CVE-2026-3388: Squirrel-lang Squirrel DOS Vulnerability

CVE-2026-3388 Overview

A vulnerability has been identified in Squirrel scripting language versions up to 3.2. The flaw exists within the SQCompiler::Factor and SQCompiler::UnaryOP functions located in the squirrel/sqcompiler.cpp file. A malicious input can trigger uncontrolled recursion, potentially leading to a denial of service condition through stack exhaustion. This vulnerability requires local access to exploit and a proof-of-concept has been made publicly available.

Critical Impact

Attackers with local access can craft malicious Squirrel scripts that trigger uncontrolled recursion in the compiler, leading to stack exhaustion and application crashes. This could affect any application embedding the Squirrel scripting engine.

Affected Products

• Squirrel scripting language versions up to and including 3.2
• Applications embedding the Squirrel scripting engine
• Game engines and embedded systems utilizing Squirrel for scripting

Discovery Timeline

• 2026-03-01 - CVE-2026-3388 published to NVD
• 2026-03-05 - Last updated in NVD database

Technical Details for CVE-2026-3388

Vulnerability Analysis

This vulnerability falls under the category of Denial of Service through uncontrolled recursion (CWE-404: Improper Resource Shutdown or Release). The issue resides in the Squirrel compiler's parsing logic, specifically within the SQCompiler::Factor and SQCompiler::UnaryOP functions in squirrel/sqcompiler.cpp.

When the compiler processes deeply nested or specially crafted expressions, the recursive nature of these functions can be exploited to cause excessive stack consumption. Without proper recursion depth limits, processing malicious input leads to stack exhaustion and subsequent application termination.

The vulnerability requires local access, meaning an attacker must be able to provide malicious Squirrel source code to a vulnerable application for compilation. Applications that accept untrusted Squirrel scripts are at risk.

Root Cause

The root cause is insufficient recursion depth validation within the Squirrel compiler's expression parsing routines. The SQCompiler::Factor and SQCompiler::UnaryOP functions recursively call each other during expression parsing without adequate bounds checking. When processing expressions with extreme nesting levels, the call stack grows uncontrollably until system limits are exceeded, resulting in a stack overflow condition.

Attack Vector

The attack requires local access to the target system. An attacker must craft a malicious Squirrel script containing deeply nested expressions or specific syntactic patterns that trigger recursive calls in the compiler. When a vulnerable application attempts to compile this script, the uncontrolled recursion exhausts the call stack.

The attack scenario involves providing a specially crafted .nut file or Squirrel code snippet to any application that uses the Squirrel scripting engine for compilation. A reproducible example has been documented in the GitHub repository.

Detection Methods for CVE-2026-3388

Indicators of Compromise

• Unexpected application crashes with stack overflow or segmentation fault errors
• Abnormally high CPU usage during Squirrel script compilation
• Presence of unusually structured .nut files with deeply nested expressions
• Application logs showing compilation failures on specific script files

Detection Strategies

• Monitor applications embedding Squirrel for unexpected termination patterns
• Implement static analysis on incoming Squirrel scripts to detect excessive nesting
• Deploy application crash monitoring to identify stack exhaustion patterns
• Review system logs for recurring crashes in Squirrel-dependent processes

Monitoring Recommendations

• Enable detailed crash logging for applications using the Squirrel scripting engine
• Set up alerts for repeated crashes in processes that compile Squirrel code
• Monitor file system activity for suspicious .nut file creation or modification
• Track resource consumption metrics during script compilation operations

How to Mitigate CVE-2026-3388

Immediate Actions Required

• Identify all applications in your environment using the Squirrel scripting engine
• Restrict access to Squirrel script compilation to trusted sources only
• Implement input validation to reject scripts with excessive nesting depth
• Consider sandboxing applications that must compile untrusted Squirrel code

Patch Information

As of the last update, the Squirrel project maintainers have been notified through GitHub Issue #312 but have not yet responded with an official patch. Organizations should monitor the official Squirrel repository for security updates and apply patches when available.

For tracking purposes, additional vulnerability details are available through VulDB #348274.

Workarounds

• Implement a custom recursion depth limit wrapper around Squirrel compilation calls
• Validate and sanitize all Squirrel scripts before compilation, rejecting those with suspicious nesting patterns
• Run Squirrel compilation in a sandboxed environment with limited stack size to fail gracefully
• Disable script compilation features for untrusted input sources until an official patch is released

# Example: Limit stack size for Squirrel processes (Linux)
# This causes controlled failure instead of system-wide impact
ulimit -s 8192
./your_squirrel_application