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CVE-2026-77806 – Unauthenticated Remote Code Execution – SPIP before 4.4.21

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Summary

CVE-2026-77806 is a critical, unauthenticated remote code execution vulnerability (CWE-94, Improper Control of Generation of Code) affecting the SPIP content management system prior to version 4.4.21. The flaw stems from mishandling of the X-Spip-Filtre HTTP request header by the analyse_resultat_skel function, and has a CVSS v3.1 base score of 9.8 (Critical). SPIP has confirmed that exploitation attempts against this vulnerability were observed in the wild in August 2026.

Technical details

  • Root cause: SPIP’s template rendering engine allows the X-Spip-Filtre HTTP header to inject a chain of PHP filter function names that are applied to user-controlled input reaching a compiled template (e.g., a forum preview texte field) via analyse_resultat_skel.
  • Trigger conditions: An attacker sends a crafted HTTP request containing a malicious X-Spip-Filtre header (e.g., chaining filters such as one that reads a request parameter into another that executes system commands) alongside a request that reaches the vulnerable rendering path; no authentication is required.
  • Attack vector: Network-based, delivered over standard HTTP(S) requests to a public-facing SPIP site (GET or POST-based submissions, including forum submission endpoints).
  • Impact: Successful exploitation results in arbitrary PHP/OS code execution on the server, with full compromise of confidentiality, integrity, and availability. SPIP has stated this flaw is not mitigated by SPIP’s built-in "security screen" protection.

Affected software

  • SPIP versions prior to 4.4.21 (including at least 4.4.19 and 4.4.20)

Severity

  • CVSS v3.1 Base Score: 9.8 (Critical)
  • Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Mitigation and recommended actions

  • Immediate: Upgrade to SPIP 4.4.21 or later, which contains the fix for this vulnerability. Updates can be obtained via the official SPIP update channel (spip_loader 7.0.0) or from get.spip.net.
  • If immediate patching is not possible: Restrict or filter incoming requests that set the X-Spip-Filtre header at a WAF or reverse proxy in front of the SPIP installation, and monitor logs for anomalous use of this header. Note that SPIP’s built-in security screen does not protect against this issue, so header filtering or upgrading are the only reliable mitigations.

References

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