[PT-2013-63] Hash Length Extension in HTMLPurifier

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(PT-2013-63) Positive Technologies Security Advisory 
       Hash Length Extension in HTMLPurifier

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---[ Vulnerable software ]

HTMLPurifier
Version: 4.5.0 and earlier

Link:
http://htmlpurifier.org/

---[ Severity level ]

Severity 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 assigned

---[ Software description ]

HTML Purifier is a standards-compliant HTML filter library written in PHP. 

---[ Vulnerability description ]

The 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 fix ]

Update your software up to the latest version

---[ Advisory status ]

03.09.2013 - Vendor gets vulnerability details
30.11.2013 - Vendor releases fixed version and details 
04.12.2013 - Public disclosure

---[ Credits ]

The vulnerability was detected by Arseny Reutov, Positive Research Center 
(Positive Technologies Company)

---[ References ]

http://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/

---[ About Positive Technologies ]

Positive Technologies www.ptsecurity.com is among the key players in the IT 
security market in Russia.

The principal activities of the company include the development of integrated 
tools for information security monitoring (MaxPatrol); providing IT security 
consulting services and technical support; development of 
the Securitylab leading Russian information security portal.

Among the clients of Positive Technologies, there are more than 40 state 
enterprises, more than 50 banks and financial organizations, 20 
telecommunication companies, more than 40 plant facilities, as well as IT, 
service and retail companies from Russia, the CIS countries, the Baltic States, 
China, Ecuador, Germany, Great Britain, Holland, Iran, Israel, Japan, Mexico, 
the Republic of South Africa, Thailand, Turkey, and the USA.

Positive Technologies is a team of highly skilled developers, advisers and 
experts with years of vast hands-on experience. The company specialists possess 
professional titles and certificates; they are the members of various 
international societies and are actively involved in the IT security field 
development.