CVE-2025-66048: 
Linux Debian vulnerability analysis and mitigation

CVE-2025-66048 is a stack-based buffer overflow vulnerability in the MFER parsing functionality of The Biosig Project libbiosig 3.9.1, specifically triggered when processing an MFER tag value of 133 (0x85). A specially crafted MFER file can lead to arbitrary code execution without requiring authentication or user interaction. It affects libbiosig versions prior to 3.9.2 and was disclosed publicly on December 11, 2025, following vendor patch release on December 10, 2025. It carries a CVSS v3.1 base score of 9.8 (Critical) (Talos Report, Red Hat Advisory).

The vulnerability is classified as CWE-121 (Stack-based Buffer Overflow) and CWE-787 (Out-of-bounds Write). Within the sopen_extended function in biosig.c, MFER file parsing uses a statically-sized 128-byte stack-allocated buffer (buf). When the tag value is 133 (0x85), the code reads len bytes from the input file directly into buf via ifread (a wrapper for fread) without validating that len does not exceed the buffer size. Because len is attacker-controlled via the crafted MFER file, supplying a value larger than 128 causes a write past the end of the stack buffer, overwriting adjacent stack memory with attacker-controlled data and enabling arbitrary code execution. This is part of a broader set of six related vulnerabilities (CVE-2025-66043 through CVE-2025-66048) affecting different MFER tag handlers in the same code (Talos Report).

Successful exploitation allows an attacker to achieve full system compromise, with high confidentiality, integrity, and availability impact. Because the attack vector is network-based and requires no privileges or user interaction, a remote attacker can supply a malicious MFER file to any system or application using libbiosig (including biosig APIs in Octave and MATLAB, sigviewer, and other biomedical signal processing tools) and execute arbitrary code in the context of the vulnerable process. This could enable data exfiltration, installation of malware, lateral movement within a network, or complete takeover of the affected host (Talos Report, Red Hat Advisory).

A proof-of-concept exploit is referenced in the Cisco Talos vulnerability report (TALOS-2025-2296), which includes AddressSanitizer crash output and debugger traces demonstrating the overflow. There is no evidence of active in-the-wild exploitation at this time. The EPSS score is approximately 0.078% (0.000780), indicating a currently low probability of exploitation in the near term. The vulnerability is not listed in the CISA Known Exploited Vulnerabilities (KEV) catalog. No specific threat actor attribution has been reported (Talos Report, Red Hat Advisory).

1. Craft a malicious MFER file: Create an MFER file with a valid magic byte header (@ MFER or @ MFR ) so that libbiosig classifies it as MFER format.
2. Set tag byte to 133 (0x85): In the MFER tag-length-value (TLV) structure, set the tag field to 0x85 to trigger the vulnerable code path in sopen_extended at line 9268 of biosig.c.
3. Encode an oversized length field: Set the length field to a value greater than 128 (the size of the stack-allocated buf). The length can be encoded using multi-octet encoding per the MFER standard to bypass simple single-byte length checks.
4. Populate the value field with payload: Fill the value field with attacker-controlled data (e.g., a ROP chain or shellcode) that will overflow the buf stack buffer and overwrite the return address or other control-flow data on the stack.
5. Deliver the file to the target: Provide the malicious MFER file to any application or service that uses libbiosig for file parsing (e.g., upload via a web interface, send over a network protocol, or place in a monitored directory).
6. Trigger parsing: When the application calls sopen_extended on the malicious file, the oversized ifread call writes attacker-controlled data past the end of buf, corrupting the stack and redirecting execution to the attacker's payload (Talos Report).

• File System: Presence of unexpected or malformed .mfer or .mfr files in directories monitored by applications using libbiosig; web shells or unexpected binaries created by the process running libbiosig.
• Process: Unusual child processes spawned by applications using libbiosig (e.g., bash, sh, curl, wget, python) following MFER file parsing events; application crashes or core dumps in the libbiosig process.
• Logs: Application logs showing Warning MFER tag2 incorrect length or Warning MFER tag131 incorrect length messages, which indicate oversized length fields being processed; AddressSanitizer or crash reports referencing sopen_extended in biosig.c.
• Network: Unexpected outbound connections from systems running libbiosig-based applications following file parsing events; unusual file upload activity to services that accept MFER files (Talos Report).

The vendor released a patched version, libbiosig 3.9.2, on December 10, 2025, which addresses all six stack-based buffer overflow vulnerabilities in the MFER parser. Users should upgrade to libbiosig 3.9.2 or later immediately. As interim mitigations, restrict network access to systems running libbiosig, implement strict input validation and allowlisting for MFER file processing, use network segmentation to limit exposure, and monitor for unusual file upload or parsing activities (Talos Report, Red Hat Advisory).

Cisco Talos researchers Mark Bereza and Lilith discovered and reported the vulnerability, publishing a detailed technical write-up as part of TALOS-2025-2296 covering all six related MFER parsing vulnerabilities. Talos also published a blog post summarizing the libbiosig findings alongside other vulnerabilities in Grassroot DICOM and Smallstep step-ca. Red Hat acknowledged the vulnerability in their security advisory. Community discussion was noted on infosec.place and Bluesky, and the vulnerability was picked up by aggregators including Vulners, VulDB, and INCIBE-CERT (Talos Report, Red Hat Advisory, Talos Blog).