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CVE-2026-56154 – Remote Code Execution / Denial of Service (Use-After-Free) – Apache HTTP Server 2.4

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Summary

CVE-2026-56154 is a use-after-free vulnerability (CWE-416) in the mod_rewrite module of Apache HTTP Server, triggered when processing lookahead expressions using the %{LA-U:HTTP:...} syntax. The flaw affects Apache HTTP Server versions 2.4.0 through 2.4.68 and has been assigned a CVSS v3.1 base score of 9.8 (Critical). The Apache HTTP Server Project has released version 2.4.69 to remediate the issue.

Technical details

  • Root cause: a use-after-free condition in mod_rewrite‘s handling of lookahead rule evaluation (%{LA-U:HTTP:...}), where memory associated with a request can be freed and subsequently accessed again during rewrite processing.
  • Trigger conditions: the affected server must have mod_rewrite enabled with a rewrite configuration (RewriteCond/RewriteRule) that uses the %{LA-U:HTTP:...} lookahead syntax; an attacker-controlled HTTP request can then trigger the unsafe memory access during rule evaluation.
  • Attack vector: network — the vulnerability is reachable via standard HTTP requests to a vulnerable, appropriately configured server, with no authentication or user interaction required.
  • Impact: use-after-free conditions of this kind can corrupt process memory, leading to a crash/denial of service, and depending on exploitability of the freed memory, potentially further memory-corruption impact.
  • Fixed upstream via commit r1938660 in the 2.4.x branch; the fix shipped in Apache HTTP Server 2.4.69.
  • Reported to the Apache HTTP Server Project by Nebula Security (@nebusecurity) on 2026-05-25; publicly disclosed and fixed on 2026-10-01.

Affected software

  • Apache HTTP Server versions 2.4.0 through 2.4.68 (inclusive), when mod_rewrite is enabled and configured with %{LA-U:HTTP:...} lookahead expressions.

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 Apache HTTP Server to version 2.4.69 or later, which contains the fix for this vulnerability.
  • If immediate patching is not possible: review mod_rewrite configurations and remove or avoid use of %{LA-U:HTTP:...} lookahead expressions in RewriteCond/RewriteRule directives until the upgrade can be applied; alternatively, disable mod_rewrite where it is not required.

References

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