CVE-2026-21876 | OWASP CRS has multipart bypass using multiple content-type parts

Exp

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.

Published: 2026-01-08 Last update: 2026-04-09 Assigner: [email protected] Source: [email protected]
NVD Status：Modified，CVE State: published
View at NVD, CVE.org, EUVD

CVE-2026-21876 is rated High Exploit Risk (82.2/100): CVSS Critical severity, with medium exploitation likelihood (EPSS 3.37%). 2 public exploit reference(s) are indexed (Exploit-DB). Public exploits are available—assess exposure, apply mitigations, and prioritize patching.

Risk is dynamic; we continuously reassess and refresh what is shown on this page as upstream context changes.

Public exploit references (Exploit-DB) for CVE-2026-21876

EDB-ID Source Kind Published Link
52558 exploit_db edb 2026-05-13 Exploit-DB ↗
— nvd_ref exploit_tag Exploit-DB ↗

Exploit prediction scoring system (EPSS) score for CVE-2026-21876

EPSS lead: Daily EPSS estimates relative likelihood of exploitation; percentile ranks this CVE among scored vulnerabilities (higher = more severe relative rank).

# Date Old EPSS score New EPSS score Delta (New - Old)
1 2026-06-05 3.98% 3.37% -0.61%
2 2026-05-22 3.59% 3.98% +0.40%
3 2026-05-14 — 3.59% —

Full EPSS history (5 records total)

Common vulnerability scoring system (CVSS) metrics for CVE-2026-21876

CVSS metrics for this CVE.

Base score Version Severity Vector Exploitability Impact Score source
9.3 3.1 CRITICAL
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:L/A:N Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:N)
No account or special rights needed—anonymous or random user is enough.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:C)
Breaking this can reach past the original component and bite other resources—bigger blast radius.
Confidentiality (C:H)
Serious risk that confidential data gets exposed in a big way.
Integrity (I:L)
Attackers could change some data, but it’s limited—not everything goes.
Availability (A:N)
Service keeps running; no real outage angle.
 3.9 4.7 [email protected]
5.3 3.1 MEDIUM
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:N)
No account or special rights needed—anonymous or random user is enough.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:U)
Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:N)
Doesn’t really leak secrets in a meaningful way.
Integrity (I:L)
Attackers could change some data, but it’s limited—not everything goes.
Availability (A:N)
Service keeps running; no real outage angle.
 3.9 1.4 [email protected]

Weakness enumeration for CVE-2026-21876

OS Trackers for CVE-2026-21876

vendor priority summary link
debian not yet assigned CVE-2026-21876 not yet assigned priority: Debian including 1 source packages (modsecurity-crs), 5 status rows across 5 suites (bookworm, bullseye, forky, sid, trixie): resolved 5. https://security-tracker.debian.org/tracker/CVE-2026-21876
ubuntu medium CVE-2026-21876 medium priority: Ubuntu including 1 source packages (modsecurity-crs), 8 status rows across 8 suites (bionic, focal, jammy, noble, plucky, questing, upstream, xenial): needs-triage 7, ignored 1. https://ubuntu.com/security/CVE-2026-21876

Affected software / configurations for CVE-2026-21876

Vendor Product Version Raw CPE
owasp owasp_modsecurity_core_rule_set < 3.3.8 cpe:2.3:a:owasp:owasp_modsecurity_core_rule_set:*:*:*:*:*:*:*:*
owasp owasp_modsecurity_core_rule_set >= 4.0.0, < 4.22.0 cpe:2.3:a:owasp:owasp_modsecurity_core_rule_set:*:*:*:*:*:*:*:*

References for CVE-2026-21876

cvelogic Threat Intelligence