How severe is CVE-2026-21876?

CVE-2026-21876 has a CVSS 3.x base score of 9.3, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity low, and availability none.

Is CVE-2026-21876 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 13% (96th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2026-21876?

CVE-2026-21876 affects Owasp Owasp Modsecurity Core Rule Set. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2026-21876?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its critical severity, prioritise patching exposed systems.

Does CVE-2026-21876 have an EU (EUVD) identifier?

Yes. CVE-2026-21876 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-1669.

When was CVE-2026-21876 published?

CVE-2026-21876 was published on 2026-01-08 and last updated on 2026-06-17.

CVE-2026-21876

CVE-2026-21876 is a critical-severity vulnerability in Owasp Owasp Modsecurity Core Rule Set with a CVSS 3.x base score of 9.3. Its EPSS exploit-prediction score of 13% places it in the 96th percentile, indicating an elevated likelihood of exploitation. The underlying weakness is classified as CWE-794.

Key facts

Severity: Critical (CVSS 3.x base score 9.3)
EPSS exploit prediction: 13% (96th percentile)
Actively exploited: Not listed in CISA KEV
EU (EUVD) id: EUVD-2026-1669
Weakness: CWE-794
Affected product: Owasp Owasp Modsecurity Core Rule Set
Published: 2026-01-08
Last modified: 2026-06-17

Description

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.

Frequently asked questions

What is CVE-2026-21876?: 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.
How severe is CVE-2026-21876?: CVE-2026-21876 has a CVSS 3.x base score of 9.3, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity low, and availability none.
Is CVE-2026-21876 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 13% (96th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2026-21876?: CVE-2026-21876 affects Owasp Owasp Modsecurity Core Rule Set. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2026-21876?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its critical severity, prioritise patching exposed systems.
Does CVE-2026-21876 have an EU (EUVD) identifier?: Yes. CVE-2026-21876 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-1669.
When was CVE-2026-21876 published?: CVE-2026-21876 was published on 2026-01-08 and last updated on 2026-06-17.

References

Affected products (1)

cpe:2.3:a:owasp:owasp_modsecurity_core_rule_set:*:*:*:*:*:*:*:*

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Other CWE-794 vulnerabilities

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