can http strict transport security meaningfully help secure

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Can HTTP Strict Transport SecurityMeaningfully Help Secure the Web?

nicolle neulistJune 2, 2012

Security B-Sides Detroit

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o hai.

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Why Think About HTTPStrict Transport Security?

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Roadmap

● what is HSTS?

● how do servers implement HSTS?

● how do browsers implement HSTS?

● what are the problems in HSTS implementation, and how can they be fixed?

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What Is HTTP Strict Transport Security?

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HSTS at 30,000 Feet

policy mechanism used to force browsersto show a website securely.

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The HSTS Header

Strict-Transport-Security: max-age=expireTime

Strict-Transport-Security: max-age=expireTime; includeSubdomains

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Things HSTS Was Designed To Address

● attackers who passively sniff traffic to gain cookies and credentials

● fake DNS server attacks

● faked websites via spoofed wireless frames

● fake sites served off of rogue wireless access points

● injection of code into insecurely embedded website content

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Things HSTS Was Not Designed To Address

● phishing attacks

● browser vulnerabilities

● malware

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How Widely IsHTTP Strict Transport Security

Implemented?

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How Many Sites Implement HSTS?

● according to SSL Labs, 12 out of 600,000 sites they found with valid SSL certificates used HSTS as of Spring 2010

● in their Spring 2011 survey, this number rose to 162 out of 1.2 million sites with valid SSL certificates

● according to SSL Pulse, a joint project of SSL Labs and Trustworthy Internet Movement, 1,697 of the 200,000 most popular SSL-enabled sites currently use HSTS.

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How Many Browsers Implement HSTS?

● Mozilla Firefox has implemented HSTS since version 4.0

● Google Chrome has implemented HSTS sinceversion 4.0.211.0

● Opera has implemented HSTS since the 2.10.239 rendering engine (Opera 12.00 Beta; not available in current stable version)

● Internet Explorer and Safari do not support HSTS.

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How IsHTTP Strict Transport Security

Implemented?

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What the Server Does

● HTTPS site sends HSTS header to let browser know to only accept that domain name, and pos-sibly its subdomains, with SSL/TLS encryption

● if the user reaches HTTP version of the site instead, 301 redirect

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What the Browser Does

The First Visit

User wants to see http://www.securesite.com↓

Browser does not see www.securesite.com in its database, so it requests http://www.securesite.com

↓Server sends 301 Permanent Redirect to

https://www.securesite.com↓

Browser requests https://www.securesite.com↓

Server sends https://www.securesite.com content,including its HSTS header

↓Browser saves HSTS header information

about www.securesite.com

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What The Browser Does

● remembering the list of HSTS sites

● user-level privileges in browser profile● Firefox: sqlite3 database● Chrome: flat text file

● handling max-age

● when user starts browser, both Firefox and Chrome check max ages and clear expired entries

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Subsequent VisitsTo Legitimatewww.securesite.com Server


Browser sees www.securesite.com in its database, so it rewrites the request to ask for https://www.securesite.com

↓Server sends https://www.securesite.com content,

including its HSTS header↓

Browser updates HSTS header information

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What If There's An Impostor PretendingTo Be The www.securesite.com Server?


Browser sees www.securesite.com in its database, so it rewrites the request to ask for https://www.securesite.com

↓Malicious server returns content purported to be from https://www.securesite.com , but it is plaintext, or the

certificate does not match the one trusted by the browser↓

Browser returns server error

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What Are The Problems WithHTTP Strict Transport Security

Implementation?

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First-Visit Issue

● HSTS provides no protection the first time a user visits the site

● how would the browser know about the header before it sees the header?

● Google Chrome has implemented a hard-coded list

● used to be an array in the C++ code; now a JSON dataset

● protect fresh installs and wiped profiles● to get on the list, email the developer

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Chrome Preloaded HSTS Issues

● scalability

● how to keep current sites on the list, and prevent denial of service if a domain stops using encryption

● Google may one day use an online database, like its Safe Browsing Database

● malicious builds

● ...who MD5s their software, anyway?

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Misimplementation of Subdomains

users are lazy.

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User wants to see http://www.securesite.com, but only typessecuresite.com into his browser

↓User has a malicious DNS server configured, so DNS

for securesite.com resolves to 10.0.0.1.↓

Browser sends GET request to 10.0.0.1 for http://securesite.com↓

Server at 10.0.0.1 sends malicous contentpassed off as http://securesite.com

↓Browser displays spoofed http://securesite.com to User

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User wants to see http://www.securesite.com, but only typessecuresite.com into his browser

↓Browser goes to http://securesite.com

↓Server sends 302 Found for https://securesite.com

↓Browser requests https://securesite.com

↓Server sends 301 Permanent Redirect to

https://www.securesite.com↓

Browser requests https://www.securesite.com ↓

Server responds with https://www.securesite.com content,including HSTS header for www.securesite.com

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if securesite.com is just a redirect to www.securesite.com, then securesite.com

needs an HSTS header.

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HSTS Database Adulteration

● HSTS lists are saved in user profile

● no root or Administrator permissions required to change max-age, or just delete the HSTS database

● why is this a problem?

● hitting the legitimate site again will resend the header information

● dangerous when paired with a rogue DNS server

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● clean_hsts.rb

● Windows Metasploit post-exploitation module● clears out HSTS databases for both Firefox and

Chrome● installs a Windows registry key to clear out the

database on boot for the user, or all users if the script has administrative privileges

● available at http://www.rogueclown.net/clean_hsts.rb

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● how can this be fixed?

● Google Chrome Preloaded HSTS● any other ways to get around this, given the design goal

of a mechanism transparent to the user?

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Any Questions?

● email: [email protected]

● Twitter: @rogueclown

● website: http://www.rogueclown.net

● or, just find me at the conference!

can http strict transport security meaningfully help secure

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