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CVE-2026-44631 – Buffer Underwrite leading to RCE – Apache HTTP Server 2.4.0 through 2.4.67

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Summary

CVE-2026-44631 is a Buffer Underwrite (CWE-124) vulnerability in Apache HTTP Server affecting all versions from 2.4.0 through 2.4.67, patched in version 2.4.68 released on June 8, 2026. The flaw is caused by a signed character overflow in the ap_regname function, triggered by crafted regular expressions in the server configuration, and carries a CVSS 9.8 Critical score — indicating full network exploitability with no authentication required and complete confidentiality, integrity, and availability impact consistent with remote code execution.

Technical details

  • Root cause: Signed character overflow in the ap_regname function causes a heap underwrite (buffer underflow) condition, writing memory before the intended buffer boundary.
  • Trigger conditions: Exploitation requires a crafted regular expression to be present in the Apache HTTP Server configuration (e.g., within mod_rewrite, mod_alias, or similar regex-processing directives). Regex-based configuration is extremely common in production Apache deployments.
  • Attack vector: Network-based, unauthenticated, requiring no user interaction. CVSS AC:L (low complexity) indicates a straightforward attack path once the triggering condition exists.
  • Impact: Full compromise of confidentiality, integrity, and availability (C:H/I:H/A:H), consistent with remote code execution potential on the affected host.
  • Discoverers: Zhenpeng (Leo) Lin at depthfirst and Bartlomiej Dmitruk.

Affected software

  • Apache HTTP Server 2.4.0 through 2.4.67 (all releases in the 2.4.x line prior to 2.4.68)

Severity

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

Mitigation and recommended actions

  • Immediate action: Upgrade to Apache HTTP Server 2.4.68, released June 8, 2026, which contains the fix (commit r1935015). This is the only vendor-recommended remediation.
  • Interim mitigation: If immediate upgrade is not feasible, review and harden Apache configuration files to audit or restrict the use of complex or externally influenced regular expressions in directives processed by ap_regname. Restrict network access to the server where operationally possible.

IONIX Status

The IONIX research team is tracking ongoing exploitation attempts and recommends immediate patching. Potentially affected assets are outlined in this post.

References

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