles,BitLockerentersitsrecoverymode,andtheusermustenterarecoverypasswordtoregainaccesstothedata.
• TPM with startup key InadditiontotheprotectionthattheTPMprovides,partoftheencryptionkeyisstoredonaUSBflashdrive,referredtoasastartup key. Data on the encryptedvolumecannotbeaccessedwithoutthestartupkey.
• TPM with PiN InadditiontotheprotectionthattheTPMprovides,BitLockerrequiresthattheuserenteraPIN.DataontheencryptedvolumecannotbeaccessedwithoutenteringthePIN.
15PROTECTING BITLOCKER-ENCRYPTED DEVICES FROM ATTACKS
• TPM with startup key and PiN InadditiontothecorecomponentprotectionthattheTPMprovides,partoftheencryptionkeyisstoredonaUSBflashdrive,andaPINisrequiredtoauthenticatetheusertotheTPM.ThisconfigurationprovidesMultifactorAuthenticationsothatiftheUSBkeyislostorstolen,itcannotbeusedforaccesstothedrive,becausethecorrectPINisalsorequired.
Formanyyears,Microsofthasrecommendedusingpre-bootauthenticationtoprotectagainstDMAandmemoryremanenceattacks.Today,Microsoftrecommendsusingpre-bootauthenticationonlyonPCsrunningWindows7thathaveanenabledDMAportoranydevicethatissusceptibletomemoryremanenceattacks.
Althougheffective,pre-bootauthenticationisinconvenienttousers.Inaddition,ifauserforgetstheirPINorlosestheirstartupkey,they’redeniedaccesstotheirdatauntiltheycancontacttheirorganization’ssupportteamtoobtainarecoverykey.Today,mostnewPCsrunningWindows8.1orWindows8providesufficientprotectionagainstDMAattackswithoutrequiringpre-bootauthentication.Forexample,mostmodernPCsincludeUSBportoptions(whicharenotvulnerabletoDMAattacks)butdonotincludeFireWireorThunderboltports(whicharevulnerabletoDMAattacks).
Infact,toachieveaWindows8InstantGo(formerlyConnectedStandby)certificationfromMicrosoft,newdevicescan’tincludeaDMAport,eliminatingtheneedforpre-bootauthenticationtomitigateagainstaDMAportattackinmosttabletsandotherWindows8–certifieddevices.Althoughthiscertificationiscurrentlyimplementedonlyontabletdevices,startingin2014,MicrosoftexpectstoseedevicessuchasconvertiblesandlaptopscertifiedforInstantGo.
BitLocker-encrypteddeviceswithDMAportsenabled,includingFireWireorThunderboltports,shouldbeconfiguredwithpre-bootauthenticationiftheyarerunningWindows7.Windows8.1devicesdonotneedpre-bootauthenticationtoprotectagainstthemostcommonlyusedDMAattackvectorsbecausenewlyattachedDMAdevicesgetDMAaccessonlyafterauserauthenticatesandsignsintoWindows.ManycustomersfindthattheDMAportsontheirdevicesareneverused,andtheychoosetoeliminatethepossibilityofanattackbydisablingtheDMAportsthemselves,eitheratthehardwarelevelorthroughGroupPolicy.
Manynewmobiledeviceshavethesystemmemorysolderedtothemotherboard,whichhelpspreventthecoldboot–styleattack,wherethesystemmemoryisfrozen,removed,andthenplacedintoanotherdevice.Thosedevices,andmostPCs,canstillbevulnerablewhenbootingtoamaliciousoperatingsystem,however.
16PROTECTING BITLOCKER-ENCRYPTED DEVICES FROM ATTACKS
Youcanmitigatetheriskofbootingtoamaliciousoperatingsystem:
• Windows 8.1 (without Secure Boot), Windows 8 (without uEFi-based Secure Boot), or Windows 7 (with or without a TPM) Disablebootingfromexternalmedia,andrequireafirmwarepasswordtopreventtheattackerfromchangingthatoption.
• Windows 8.1 or Windows 8 (certified or with Secure Boot) Passwordprotectthefirmware,anddonotdisableSecureBoot.
Protection during startup
Duringthestartupprocess,Windows8.1andWindows8useTrustedBootandEarlyLaunchAnti-Malware(ELAM)toexaminetheintegrityofeverycomponent.Thesectionsthatfollowdescribethesetechnologiesinmoredetail.
Trusted Boot
TrustedBoottakesoverwhereUEFI-basedSecureBootleavesoff—duringtheoperatingsysteminitializationphase.ThebootloaderverifiesthedigitalsignatureoftheWindows8kernelbeforeloadingit.TheWindows8kernel,inturn,verifieseveryothercomponentoftheWindowsstartupprocess,includingthebootdrivers,startupfiles,andELAMdriver.IfafilehasbeenmodifiedorisnotproperlysignedwithaMicrosoftsignature,Windowsdetectstheproblemandrefusestoloadthecorruptedcomponent.Often,Windows8canautomaticallyrepairthecorruptedcomponent,restoringtheintegrityofWindowsandallowingthePCtostartnormally.
