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CVE-2026-43631 – Unauthenticated Remote Code Execution (Use-After-Free) – llama.cpp llama-server bui

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Summary

CVE-2026-43631 is a use-after-free vulnerability in the llama-server component of llama.cpp (ggml-org). When the --sleep-idle-seconds feature is enabled, concurrent worker threads dereference a freed vocab pointer that can be reclaimed with attacker-controlled data, allowing an unauthenticated remote attacker to execute arbitrary code. It is rated Critical (CVSS v4.0 base score 9.2).

Technical details

  • Root cause: Use-after-free (CWE-416) combined with a race condition (CWE-362) affecting the vocab pointer in llama-server.
  • Trigger conditions: The --sleep-idle-seconds feature must be enabled; the flaw is reachable as the server transitions to sleep mode while concurrent worker threads are active.
  • Attack vector: Network. No authentication or user interaction is required; attack complexity is high.
  • Impact: Concurrent worker threads dereference a freed vocab pointer that can be reclaimed with attacker-controlled data, leading to remote code execution with high impact to confidentiality, integrity, and availability.

Affected software

  • llama.cpp llama-server, builds b7492 through b9060 (inclusive).

Severity

  • CVSS v4.0: 9.2 (Critical) — CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N
  • CVSS v3.1: 8.1 (High) — CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H

Mitigation and recommended actions

  • Immediate: Upgrade llama.cpp to a build later than b9060 that includes the fix; refer to the published security patches.
  • If no patch: Disable the --sleep-idle-seconds feature, and restrict network access to llama-server so it is not exposed to untrusted networks.

References

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