CVE-2009-5017 in Firefox
Summary
by MITRE
Mozilla Firefox before 3.6 Beta 3 does not properly handle overlong UTF-8 encoding, which makes it easier for remote attackers to bypass cross-site scripting (XSS) protection mechanisms via a crafted string, a different vulnerability than CVE-2010-1210.
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Analysis
by VulDB Data Team • 09/29/2021
The vulnerability described in CVE-2009-5017 represents a critical flaw in Mozilla Firefox's handling of UTF-8 encoding that significantly undermines the browser's security model. This issue affects Firefox versions prior to 3.6 Beta 3 and stems from the browser's improper validation of overlong UTF-8 sequences, which are valid but non-canonical representations of Unicode characters. The flaw allows attackers to craft malicious strings that can bypass the browser's cross-site scripting protection mechanisms, creating a vector for sophisticated attacks that exploit the inconsistency between how the browser processes UTF-8 encoding and how it enforces security policies.
The technical root cause of this vulnerability lies in Firefox's failure to properly normalize UTF-8 sequences during input processing. Overlong UTF-8 encoding occurs when Unicode characters are represented using more bytes than necessary, such as representing a single-byte character using two or more bytes. This non-standard encoding can confuse security filters that expect canonical representations, allowing malicious payloads to slip through validation checks that would normally detect XSS attempts. The vulnerability operates at the protocol level where UTF-8 encoding is processed, making it particularly insidious as it can bypass multiple layers of security controls that rely on proper character encoding validation.
From an operational perspective, this vulnerability creates significant risk for users of affected Firefox versions as it enables attackers to craft XSS payloads that exploit the browser's inconsistent handling of character encoding. Attackers can leverage this flaw to bypass security mechanisms such as Content Security Policy (CSP) and other XSS protection features, potentially leading to session hijacking, data theft, or malicious code execution. The impact extends beyond simple cross-site scripting as the vulnerability can be combined with other techniques to create more sophisticated attack vectors that can circumvent modern web security defenses. This type of vulnerability is particularly dangerous in environments where users access untrusted websites or where security boundaries are already compromised.
The security implications of CVE-2009-5017 align with CWE-116, which addresses the improper handling of encoding in security controls, and can be mapped to ATT&CK technique T1211 for exploitation of encoding flaws. Organizations should implement immediate mitigations including updating to Firefox 3.6 Beta 3 or later versions, deploying web application firewalls that can detect and block overlong UTF-8 sequences, and implementing additional input validation measures. Security teams should also conduct thorough vulnerability assessments to identify systems running affected browser versions and ensure proper patch management procedures are in place. The vulnerability demonstrates the critical importance of proper encoding normalization in security-critical applications and highlights the need for comprehensive testing of edge cases in character encoding handling.