Windows8usesTrustedBootonanyhardwareplatform:ItrequiresneitherUEFInoraTPM.However,withoutSecureBoot,it’spossibleformalwaretocompromisethestartupprocesspriortoWindowsstarting,atwhichpointTrustedBootprotectionscouldbebypassedorpotentiallydisabled.
Early Launch Anti-Malware
BecauseUEFI-basedSecureBoothasprotectedthebootloaderandTrustedBoothasprotectedtheWindowskernelorotherWindowsstartupcomponents,thenextopportunityformalwaretostartisbyinfectinganon-Microsoftboot-relateddriver.Traditionalantimalwareappsdon’tstartuntilaftertheboot-relateddrivershavebeenloaded,givingarootkitdisguisedasadrivertheopportunitytowork.
ThepurposeofELAMistoloadanantimalwaredriverbeforedriversthatareflaggedasboot-startcanbeexecuted.Thisapproachprovidestheabilityforanantimalwaredrivertoregisterasa
17PROTECTING BITLOCKER-ENCRYPTED DEVICES FROM ATTACKS
trustedboot-criticaldriver.ItislaunchedduringtheTrustedBootprocess,andwiththat,Windowsensuresthatitisloadedbeforeanyothernon-Microsoftsoftware.
Withthissolutioninplace,bootdriversareinitializedbasedontheclassificationthattheELAMdriverreturnsaccordingtoaninitializationpolicy.ITproshavetheabilitytochangethispolicythroughGroupPolicy.
ELAMclassifiesdriversasfollows:
• good Thedriverhasbeensignedandhasnotbeentamperedwith.
• Bad Thedriverhasbeenidentifiedasmalware.Itisrecommendedthatyounotallowknownbaddriverstobeinitialized.
• Bad but required for boot Thedriverhasbeenidentifiedasmalware,butthecomputercannotsuccessfullybootwithoutloadingthisdriver.
• unknown Thisdriverhasnotbeenattestedtobyyourmalware-detectionapplicationorclassifiedbytheELAMboot-startdriver.
Protection after startup: DMA attack protection
Windows8.1minimizestheriskofDMAattacksbypreventingnewlyattachedDMAdevicesfromgainingDMAuntilauserauthenticatesbysigning-in.Thisdoesn’teliminatetherisk,butitdoesreducetheriskofanattackerconnectingaDMAdevicetoaPCandretrievingtheencryptionkeywhiletheuserisawayfromthePC.
TosuccessfullyperformaDMAattackonaWindows8.1device,theattackerwouldneedamaliciousDMAdeviceconnectedtothePCwhiletheuserwasloggedon.TheattackerwouldnotsimplybeabletoattachaDMAdevicewhentheuserwasatthePC,retrievetheencryptionkey,andthenleavewiththedevice.Theattackerwouldeitherneedto:
• Attachthedevicewhiletheuserwasloggedon
• Attachthedeviceatanytime,waitfortheusertologon,andthenreturntoretrievethedevice
Windows8InstantGo–certifieddevicesdonothaveDMAports,eliminatingtheriskofDMAattacks.Onotherdevices,youmightbeabletodisableFireWire,Thunderbolt,orotherportsthatsupportDMA.
18PROTECTING BITLOCKER-ENCRYPTED DEVICES FROM ATTACKS
Choosing the right countermeasuresFigure 2,Figure3onpage19,andFigure 4 on page 20 summarize the recommended mitigationsfordifferenttypesofattacksagainstPCsrunningrecentversionsofWindows.Theorangeblocksindicatethatthesystemrequiresadditionalconfigurationfromthedefaultsettings.
