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Fencer Vulns
critical 9.3

CVE-2026-21876 in the critical feed json

OWASP CRS has multipart bypass using multiple content-type parts

published 2026-01-08 10 mo ago modified 2026-06-17 4 mo ago also EUVD-2026-1669

cvss
9.3
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
17.3%
EPSS 17.3% chance of exploitation in 30 days, percentile 97%
public exploits
1
Exploit-DB

Summary

The OWASP core rule set (CRS) is a set of generic attack detection rules for use with compatible web application firewalls. Prior to versions 4.22.0 and 3.3.8, the current rule 922110 has a bug when processing multipart requests with multiple parts. When the first rule in a chain iterates over a collection (like `MULTIPART_PART_HEADERS`), the capture variables (`TX:0`, `TX:1`) get overwritten with each iteration. Only the last captured value is available to the chained rule, which means malicious charsets in earlier parts can be missed if a later part has a legitimate charset. Versions 4.22.0 and 3.3.8 patch the issue.

Patch / fix

  • owasp coreruleset 3.3.8, 4.22.0
  • owasp owasp_modsecurity_core_rule_set 3.3.8, 4.22.0
  • modsecurity-crs Debian 12 3.3.4-1+deb12u1
  • modsecurity-crs Debian 13 3.3.7-1+deb13u1
  • modsecurity-crs Debian 14 3.3.8-1

debian advisories DEBIAN-CVE-2026-21876

Timeline

  1. Published
  2. First public exploit (EDB-52558)

CISA SSVC

Exploitation
none
Automatable
yes
Technical impact
partial
Assessed
10 mo ago

Exploit prediction

17.35% chance of exploitation in the next 30 days, higher than 97% of all scored CVEs (FIRST EPSS, 2026-11-08).

Exploits and checks

  • EDB-52558 coreruleset 4.21.0 - Firewall Bypass webapps, 2026-05-13
  • github.com https://github.com/daytriftnewgen/CVE-2026-21876 referenced PoC, tagged "exploit" by a source, unverified
  • github.com https://github.com/coreruleset/coreruleset/security/advisories/GHSA-36fv-25j3-r2c5 referenced PoC, tagged "exploit" by a source, unverified

CVSS scores

assigned by version score vector
CNA 3.1 9.3 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:L/A:N
NVD 3.1 5.3 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N

Affected products

  • coreruleset coreruleset

Classification

Weakness
  • CWE-794 Incomplete Filtering of Multiple Instances of Special Elements

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

CPEs and fixed versions use Anchore's correction of NVD's data: Added CPE configurations because not yet analyzed by NVD.