CVE-2026-20777: 
Linux Debian vulnerability analysis and mitigation

CVE-2026-20777 is a heap-based buffer overflow vulnerability in the Nicolet WFT file parsing functionality of The Biosig Project libbiosig, an open-source library for processing biomedical signal data (EKG, EEG, etc.). Affected versions include libbiosig 3.9.2 and the Master Branch at commit db9a9a63. The vulnerability was disclosed to the vendor on February 12, 2026, a patch was released on February 15, 2026, and public disclosure occurred on March 3, 2026. It carries a CVSS v3.1 base score of 8.1 (High) (Talos Advisory).

The root cause is CWE-122 (Heap-based Buffer Overflow) in the sopen_extended function within biosig.c. When parsing a Nicolet WFT file (identified by the magic bytes 0x33 0x00 0x32 0x00 0x31 0x00 0x30 0x00), the code reads the Nicolet_Digitizer_Type field from the file header and copies it into the fixed-size hdr->ID.Manufacturer._field buffer (129 bytes total, MAX_LENGTH_MANUF+1) using an unchecked strcpy call at offset +8, leaving only 120 bytes of usable space. Because the parser navigates fields by seeking to the next null byte rather than using fixed offsets, an attacker can craft a WFT file where the Nicolet_Digitizer_Type field exceeds 120 bytes, causing strcpy to overflow the destination buffer and overwrite adjacent heap memory — including the metadata of neighboring heap chunks. A PoC file demonstrating a 735-byte write (overflowing the entire 0x400-byte hdr heap chunk) is referenced in the Talos advisory (Talos Advisory).

Successful exploitation can result in arbitrary code execution with the privileges of the process loading the malicious WFT file, enabling full system compromise, data theft, or denial of service. Because libbiosig underpins biosig APIs used in Octave, MATLAB, sigviewer, and other scientific software, the attack surface extends to any application or pipeline that processes user-supplied biomedical signal files. On platforms where _FORTIFY_SOURCE is enabled, exploitation is reduced to a denial-of-service (process abort), but platforms without this protection or builds that explicitly disable it remain fully exploitable (Talos Advisory).

A proof-of-concept file demonstrating the heap-buffer-overflow condition is referenced in the Cisco Talos advisory (TALOS-2026-2362), and Feedly threat intelligence confirms PoC exploits are available on talosintelligence.com. There is no evidence of active in-the-wild exploitation as of the time of disclosure. The vulnerability is not listed in the CISA Known Exploited Vulnerabilities (KEV) catalog. The EPSS score is 0.001 (0.1%), indicating low current probability of exploitation in the wild. No threat actor attribution has been reported (Talos Advisory).

1. Craft a malicious WFT file: Create a file whose first 8 bytes match the Nicolet WFT magic sequence (0x33 0x00 0x32 0x00 0x31 0x00 0x30 0x00) to pass the getfiletype check in libbiosig.
2. Set a valid Header_size field: At offset 8, place a valid ASCII integer representing the header size so that hdr->HeadLen is set and the header buffer is allocated/resized correctly.
3. Populate preceding fields: Fill in the File_size, Version, and any other WFT header fields that are parsed before Nicolet_Digitizer_Type, each terminated by a null byte, to advance the next pointer to the target field.
4. Inject an oversized Nicolet_Digitizer_Type value: Place a string longer than 120 bytes (without an embedded null) in the position corresponding to the Nicolet_Digitizer_Type field. This causes the unchecked strcpy(hdr->ID.Manufacturer._field+8, next) call in biosig.c:5747 to overflow the 129-byte _field buffer and overwrite adjacent heap memory.
5. Trigger heap corruption: Depending on heap layout, the overflow can corrupt neighboring heap chunk metadata or function pointers, potentially enabling arbitrary code execution. On hardened builds, this results in a SIGABRT via _FORTIFY_SOURCE.
6. Deliver the file: Provide the malicious .wft file to any application using the vulnerable libbiosig (e.g., via file upload, shared storage, or network-accessible processing pipeline) to trigger parsing and exploitation (Talos Advisory).

• File System: Presence of unexpected or externally sourced .wft files in directories processed by libbiosig-based applications; files beginning with the byte sequence 33 00 32 00 31 00 30 00 that contain anomalously long field values.
• Process: Abnormal termination (SIGABRT) of applications using libbiosig with error messages such as *** buffer overflow detected ***: terminated; unexpected child processes spawned by biomedical signal processing applications (e.g., shells, network utilities).
• Logs: AddressSanitizer output referencing heap-buffer-overflow in strcpy at biosig.c:5747 within sopen_extended; application crash logs referencing constructHDR or sopen_extended stack frames.
• Network: Unexpected outbound connections from biomedical signal processing servers following ingestion of .wft files from untrusted sources (Talos Advisory).

The vendor released a patch on February 15, 2026 (prior to public disclosure on March 3, 2026); users should update libbiosig to a version beyond the Master Branch commit db9a9a63 or version 3.9.2. The Talos advisory recommends replacing the vulnerable strcpy call with strncpy bounded to the available buffer size, and ideally refactoring the WFT parser to use fixed field offsets and sizes per the format specification rather than null-byte navigation. As interim workarounds: restrict processing of .wft files to trusted sources only, disable WFT file parsing if not required, ensure _FORTIFY_SOURCE is enabled in build configurations (which degrades exploitation to denial-of-service), and implement file type and input validation at application boundaries (Talos Advisory).

The vulnerability was discovered and reported by Mark Bereza and Lilith of Cisco Talos, with coordinated disclosure following a vendor patch release. Detection support was added to Tenable Nessus (plugin 300634). Social media activity was limited to automated CVE notification accounts on Mastodon and Bluesky shortly after public disclosure, with no notable broader community debate or media coverage identified (Talos Advisory).