CVE-2013-4567 in MediaWiki
Summary
by MITRE
Incomplete blacklist vulnerability in Sanitizer::checkCss in MediaWiki before 1.19.9, 1.20.x before 1.20.8, and 1.21.x before 1.21.3 allows remote attackers to conduct cross-site scripting (XSS) attacks via a \b (backspace) character in CSS.
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Analysis
by VulDB Data Team • 06/02/2021
The vulnerability identified as CVE-2013-4567 represents a critical security flaw in MediaWiki's CSS sanitization mechanism that persisted across multiple versions including 1.19.9, 1.20.8, and 1.21.3. This issue stems from an incomplete blacklist implementation within the Sanitizer::checkCss function, which is responsible for filtering potentially malicious CSS content to prevent cross-site scripting attacks. The vulnerability specifically manifests when a backspace character is included in CSS content, allowing attackers to bypass the sanitization process and execute malicious code in the context of a victim's browser.
The technical root cause of this vulnerability lies in the insufficient filtering of special characters within CSS content processing. The Sanitizer::checkCss function in MediaWiki relies on a blacklist approach to identify and remove dangerous CSS properties and characters, but the implementation fails to properly handle the backspace character which can be used to manipulate the CSS parsing process. This incomplete blacklist approach creates a security gap where certain characters that should be filtered out are permitted to pass through the sanitization process, thereby enabling attackers to inject malicious CSS that can execute JavaScript code in the victim's browser context.
The operational impact of this vulnerability is significant as it allows remote attackers to conduct cross-site scripting attacks without requiring any authentication or privileged access. When a malicious user submits CSS content containing a backspace character, the sanitization process fails to properly filter this character, enabling the injection of malicious CSS that can execute arbitrary JavaScript code. This creates a persistent threat vector where attackers can compromise user sessions, steal cookies, redirect users to malicious sites, or perform other malicious activities that target users of the vulnerable MediaWiki installations. The vulnerability affects all versions prior to the specified patches, making it particularly dangerous for organizations running older MediaWiki deployments.
The security implications of this vulnerability align with CWE-79, which describes cross-site scripting flaws, and can be mapped to ATT&CK technique T1566.001 for the initial access phase of a cyberattack. Organizations affected by this vulnerability should prioritize immediate patching of their MediaWiki installations to the recommended versions that contain the fixed sanitization logic. Additional mitigations include implementing strict content filtering at the network level, monitoring for suspicious CSS content submissions, and educating users about the risks of submitting untrusted content to MediaWiki platforms. The vulnerability demonstrates the critical importance of comprehensive input validation and the dangers of relying solely on blacklist-based approaches for security filtering, as opposed to more robust whitelist validation methods that explicitly define what content is allowed rather than what content is forbidden.