CVE-2026-3392: Lily-lang Lily Use-After-Free Vulnerability

CVE-2026-3392 Overview

A null pointer dereference vulnerability has been identified in FascinatedBox Lily programming language interpreter up to version 2.3. The vulnerability exists in the eval_tree function within the file src/lily_emitter.c. When triggered, this manipulation causes a null pointer dereference condition that can lead to application crashes and denial of service. The attack requires local execution and has been publicly disclosed with exploit details made available.

Critical Impact

This vulnerability allows local attackers to crash the Lily interpreter through crafted input, potentially disrupting development workflows or applications relying on the Lily scripting language.

Affected Products

• lily-lang lily up to version 2.3
• Lily programming language interpreter
• Applications embedding the Lily interpreter

Discovery Timeline

• 2026-03-01 - CVE CVE-2026-3392 published to NVD
• 2026-03-04 - Last updated in NVD database

Technical Details for CVE-2026-3392

Vulnerability Analysis

This vulnerability is classified as a null pointer dereference (CWE-404) affecting the Lily programming language interpreter. The flaw resides in the eval_tree function located in src/lily_emitter.c, which is responsible for evaluating parsed syntax tree nodes during the compilation or interpretation phase. When the function encounters a malformed or unexpected tree structure, it fails to properly validate pointer references before dereferencing them, leading to a crash.

The vulnerability requires local access to exploit, meaning an attacker must be able to execute code or provide input to the Lily interpreter on the target system. While this limits the attack surface, it poses risks in scenarios where Lily is used as an embedded scripting language in larger applications that process untrusted input files.

Root Cause

The root cause of this vulnerability lies in insufficient input validation within the eval_tree function. The function processes syntax tree nodes but does not adequately verify that all required tree node references are non-null before attempting to access them. This can occur when parsing malformed Lily source code that creates an incomplete or invalid abstract syntax tree structure.

Attack Vector

The attack is executed locally by providing a specially crafted Lily source file to the interpreter. An attacker constructs malicious input that generates a syntax tree with missing or null node references. When the eval_tree function processes this malformed tree, it attempts to dereference a null pointer, causing the application to crash. The exploit has been made publicly available, lowering the barrier for exploitation.

A proof-of-concept file demonstrating this vulnerability has been published and can be found in the GitHub PoC repository. The vulnerability was reported to the project maintainers via the GitHub issue tracker, but no response has been received at the time of disclosure.

Detection Methods for CVE-2026-3392

Indicators of Compromise

• Unexpected crashes or segmentation faults in the Lily interpreter process
• Application logs showing null pointer access violations in lily_emitter.c
• Core dump files generated from Lily interpreter crashes
• Unusual or malformed .lily source files being processed

Detection Strategies

• Monitor for repeated crashes of the Lily interpreter or applications embedding it
• Implement crash dump analysis to identify null pointer dereferences in eval_tree
• Deploy file integrity monitoring on Lily source files in production environments
• Use application sandboxing to contain potential denial of service impacts

Monitoring Recommendations

• Set up process monitoring for Lily interpreter instances to detect abnormal terminations
• Configure logging to capture interpreter error messages and crash events
• Implement rate limiting on Lily script execution in multi-tenant environments
• Review access logs for attempts to upload or execute unusual Lily source files

How to Mitigate CVE-2026-3392

Immediate Actions Required

• Restrict execution of untrusted Lily source files until a patch is available
• Implement input validation on Lily source files before processing
• Run Lily interpreter processes in sandboxed or isolated environments
• Monitor the GitHub repository for security updates

Patch Information

At the time of this advisory, the Lily project maintainers have not yet responded to the vulnerability report submitted via GitHub Issue #384. Users are advised to monitor the official repository for patch releases and apply updates as soon as they become available.

Workarounds

• Avoid processing Lily source files from untrusted sources
• Implement process isolation using containers or sandboxing technologies
• Deploy crash handlers to gracefully recover from interpreter failures
• Consider implementing a pre-parser to validate Lily syntax tree structures before full evaluation

# Configuration example - Run Lily interpreter in restricted environment
# Using firejail to sandbox Lily interpreter execution
firejail --private --net=none lily suspicious_script.lily

# Alternative: Run in Docker container with resource limits
docker run --rm --memory=256m --cpus=0.5 -v /path/to/scripts:/scripts:ro lily-container lily /scripts/input.lily