CVE-2026-43631 in llama.cpp
Summary
by MITRE • 08/07/2026
llama.cpp builds b7492 through the latest b9060 contains a use-after-free vulnerability in the vocab pointer of llama-server when the --sleep-idle-seconds feature is enabled, allowing unauthenticated remote attackers to execute arbitrary code. Attackers can trigger the vulnerability by sending requests to affected endpoints while the server transitions to sleep mode, causing concurrent worker threads to dereference a freed vocab pointer that can be reclaimed with attacker-controlled data to achieve remote code execution.
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Analysis
by VulDB Data Team • 08/07/2026
The vulnerability identified in llama.cpp versions b7492 through b9060 represents a critical use-after-free condition that fundamentally compromises the security integrity of the llama-server implementation. This flaw specifically targets the vocab pointer management within the server's memory architecture, creating a dangerous scenario where freed memory locations can be accessed and manipulated by concurrent threads during idle state transitions. The vulnerability manifests exclusively when the --sleep-idle-seconds feature is actively enabled, indicating that the risk is directly tied to the server's power management functionality rather than general operational use.
The technical exploitation of this vulnerability occurs through a carefully orchestrated sequence involving concurrent thread execution and memory management timing issues. When the server enters its sleep mode transition period, multiple worker threads attempt to access the vocab pointer that has already been freed from memory. This race condition creates an opportunity where the freed memory block can be reallocated and populated with attacker-controlled data before the subsequent dereference operation occurs. The use-after-free scenario follows the common pattern documented in CWE-416 where memory is accessed after it has been freed, but the specific context within the llama-server's vocabulary handling system makes this particularly dangerous.
The operational impact of this vulnerability extends far beyond simple denial-of-service conditions, as it enables unauthenticated remote code execution capabilities that can be leveraged by attackers without requiring any privileged access. The attack surface is particularly concerning given that the vulnerability can be triggered through standard API endpoints that are typically exposed to external networks, making the exploit accessible to anyone with network connectivity to the affected server. This represents a severe escalation from typical memory corruption vulnerabilities since it provides direct execution capabilities rather than merely system instability or data compromise.
The exploitation methodology relies on understanding the precise timing window during which the sleep transition occurs and the subsequent memory allocation patterns that allow attackers to control the contents of the freed memory location. The vulnerability's presence in the vocab pointer handling specifically indicates issues with how the server manages its language model vocabulary data structures during state transitions, suggesting that proper memory lifecycle management protocols are not adequately implemented for this particular feature set. This type of vulnerability aligns with ATT&CK technique T1059.007 for remote code execution through memory corruption attacks.
Mitigation strategies should focus on immediate patching of the affected llama.cpp versions to address the memory management race condition in the sleep mode implementation. The most effective approach involves implementing proper synchronization mechanisms around the vocab pointer access during state transitions, ensuring that memory deallocation occurs only after all concurrent threads have completed their operations. Additionally, input validation and bounds checking should be strengthened throughout the vocabulary handling code path to prevent any potential manipulation of freed memory locations. Organizations should also consider disabling the --sleep-idle-seconds feature until proper security patches are applied, as this removes the vulnerable execution path entirely while maintaining service functionality.