CVE-2025-66043: 
Linux Debian vulnerability analysis and mitigation

CVE-2025-66043 is one of several stack-based buffer overflow vulnerabilities in the MFER file parsing functionality of The Biosig Project libbiosig version 3.9.1. Discovered and disclosed by Cisco Talos researchers Mark Bereza and Lilith on December 11, 2025, it allows an attacker to supply a specially crafted MFER file to trigger 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 root cause is a stack-based buffer overflow (CWE-121 / CWE-787) in the sopen_extended function within biosig4c++/biosig.c. Specifically, when parsing an MFER file with Tag value 3 (character code), libbiosig allocates a fixed 17-byte stack buffer char v[17] but reads up to len bytes from the file into it via ifread (a wrapper for fread) without enforcing the buffer boundary — even though a warning is printed when len > 16, execution continues regardless. An attacker crafts an MFER file encoding a Tag 3 field with an oversized length (e.g., 0x44 = 68 bytes), causing attacker-controlled data to overwrite adjacent stack memory, potentially redirecting control flow to achieve arbitrary code execution. The vulnerability is triggered simply by opening a malicious file, requiring no privileges or user interaction beyond file delivery (Talos Advisory).

Successful exploitation results in complete system compromise, with full impact to confidentiality, integrity, and availability. An attacker who can deliver a malicious MFER file to a system running libbiosig 3.9.1 — including applications built on it such as biosig APIs for Octave/Matlab, sigviewer, and other biomedical signal processing tools — can achieve arbitrary code execution in the context of the processing application. Given libbiosig's use in medical and scientific environments, exploitation could expose sensitive patient data, corrupt medical signal records, or disrupt clinical workflows (Talos Advisory, Red Hat CVE).

A proof-of-concept exploit is publicly available via the Cisco Talos vulnerability report, which includes AddressSanitizer crash output and debugger traces 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 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 or @ MFR to pass libbiosig's file type detection. Include a Tag 3 (character code) field with an oversized length value encoded in the length field — for example, set len = 0x44 (68 bytes) to overflow the 17-byte stack buffer v.
2. Populate the payload: Fill the bytes following the length field with attacker-controlled data (e.g., a ROP chain or shellcode) sized to overflow v[17] and overwrite adjacent stack variables, saved frame pointers, or return addresses.
3. Deliver the file: Provide the malicious MFER file to a target system running libbiosig 3.9.1 — this could be via email attachment, file upload to a medical data processing portal, shared network storage, or any other vector that causes the application to open the file.
4. Trigger parsing: The target application calls sopen_extended, which invokes getfiletype to identify the file as MFER, then enters the MFER parsing loop. When Tag 3 is encountered, ifread reads 68 bytes into the 17-byte buffer v, overflowing the stack.
5. Achieve code execution: The overwritten return address or function pointer redirects execution to attacker-controlled code, enabling arbitrary command execution in the context of the vulnerable process (Talos Advisory).

• File System: Presence of unexpected or externally sourced .mfer or .mfr files in directories processed by libbiosig-based applications; new or modified files in application directories following MFER file processing.
• Process: Unexpected child processes spawned by biomedical signal processing applications (e.g., sigviewer, Octave, Matlab biosig plugins); crashes or abnormal termination of libbiosig-based applications with stack corruption errors.
• Logs: Application logs showing Warning MFER tag2 incorrect length messages with large length values (e.g., >16); AddressSanitizer or crash dump output referencing sopen_extended in biosig.c around line 8775; repeated application crashes when processing specific MFER files.
• Network: Unusual inbound file transfers of MFER-formatted data to systems running libbiosig; outbound connections from biomedical processing applications to unknown external hosts following file parsing events (Talos Advisory).

The vendor released a patched version, libbiosig 3.9.2, on December 10, 2025 — the same day as vendor disclosure — which addresses this and the related MFER parsing vulnerabilities (CVE-2025-66044 through CVE-2025-66048). Users should upgrade to libbiosig 3.9.2 or later immediately. As interim mitigations: disable MFER file processing if not required, implement strict input validation and file allowlisting before passing files to libbiosig, apply network segmentation to restrict access to systems running the vulnerable library, and monitor for anomalous MFER file submissions (Talos Advisory, Red Hat CVE).

Cisco Talos published a detailed technical advisory (TALOS-2025-2296) on December 11, 2025, crediting researchers Mark Bereza and Lilith for the discovery across the full set of six related MFER parsing vulnerabilities. Red Hat acknowledged the CVE and published a tracking entry. The vulnerability was noted in CISA's weekly vulnerability bulletin (SB25-349) and covered by security aggregators including Tenable (Nessus plugin 278396) and VulDB. Community discussion was observed on infosec.place and security news aggregators, with general acknowledgment of the critical severity given the medical/scientific software context (Talos Advisory, Red Hat CVE).