Sanitizer bypass via trailing comment removal
Published Jan 7, 2021 · Updated Aug 4, 2024
CSS sanitizer bypass in Firefox before 84, Firefox ESR before 78.6, and Thunderbird before 78.6 lets attackers preserve disallowed CSS. nsTreeSanitizer::SanitizeStyleSheet uses an imprecise CSS rule start and removes a closing comment token, causing a rejected @import rule to survive sanitization. Exploitation requires crafted CSS to enter a sanitization path through user interaction, after which attacker-controlled CSS applies in the sanitized document.
Summary
What happened
CSS sanitizer bypass in Firefox before 84, Firefox ESR before 78.6, and Thunderbird before 78.6 lets attackers preserve disallowed CSS. nsTreeSanitizer::SanitizeStyleSheet uses an imprecise CSS rule start and removes a closing comment token, causing a rejected @import rule to survive sanitization. Exploitation requires crafted CSS to enter a sanitization path through user interaction, after which attacker-controlled CSS applies in the sanitized document.
The record
- CVE
- CVE-2020-26973
- Published
- Jan 7, 2021
- Updated
- Aug 4, 2024
- Vendor
- Mozilla
- Product
- Firefox
- Classifications
- Unavailable
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Jan 7, 2021CVE publishedPublication date reported by the CVE source.
- Aug 4, 2024Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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Is a vulnerable build present?
Compare these published version ranges with your installed build and any vendor patches.
- Affected versionversion=unspecified <84
What conditions does exploitation require?
What is affected?
Attacks
What attackers are doing with it
Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.
Weakness, pattern, technique
No sourced classifications are available.
Public exploit references
- Mozilla Bugzilla minimal testcaseproof of concept · demonstrated
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Technologies
Your stack
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Your stack
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