CVE-2023-5941: FreeBSD stdio Buffer Overflow Vulnerability

CVE-2023-5941 Overview

CVE-2023-5941 is a heap buffer overflow vulnerability in FreeBSD's libc standard I/O library affecting the __sflush() function. The vulnerability occurs when the stdio function fails to correctly update FILE objects' write space members for write-buffered streams when the write(2) system call returns an error. This flaw can lead to heap buffer overflows that may result in data corruption or arbitrary code execution at the privilege level of the calling program.

Critical Impact

This heap buffer overflow vulnerability enables potential arbitrary code execution with the privileges of the affected application, posing a significant risk to systems running vulnerable FreeBSD versions.

Affected Products

• FreeBSD 12.4-RELEASE prior to 12.4-RELEASE-p7
• FreeBSD 13.2-RELEASE prior to 13.2-RELEASE-p5
• FreeBSD 12.4 patch levels p1 through p6

Discovery Timeline

• November 8, 2023 - CVE-2023-5941 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2023-5941

Vulnerability Analysis

This vulnerability resides in the __sflush() function within FreeBSD's libc stdio implementation. The function is responsible for flushing write-buffered streams to their underlying file descriptors. When the write(2) system call returns an error condition, the function fails to properly update the internal FILE object structures that track the available write buffer space.

The improper calculation falls under CWE-131 (Incorrect Calculation of Buffer Size), where the write space members of FILE objects become inconsistent with the actual buffer state. This miscalculation creates a condition where subsequent write operations may overflow the heap-allocated buffer boundaries.

Applications that rely heavily on stdio buffered write operations and encounter write errors—whether from disk full conditions, network socket errors, or custom stdio write routines—are particularly susceptible. The exploitation potential depends on the nature of the calling application and how it handles error conditions from write operations.

Root Cause

The root cause is an incorrect calculation of buffer size (CWE-131) in the __sflush() function. When the underlying write(2) system call fails and returns an error, the function does not properly adjust the FILE structure's write space tracking members. This leaves the buffer management in an inconsistent state where the tracked available space does not match the actual buffer boundaries.

The vulnerability can also be triggered through applications that override the default stdio write routine with custom implementations that return errors under specific conditions.

Attack Vector

The attack vector is network-accessible with low complexity, requiring no privileges or user interaction. Exploitation scenarios include:

1. Network Services: FreeBSD-based servers running applications with network-facing stdio operations where write errors can be induced through connection manipulation
2. Error-Triggered Exploitation: Attackers may deliberately cause write failures by exhausting system resources or manipulating file descriptors
3. Custom Write Routines: Applications using overridden stdio write routines that can be influenced to return error conditions

The vulnerability manifests when an application's stdio-based write operations encounter errors from the underlying write(2) call. The improper buffer state tracking can then lead to heap memory corruption on subsequent write operations. For detailed technical analysis, see the FreeBSD Security Advisory SA-23:15.

Detection Methods for CVE-2023-5941

Indicators of Compromise

• Unexpected application crashes or segmentation faults in processes performing heavy stdio write operations
• Heap corruption errors or memory-related warnings in system logs
• Unusual process terminations following write error conditions
• Anomalous memory allocation patterns in FreeBSD system processes

Detection Strategies

• Monitor system logs for signs of memory corruption in applications utilizing stdio functions
• Implement heap integrity checking mechanisms to detect buffer overflows
• Deploy runtime application self-protection (RASP) tools that monitor for heap manipulation
• Audit applications that use custom stdio write routines for error handling vulnerabilities

Monitoring Recommendations

• Enable FreeBSD's kernel crash dump analysis to investigate potential exploitation attempts
• Configure monitoring for unusual write(2) error patterns across critical applications
• Implement file integrity monitoring on critical system binaries that could be targets of memory corruption attacks
• Deploy network-level monitoring to detect potential remote exploitation attempts against exposed services

How to Mitigate CVE-2023-5941

Immediate Actions Required

• Upgrade FreeBSD 12.4-RELEASE systems to 12.4-RELEASE-p7 or later
• Upgrade FreeBSD 13.2-RELEASE systems to 13.2-RELEASE-p5 or later
• Review and restart all critical services after patching to ensure they utilize the updated libc
• Audit custom applications that implement overridden stdio write routines

Patch Information

FreeBSD has released security patches addressing this vulnerability. The fixes are available in:

• FreeBSD 12.4-RELEASE-p7 - Contains the corrected __sflush() implementation
• FreeBSD 13.2-RELEASE-p5 - Contains the corrected __sflush() implementation

Administrators should apply the official patches using the FreeBSD update mechanism. Detailed patch information is available in the FreeBSD Security Advisory SA-23:15. NetApp customers should also review the NetApp Security Advisory ntap-20231214-0004 for product-specific guidance.

Workarounds

• Limit network exposure of FreeBSD services running vulnerable versions until patches can be applied
• Implement network segmentation to restrict access to critical FreeBSD-based infrastructure
• Monitor applications for abnormal behavior related to write operations
• Consider using application-level sandboxing to limit the impact of potential exploitation

# Update FreeBSD to patched version
freebsd-update fetch
freebsd-update install

# Verify the installed FreeBSD version after patching
freebsd-version -k

# Restart critical services to apply updated libc
service -R