Affects widely-used OWASP CRS; enables WAF evasion across many orgs.
CVE-2026-21876
CRS multipart processing bug overwrites captures, risking charset-based evasion.
Is CVE-2026-21876 being exploited?
Not confirmed. CVE-2026-21876 does not appear in CISA's Known Exploited Vulnerabilities catalog, which records only exploitation that has been observed and reported publicly. That is evidence of absence of a report, not evidence the flaw is unattacked. EPSS currently estimates a 14% probability of exploitation in the next 30 days.
How severe is CVE-2026-21876?
CVE-2026-21876 is rated Critical with a CVSS score of 9.3. Severity describes how bad exploitation would be, not how likely it is: pair it with exploitation evidence before deciding what to patch first.
Is there a patch for CVE-2026-21876?
Yes. A fix has been recorded for CVE-2026-21876. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2026-21876 affect?
CVE-2026-21876 affects OWASP Core Rule Set (CRS), Web Application Firewall with ModSecurity or compatible engines. Confirm the exact affected versions against the vendor advisory before deciding you are exposed.
What should I do about CVE-2026-21876?
Patch CRS to 4.22.0 or 3.3.8+
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
WAF detection can be bypassed as earlier multipart captures are overwritten, enabling malicious payloads to reach apps.
Immediate action required
- affected< 4.22.0
- affected< 3.3.8
As published in the CVE Program record. A version outside these ranges is not a statement that it is unaffected — vendors sometimes understate a range, and distribution-backported builds carry upstream numbers that do not reflect what was patched into them.