Security readout for executives and security teams
Plain-English summary
This flaw lets specially structured file-upload style requests bypass an OWASP Core Rule Set detection rule. Organizations using affected CRS versions may have less WAF protection than expected, especially for internet-facing applications that rely on CRS to block suspicious multipart content.
Executive priority
High priority. Patch or confirm vendor-managed remediation quickly for public applications relying on OWASP CRS. The issue weakens a security control rather than directly compromising an application, but the CVSS score and bypass nature justify urgent handling.
Technical view
Before CRS 4.22.0 and 3.3.8, rule 922110 mishandles chained rule captures while iterating MULTIPART_PART_HEADERS. Earlier captured charset values can be overwritten by later parts, so a malicious charset in an earlier multipart section may be missed when a later section looks legitimate.
Likely exposure
Exposure is likely where OWASP CRS versions below 4.22.0 or below 3.3.8 protect web applications and process multipart requests. Risk is higher for public applications accepting uploads or form submissions. Bundled CRS copies in WAF products should be checked against vendor packaging.
Exploitation context
The CVE is not listed in KEV, and the provided sources do not prove active exploitation. A public GitHub reference is tagged as exploit in the source bundle, so defenders should assume technical details may be accessible and prioritize remediation.
Researcher notes
Focus validation on CRS rule 922110 and multipart header handling. The advisory describes capture variable overwrite across chained rule iteration. Avoid assuming backend compromise; the demonstrated security impact is WAF rule bypass. Evidence for real-world exploitation is not provided in the bundle.
Mitigation direction
- Upgrade OWASP CRS to 4.22.0 or 3.3.8.
- Check WAF vendor guidance if CRS is bundled or managed.
- Prioritize internet-facing applications accepting multipart form data.
- Review temporary controls with the vendor if immediate upgrade is impossible.
Validation and detection
- Inventory deployed OWASP CRS versions across WAF environments.
- Confirm affected versions are not below 4.22.0 or 3.3.8.
- Verify managed WAF packages include the patched CRS release.
- Review WAF logs for unusual multipart requests with multiple part headers.
Public sources used
Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.
Conservative CVE-to-ATT&CK context
These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.
ATT&CK lookup starting points
Use these exact CWE pages and searches to review the Glexia ATT&CK library from this CVE's weakness and description context.
CWE-794: Exact CWE lookup
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Open ATT&CK lookupCVE-2026-21876 mapping review
Open the CVE-to-ATT&CK bridge for reviewed, inferred, or future official mappings tied to this CVE.
Open ATT&CK lookup- Severity
- Critical
- CVSS
- 9.3 (3.1)
- Known Exploited
- No
- Published
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:L/A:N
CNA and ADP enrichment extracted from CVE v5
These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.
CVSS vector scores
1 official scoreWe collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:L/A:N3.94.7Primary CVE scoreVulnerability scoring details
Base CVSS 3.1 score
9.3CriticalVector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:L/A:N
Source materials
- CVE List V5 sourceCVE List V5
- https://github.com/coreruleset/coreruleset/security/advisories/GHSA-36fv-25j3-r2c5CVE reference · x_refsource_CONFIRM
- https://github.com/coreruleset/coreruleset/commit/80d80473abf71bd49bf6d3c1ab221e3c74e4eb83CVE reference · x_refsource_MISC
- https://github.com/coreruleset/coreruleset/commit/9917985de09a6cf38b3261faf9105e909d67a7d6CVE reference · x_refsource_MISC
- https://github.com/coreruleset/coreruleset/releases/tag/v3.3.8CVE reference · x_refsource_MISC
- https://github.com/coreruleset/coreruleset/releases/tag/v4.22.0CVE reference · x_refsource_MISC
- https://github.com/daytriftnewgen/CVE-2026-21876CVE reference · exploit
Products and packages named in the record
CWE details
CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.
Incomplete Filtering of Multiple Instances of Special Elements
Incomplete Filtering of Multiple Instances of Special Elements represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.