FigurE 2 How to choose the best countermeasures for Windows 7
Windows 7without TPM
Bootkits and Rootkits
Without TPM, bootintegrity checking isnot available
Secure by default, and canbe improved with accountlockout Group Policy
Check devices for DMAports. Consider disablingports if not in use or requireBitLocker with pre-boot authentication
Secure by default,hyberfil.sys secured onencrypted volume
Require a BIOS password and disable booting from external media. If an attack is viable, consider pre-boot authentication
Secure by default. RequireBitLocker with TPM for bootintegrity validation
Secure by default, and canbe improved with accountlockout Group Policy
Secure by default,hyberfil.sys secured onencrypted volume
Require a BIOS password and disable booting from external media. If an attack is viable, consider pre-boot authentication
Brute ForceSign-in
DMA Attacks
Hyberfil.sys Attacks
Memory Remanence Attacks
Windows 7with TPM
Check devices for DMAports. Consider disablingports if not in use or requireBitLocker with pre-boot authentication
19PROTECTING BITLOCKER-ENCRYPTED DEVICES FROM ATTACKS
FigurE 3 How to choose the best countermeasures for Windows 8
Bootkits and Rootkits
Without TPM, bootintegrity checking isnot available
Secure by default, and canbe improved with accountlockout Group Policy
Check devices for DMAports. Consider disablingports if not in use or requireBitLocker with pre-boot authentication
Secure by default,hyberfil.sys secured onencrypted volume
Require a BIOS passwordand disable booting fromexternal media. If an attack is viable, consider pre-boot authentication
Secure by default whenUEFI-based Secure Bootis enabled and a password is required to change settings
Secure by default, and canbe improved with accountlockout and device lockoutGroup Policy settings
Check devices for DMAports. Consider disablingports if not in use or requireBitLocker with pre-boot authentication
Secure by default,hyberfil.sys secured onencrypted volume
Password protect thefirmware and ensureSecure Boot is enabled. If an attack is viable, consider pre-boot authentication
Brute ForceSign-in
DMA Attacks
Hyberfil.sys Attacks
Memory Remanence Attacks
Windows 8 without TPM
Windows 8 Certified
20PROTECTING BITLOCKER-ENCRYPTED DEVICES FROM ATTACKS
FigurE 4 How to choose the best countermeasures for Windows 8.1
Bootkits and Rootkits
Secure by default, and canbe improved with accountlockout Group Policy
Secure by default for all lostor stolen devices becausenew DMA devices aregranted access only when anauthorized user is signed in
Secure by default,hyberfil.sys secured onencrypted volume
Require a BIOS passwordand disable booting fromexternal media. If an attack is viable, consider pre-boot authentication
Secure by default, and canbe improved with accountlockout and device lockoutGroup Policy settings
Secure by default for all lostor stolen devices becausenew DMA devices aregranted access only when anauthorized user is signed in
Secure by default,hyberfil.sys secured onencrypted volume
Password protect thefirmware and ensureSecure Boot is enabled. If an attack is viable, consider pre-boot authentication
Brute ForceSign-in
DMA Attacks
Hyberfil.sys Attacks
Memory Remanence Attacks
Windows 8.1 without TPM
Windows 8.1 Certified
Without TPM, bootintegrity checking isnot available
Secure by default whenUEFI-based Secure Bootis enabled and a password is required to change settings
21PROTECTING BITLOCKER-ENCRYPTED DEVICES FROM ATTACKS
SummaryYoucanuseBitLockertoprotectyourWindows8.1,Windows8,andWindows6clientPCs.Whicheveroperatingsystemyou’reusing,MicrosoftandWindows-certifieddevicesprovidecountermeasurestoaddressattacksandimproveyourdatasecurity.Inmostcases,particularlyonWindows8devices,thisprotectioncanbeimplementedwithouttheneedforpre-bootauthentication.
ThelatestWindows8.1InstantGodevices,primarilytablets,aredesignedtobesecurebydefaultagainstallattacksthatmightcompromisetheBitLockerencryptionkey.OtherWindows8.1devicescanbe,too.DMAport–basedattacks,whichrepresenttheattackvectorofchoice,arenotpossibleonInstantGodevices,onwhichtheseporttypesareprohibited.DMAportsonevennon-InstantGodevicesisincreasinglyrare,particularlyonmobiledevices.Regardlessofthehardwareconfiguration,theriskofDMAattackshasbeenaddressedinWindows8.1itself,whichhasbeenupdatedtopreventnewDMAdevicesthathavebeenattachedtoadevicefromgainingDMAuntilanauthorizedusersigns-in.DMAportscanevenbedisabledentirely,whichisincreasinglypopularoptionbecausetheuseofDMAportsisrareinthenon-developerspace.
Memoryremanenceattackscanbemitigatedwithproperconfiguration;incaseswherethesystemmemoryisfixedandnon-removable,theyarenotpossibleusingpublishedtechniques.Evenincaseswheresystemmemorycanberemovedandloadedintoanotherdevice,attackerswillfindtheattackvectorextremelyunreliable,ashasbeenshownintheDRDCValcartiergroup’sanalysis(see“Anin-depthanalysisofthecoldbootattack”athttp://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA545078).
Windows7PCssharethesamesecurityrisksasWindows8devicesbutarefarmorevulnerabletoDMAandmemoryremanenceattacks,becauseWindows7devicesaremorelikelytoincludeDMAports,lacksupportforUEFI-basedSecureBoot,andrarelyhavefixedmemory.Toeliminatetheneedforpre-bootauthenticationonWindows7devices,disabletheabilitytoboottoexternalmedia,password-protecttheBIOSconfiguration,anddisabletheDMAports.Ifyoubelievethatyourdevicesmaybeatargetofamemoryremanenceattack,wherethesystemmemorymayberemovedandputintoanothermachinetogainaccesstoitscontents,considertestingyourdevicestodeterminewhethertheyaresusceptibletothistypeofattack.
Intheend,manycustomerswillfindthatpre-bootauthenticationimprovessecurityonlyforashrinkingsubsetofdeviceswithintheirorganization.Microsoftrecommendsacarefulexaminationoftheattackvectorsandmitigationsoutlinedinthisdocumentalongwithanevaluationofyourdevicesbeforechoosingtoimplementpre-bootauthentication,whichmaynotenhancethesecurityofyourdevicesandinsteadwillonlycompromisetheuserexperienceandaddtosupportcosts.
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