CVE-2026-42250 Off-by-One Leading to Out-of-Bounds Write in bzip2

CVE-2026-42250 is an off-by-one vulnerability in bzip2 that can lead to an out-of-bounds write. The specific details of the vulnerability are not provided in the source; however, the Microsoft Security Response Center has released information about it, suggesting it impacts systems where bzip2 is utilized. Defenders need to monitor for exploitation attempts targeting this vulnerability after exploitation details become public. The lack of specifics necessitates broad monitoring for anomalies related to bzip2 processing until further details emerge.

Attack Chain

1. An attacker crafts a malicious bzip2 compressed file.
2. The file is delivered to the target system through a vulnerable application or service.
3. The target application attempts to decompress the malicious bzip2 file using the vulnerable bzip2 library.
4. Due to the off-by-one error, the decompression process writes data beyond the allocated buffer.
5. This out-of-bounds write corrupts adjacent memory regions.
6. The memory corruption can lead to arbitrary code execution.
7. The attacker gains control of the affected process.
8. The attacker pivots to further compromise the system.

Impact

Successful exploitation of CVE-2026-42250 can lead to arbitrary code execution within the context of the application processing the malicious bzip2 file. This could lead to complete system compromise, data breaches, or denial-of-service conditions. The scope of impact depends on the specific application utilizing the vulnerable bzip2 library.

Recommendation

• Monitor process creation events for applications decompressing bzip2 files followed by suspicious activity (see Sigma rule Detect Suspicious bzip2 Decompression Followed by Shell).
• Implement file integrity monitoring (FIM) on bzip2 library files to detect unauthorized modifications.
• Investigate any unexpected crashes or errors related to bzip2 decompression operations.
• Review and harden applications that handle bzip2 compressed files.