CVE-2025-66044: 
Linux Debian vulnerability analysis and mitigation

CVE-2025-66044 is a stack-based buffer overflow vulnerability in the MFER parsing functionality of The Biosig Project libbiosig version 3.9.1. It is one of six related vulnerabilities (CVE-2025-66043 through CVE-2025-66048) discovered by Cisco Talos researchers Mark Bereza and Lilith. A specially crafted MFER file can trigger the overflow, potentially leading to arbitrary code execution. All versions of libbiosig prior to 3.9.2 are affected. It carries a CVSS v3.1 base score of 9.8 (Critical) (Talos Advisory, Red Hat CVE).

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, when processing an MFER-formatted file with Tag value 64 (0x40, the "preamble" tag), a 256-byte stack-allocated buffer named tmp is declared and populated via ifread(tmp, 1, len, hdr) at line 9151. The len value is read directly from the attacker-controlled input file without adequate bounds checking — when len is encoded using multiple octets and exceeds 256, the ifread call (a wrapper for fread) writes attacker-controlled data past the end of the stack buffer. The MFER file type is identified by magic bytes (@ MFER or @ MFR ), and the vulnerability is triggered during the subsequent tag-length-value parsing loop. Cisco Talos confirmed the overflow using AddressSanitizer, demonstrating a write of attacker-controlled data beyond the buffer boundary (Talos Advisory).

Successful exploitation allows an unauthenticated remote attacker to achieve arbitrary code execution on systems running libbiosig 3.9.1, with no user interaction required. Since libbiosig is a core component of biosig APIs used in Octave, MATLAB, SigViewer, and other biomedical signal processing software, exploitation could compromise scientific and medical data processing environments. The full CIA triad is impacted — confidentiality, integrity, and availability are all rated HIGH — meaning an attacker could exfiltrate sensitive medical signal data, corrupt processing results, or crash affected applications (Talos Advisory, Red Hat CVE).

A proof-of-concept exploit is publicly available via the Cisco Talos vulnerability report, which includes detailed crash information and AddressSanitizer output demonstrating the overflow (Talos Advisory). There is no current evidence of in-the-wild exploitation. The EPSS score is approximately 0.078% (0.000780), indicating a low but non-zero probability of exploitation in the near term. The vulnerability is not currently listed in the CISA Known Exploited Vulnerabilities (KEV) catalog. No threat actor attribution has been reported.

1. Craft a malicious MFER file: Create a file beginning with the magic bytes @ MFER (8 bytes) or @ MFR (6 bytes) to pass libbiosig's file type detection in getfiletype.
2. Set Tag byte to 0x40 (64): In the MFER tag-length-value structure, set the tag byte to 0x40 to trigger the preamble parsing branch at line 9151 of biosig.c.
3. Encode an oversized length field: Use multi-octet length encoding (per MFER spec, with the high bit set) to encode a len value greater than 256 — for example, encoding 0x101 (257) or larger — which exceeds the size of the stack-allocated tmp[256] buffer.
4. Populate the payload data: Fill the remaining bytes of the crafted file with attacker-controlled data (e.g., shellcode or ROP chain) that will be written past the end of tmp via ifread(tmp, 1, len, hdr).
5. Deliver the file to the target: Provide the malicious MFER file to any application that uses libbiosig for file parsing (e.g., SigViewer, a biosig-based Octave/MATLAB script, or a web service accepting medical waveform uploads).
6. Trigger code execution: When sopen_extended processes the file, the stack buffer overflow overwrites adjacent stack memory with attacker-controlled data, potentially redirecting execution flow to achieve arbitrary code execution (Talos Advisory).

• File System: Presence of MFER files (magic bytes @ MFER or @ MFR ) with anomalously large length fields in tag-length-value structures; unexpected MFER files in upload directories or temporary folders.
• Logs: Application crash logs or core dumps referencing sopen_extended in biosig.c; AddressSanitizer output indicating stack-buffer-overflow at biosig.c:9151; stderr output containing Warning MFER messages from libbiosig.
• Process: Unexpected child processes spawned by applications using libbiosig (e.g., SigViewer, Octave, MATLAB biosig scripts); abnormal process termination (SIGABRT, SIGSEGV) in biosig-linked applications.
• Network: Unusual file uploads of MFER-formatted data to services that process biomedical signal files, particularly files with oversized length fields in the MFER structure (Talos Advisory).

Upgrade libbiosig to version 3.9.2 or later, which was released on 2025-12-10 (the same day as vendor disclosure) and addresses all six related MFER parsing vulnerabilities (Talos Advisory). As a workaround where patching is not immediately possible, restrict and monitor file uploads from untrusted sources, implement strict input validation for MFER files before passing them to libbiosig, and avoid exposing biosig-based services to untrusted networks. Applications embedding libbiosig (Octave biosig API, MATLAB biosig, SigViewer) should also be updated to versions linking against libbiosig 3.9.2+.

Cisco Talos publicly disclosed the vulnerability on December 11, 2025, crediting researchers Mark Bereza and Lilith. Red Hat tracked the CVE and published their advisory on December 12, 2025 (Red Hat CVE). CISA included the vulnerability in its weekly bulletin (SB25-349). Community aggregators including Vulners, VulDB, and CVEFeed.io indexed the vulnerability shortly after disclosure. No significant social media controversy or broader industry debate has been noted given the specialized nature of the affected library.