CVE-2026-9541

Vulnerability

The vulnerability is in Squirrel up to version 3.2, in the function ReadObject within squirrel/sqobject.cpp (commit f9267f2). When parsing OT_STRING elements from a .cnut bytecode file, the function reads a string length as a signed SQInteger and passes it to GetScratchPad() and SafeRead() without validating for negative values. A negative length causes GetScratchPad to return the existing scratchpad buffer (since it only resizes for positive sizes), and then SafeRead passes the negative length to fread, which interprets it as a large unsigned size_t, resulting in a heap buffer overflow. The bug is reachable via sq_static or any embedder using sqstd_loadfile / sq_readclosure [1].

Exploitation

An attacker needs only local access to load a crafted .cnut bytecode file. No authentication or special privileges are required beyond the ability to execute a Squirrel program that loads the malicious bytecode. The attacker provides a bytecode file containing an OT_STRING with a negative length value (e.g., -1). The ReadObject function reads this value and triggers the heap overflow when SafeRead writes beyond the intended buffer, corrupting adjacent heap memory [1]. A proof-of-concept exists and is publicly available [2].

Impact

Successful exploitation leads to a heap-based buffer overflow, which can allow an attacker to overwrite heap metadata or adjacent objects. This can potentially lead to arbitrary code execution within the context of the Squirrel process. The impact is local code execution, with confidentiality, integrity, and availability all potentially compromised depending on the exploit payload [1].

Mitigation

The project was informed of the problem but has not yet responded (as of the advisory date). No official patch exists for Squirrel 3.2 or earlier; the maintainer has been unresponsive. The vulnerability is not listed in KEV (as of publication). As a mitigation, users should avoid loading .cnut bytecode from untrusted sources. If possible, restrict local file loading permissions to trusted users only. Since no fix is available, upgrading to a future patched version (if released) is recommended [1].