PT-2013-63: Hash Length Extension in HTMLPurifier Vulnerable softwareHTMLPurifier Version: 4.5.0 and earlierLink: http://htmlpurifier.org/Severity levelSeverity level: Medium Impact: Signature Forgery, Arbitrary Value Injection into Signed Data Access Vector: Remote CVSS v2: Base Score: 5.0 Vector: (AV:N/AC:L/Au:N/C:N/I:P/A:N)CVE: not assignedSoftware descriptionHTML Purifier is a standards-compliant HTML filter library written in PHP. Vulnerability descriptionThe specialists of the Positive Research center have detected a vulnerability in HTMLPurifier that allows attackers to carry out a "Hash Length Extension" attack..Class HTMLPurifier_URIFilter_Munge implements a URI filter that replaces all links with a formatted URL, for example, it can be used to rewrite links to pass them into redirection script. To prevent links from being altered HTMLPurifier allows specifying a secret key via "URI.MungeSecretKey" configuration directive. However, this signature generation method is susceptible to a hash length extension attack.The vulnerability allows attackers to append an arbitrary value to the signed data without knowing the secret key. This may cause other vulnerabilities in the web applications, which use signature generated by HTMLPurifier to keep the links from being altered.How to fixUpdate your software up to the latest versionAdvisory status 03.09.2013 - Vendor gets vulnerability details 30.11.2013 - Vendor releases fixed version and details 04.12.2013 - Public disclosureCreditsThe vulnerability was detected by Arseny Reutov, Positive Research Center (Positive Technologies Company)Referenceshttp://en.securitylab.ru/lab/PT-2013-63 Reports on the vulnerabilities previously discovered by Positive Research:http://www.ptsecurity.com/research/advisory/ http://en.securitylab.ru/lab/
