csrf final
TRANSCRIPT
Agenda
• Why CSRF is interesting?
• Refresher on HTML forms
• Anatomy of CSRF attack
• Obstacles for attacker
• Existing CSRF defenses
Existing CSRF defenses
• Secret Validation token• We use a validation token to determine whether the request came from
an authorized source.
• Validation token must be hard to guess by the attacker
• If the request is missing a validation token or the token does not matchthe expected value the server should reject the request.
• Where the Secret validation token fails• Many websites and CSRF frameworks fail to implement secret token
defense correctly.
• One common mistake is to leak the CSRF token during cross site request.
• Eg. If the honest site appends the CSRF token to the hyperlinks to another website then that website gains the ability to forge cross-site requests against the honest site.
• The Referer Header• When the browser issues an HTTP request, it includes a referer header that
indicates which URL initiated the request.
• This information in the Referer header could be used to distinguish between same site request and cross site request.
Referer: http://www.facebook.com/
Referer: http://www.attacker.com/evil.html
Referer:
?
• Privacy Issues with Referer header
• The referer contains senstive information that impinges on the privacy
• The referer header reveals contents of the search query that lead to visit a website.
• Some organizations are concerned that confidential information about their corporate intranet might leak to external websites via Referer header
• Referer header Implementation
Lenient Referer Validation
• The site blocks request whose referer header has incorrect value
• If the request lacks the header then the site accepts the request.
• Disadvantage is that the attacker can cause the web browser to suppress the referer header.
• Eg. Request issued from ftp and data URLs do not carry Referer headers.
• Referer header Implementation
Strict Referer Validation
• The site blocks the request whose referer header has incorrect value and also blocks request that lack a refererheader.
• Disadvantage is that some browsers and network configurations suppress referer header for legitimate requests.
• Custom HTTP Headers
• Browsers prevent sites from sending custom HTTP headers to another site but allow sites to send custom HTTP headers to themselves.
• Cookie value is not actually required to prevent CSRF attacks, the mere presence of the header is sufficient.
• To use this scheme as a CSRF Defense, a site must issue all state modifying requests using XMLHttpRequest, attach the header and reject all requests that do not accompany the header .