Unauthenticated WAF bypass via overwritten charset captures
Published Jan 8, 2026 · Updated Jan 8, 2026
Security-control bypass in OWASP CRS before 3.3.8 and 4.22.0 allows remote attackers to evade multipart charset validation. Rule 922110 overwrites TX capture variables while iterating over multipart headers, so its chained check validates only the final part. The rule is enabled at default Paranoia Level 1; encoded input reaches the backend only when a later part supplies an approved charset.
Summary
What happened
Security-control bypass in OWASP CRS before 3.3.8 and 4.22.0 allows remote attackers to evade multipart charset validation. Rule 922110 overwrites TX capture variables while iterating over multipart headers, so its chained check validates only the final part. The rule is enabled at default Paranoia Level 1; encoded input reaches the backend only when a later part supplies an approved charset.
The record
- CVE
- CVE-2026-21876
- Published
- Jan 8, 2026
- Updated
- Jan 8, 2026
- Vendor
- OWASP ModSecurity
- Product
- OWASP CRS
- Classifications
- CWE-794, T1211
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Jan 8, 2026CVE publishedPublication date reported by the CVE source.
- Jan 8, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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Is a vulnerable build present?
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- Affected versionversion=< 3.3.8
- Affected versionversion=< 4.22.0
What conditions does exploitation require?
What is affected?
Published CVSS scores
CVSS describes severity. EPSS estimates exploitation probability.
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
Public exploit references
- Rule 922110 multipart charset bypass proof of conceptproof of concept · demonstrated
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Technologies
Your stack
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Your stack
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