CVE-2026-4012 Overview
A vulnerability has been identified in rxi fe, a tiny embedded Lisp programming language implementation. The vulnerability exists in the read_ function within src/fe.c and can be triggered by processing specially crafted input, resulting in an out-of-bounds read condition. This memory corruption vulnerability requires local access to exploit and has been publicly disclosed.
Critical Impact
Local attackers with access to the fe interpreter can trigger out-of-bounds memory reads, potentially leaking sensitive information from adjacent memory regions or causing application instability.
Affected Products
- rxi fe (all versions up to commit ed4cda96bd582cbb08520964ba627efb40f3dd91)
- Applications embedding the rxi fe Lisp interpreter
- Projects using the fe library for script evaluation
Discovery Timeline
- 2026-03-12 - CVE-2026-4012 published to NVD
- 2026-03-12 - Last updated in NVD database
Technical Details for CVE-2026-4012
Vulnerability Analysis
The vulnerability resides in the read_ function located in src/fe.c, which is responsible for parsing and reading input within the fe embedded Lisp interpreter. When processing certain malformed input (specifically the input value "1"), the function fails to properly validate memory boundaries before accessing data, resulting in an out-of-bounds read condition.
This type of memory corruption vulnerability (CWE-119: Improper Restriction of Operations within the Bounds of a Memory Buffer) occurs when the software performs operations outside the intended boundaries of a memory buffer. In this case, the read operation accesses memory beyond the allocated buffer limits, which can expose sensitive data from adjacent memory locations.
The rxi fe project uses a rolling release model, meaning there are no discrete version numbers to track. The vulnerability affects all code up to and including commit ed4cda96bd582cbb08520964ba627efb40f3dd91.
Root Cause
The root cause of this vulnerability is insufficient bounds checking in the read_ function when processing input data. The function does not adequately verify that memory access operations remain within the allocated buffer boundaries before attempting to read data. This lack of input validation allows an attacker to craft specific input that causes the function to read beyond the intended memory region.
Attack Vector
The attack requires local access to the system running the fe interpreter. An attacker would need the ability to provide malicious input to the read_ function, either directly through the interpreter or via an application that embeds the fe library. The exploitation involves supplying crafted input that triggers the boundary violation in the memory read operation.
The vulnerability mechanism involves improper handling of input within the read_ function in src/fe.c. When specific input patterns are processed, the function performs memory read operations without adequate boundary validation, leading to out-of-bounds memory access. Technical details and a proof-of-concept can be found in the GitHub Issue #34 Discussion and the associated PoC repository.
Detection Methods for CVE-2026-4012
Indicators of Compromise
- Unexpected crashes or memory access violations in applications using the fe interpreter
- Abnormal memory read patterns or segmentation faults in src/fe.c related operations
- Error logs indicating out-of-bounds memory access during script evaluation
Detection Strategies
- Monitor application logs for memory access violations or segmentation faults originating from fe library functions
- Implement memory sanitizers (ASan, Valgrind) in development and testing environments to detect out-of-bounds reads
- Review code for usage of the rxi fe library, particularly any functions that process untrusted input
Monitoring Recommendations
- Enable detailed logging for applications embedding the fe interpreter to capture memory-related errors
- Deploy runtime application self-protection (RASP) solutions to detect anomalous memory access patterns
- Implement file integrity monitoring on systems where fe is deployed to detect unauthorized modifications
How to Mitigate CVE-2026-4012
Immediate Actions Required
- Review all applications using the rxi fe library and assess exposure to untrusted input
- Restrict local access to systems running applications that embed the fe interpreter
- Consider disabling or sandboxing fe interpreter functionality until a patch is available
- Monitor the GitHub repository for updates and patches
Patch Information
As of the last update on 2026-03-12, no official patch has been released by the maintainer. The project was informed of the vulnerability through GitHub Issue #34 but has not yet responded. Due to the rolling release model used by this project, users should monitor the repository for commits addressing this vulnerability and update to the latest code once a fix is available.
For additional technical details, refer to the VulDB Advisory #350534.
Workarounds
- Limit the use of the fe interpreter to trusted input sources only until a patch is released
- Implement input validation and sanitization at the application layer before passing data to the fe interpreter
- Consider running applications using fe in sandboxed or containerized environments to limit the impact of potential exploitation
- Evaluate alternative embedded Lisp implementations if security is critical and no patch is forthcoming
# Example: Running fe-based applications in a restricted environment
# Use resource limits and sandboxing to contain potential exploitation
# Create restricted user for running fe applications
useradd -r -s /sbin/nologin fe_sandbox
# Run with limited capabilities using firejail (if available)
firejail --quiet --private --net=none ./fe_application
# Alternative: Use systemd sandboxing for services
# Add these options to the service unit file:
# MemoryDenyWriteExecute=yes
# ProtectSystem=strict
# PrivateTmp=yes
# NoNewPrivileges=yes
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
