How do I fix CVE-2025-54494 in libbiosig?

libbiosig is an open-source C library, so the fix is a code-level patch in biosig.c followed by a rebuild. Update to a Biosig Project release published after the TALOS-2025-2234 advisory (2025-08-25) that bounds the tag 133 (0x85) read at line 9205, rebuild every application that statically links or bundles libbiosig, and redeploy. There is no apt or winget package update that covers a vendored copy you compiled yourself.

What input triggers CVE-2025-54494?

A specially crafted MFER (Medical waveform Format Encoding Rules) file. When libbiosig parses an MFER tag value of 133 (0x85) it reads len bytes into a fixed stack buffer without a bounds check, overflowing the stack and allowing arbitrary code execution. Any workflow that opens untrusted MFER files with libbiosig is exposed.

● Critical · CVSS 9.8

How to Fix CVE-2025-54494: libbiosig MFER Stack Overflow

Q: Is CVE-2025-54494 actively exploited?

No. There is no CISA KEV entry for CVE-2025-54494 and the CISA SSVC assessment records Exploitation as none. No public exploit or Metasploit module is referenced in the Talos advisory. Because the flaw is rated automatable with total technical impact, treat it as urgent on a severity basis even though in-the-wild exploitation is not confirmed.

⚡ At a glance

Severity	CVSS 3.1 base 9.8, Critical (AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H)
Actively exploited?	No (CISA SSVC Exploitation: none; no KEV entry)
Affected	libbiosig 3.9.0 and master branch commit 35a819fa
Remediation	Update to a Biosig Project release published after 2025-08-25, then rebuild and redeploy linking apps
Type (CWE)	CWE-121: Stack-based Buffer Overflow (MFER parsing, tag 133)

Exploitation status

There is no CISA KEV entry for CVE-2025-54494 at present, so active in-the-wild exploitation has not been officially confirmed for this CVE. Do not read that as all-clear: the KEV catalog often trails real-world attacks, so prioritise this on its severity rather than waiting for a listing.

Public exploit availability: no public proof-of-concept or Metasploit module is referenced in this record yet. That says nothing about private exploit code, so do not treat the issue as low risk just because none is published.

CVE-2025-54494 is a critical stack-based buffer overflow in libbiosig, the open-source C library from The Biosig Project for reading and writing biomedical signal files such as EEG, ECG, and polysomnography recordings. Cisco Talos researchers Mark Bereza and Lilith reported it as TALOS-2025-2234, published on 25 August 2025. The bug lives in the MFER parser and earns a CVSS 3.1 base score of 9.8, the near-ceiling that buffer overflows reach when they are remotely reachable, need no privileges, and require no user interaction beyond opening a file.

The short version: if an application built on libbiosig 3.9.0 (or the master branch at commit 35a819fa) parses an attacker-supplied MFER file, that file can corrupt the stack and run code of the attacker's choosing. The fix is a source-level patch in biosig.c followed by a rebuild of every binary that bundles the library. There is no system package you can simply apt upgrade if you vendored your own copy, which is exactly how most labs and device firmware ship it.

What is the vulnerability, exactly?

MFER stands for Medical waveform Format Encoding Rules, a tag-length-value (TLV) container for medical waveforms. libbiosig walks the file tag by tag. The Talos advisory pins the flaw to a single code path. On the master branch (35a819fa), line 9205 of biosig.c handles the tag value 133 (hex 0x85):

else if (tag==133)    // 0x85
{
    curPos += ifread(buf, 1, len, hdr);

Here len comes straight out of the file, and buf is a fixed-size buffer on the stack. The code reads len bytes into buf with no check that len is smaller than the buffer. A crafted MFER file that declares an oversized length for the 0x85 tag writes past the end of buf, smashing the return address and adjacent locals. That is the textbook signature of CWE-121: Stack-based Buffer Overflow, and because the attacker controls the overflowing bytes, it escalates from a crash to arbitrary code execution.

Two builds are confirmed affected by the record: the tagged release 3.9.0 and the development snapshot at commit 35a819fa. If you forked or vendored libbiosig at or before that commit, assume you carry the bug until you confirm the bounds check is present in your tree.

Who is exposed and how bad is it

The CVSS vector is AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H. Network attack vector here does not mean a listening service; it means the malicious MFER file can arrive over any channel and the parser does the rest. Think email attachments, a shared research dataset, a file uploaded to a web service that runs libbiosig server-side, or a recording handed off from another device. No authentication and no user interaction beyond loading the file are required, and a successful hit gives full read, write, and denial-of-service control of the process.

The practical exposure depends on where libbiosig sits in your stack:

Desktop signal-analysis tools (BioSig for Octave/MATLAB, SigViewer, and similar) link libbiosig to import recordings. A researcher who opens an untrusted MFER file is the most common path.
Server-side conversion or ingestion pipelines that accept uploaded recordings are the highest-value target, because the parser runs unattended on potentially many hostile files.
Embedded and device firmware that statically links libbiosig is the hardest to fix, because the patched object has to be recompiled into the firmware image and reflashed.

The CISA SSVC assessment in the record marks this Automatable: yes with Technical Impact: total. Automatable means an attacker can reliably script the trigger across many targets once they have a working exploit. That combination is why you should not let the absence of a KEV listing lull you into deferring the patch.

How to fix CVE-2025-54494

libbiosig is source code, not a vendor binary on a patch-Tuesday cadence. The remediation is the open-source pattern: get a fixed tree, rebuild, redeploy. Do not paste in a generic apt upgrade or winget command, because they will not touch a copy you compiled and shipped yourself.

1. Update to a fixed source tree

Pull a Biosig Project release published after the advisory date (2025-08-25) that addresses the tag 133 read at line 9205, or update your vendored copy to a commit that includes the bounds check. The authoritative pointer to the fix is the Talos advisory and the project's own commit history; confirm the patched code rejects or clamps an oversized len for the 0x85 tag before it calls ifread into the stack buffer.

2. Rebuild everything that links the library

Recompile libbiosig from the fixed source and rebuild every application, container image, or firmware image that statically links or bundles it. A patched libbiosig.so on disk does nothing for a binary that baked in the vulnerable object at build time. Inventory your build manifests for libbiosig before you declare the work done.

3. Redeploy and replace the vulnerable artifacts

Roll the rebuilt binaries to every host, replace affected container layers, and reflash firmware that carried the static copy. For server-side pipelines, restart the worker processes so they load the new code.

Interim mitigation until you can rebuild

If a rebuild has to wait for a maintenance window, reduce exposure rather than ignore it:

Stop parsing untrusted MFER files. Restrict input to recordings from sources you control, and quarantine anything that arrives from outside.
Sandbox the parser. Run any libbiosig-backed conversion in a low-privilege, isolated container or seccomp-confined process so a successful overflow cannot reach the rest of the host.
Confirm hardening flags. Stack canaries (-fstack-protector-strong), ASLR, and non-executable stacks raise the bar for turning the overflow into reliable code execution. They are mitigations, not a fix.

Confirming you are patched and hunting for abuse

After you rebuild, verify the fix is actually deployed and look back for signs the flaw was hit before you closed it:

Prove the version. Check the libbiosig version compiled into each binary and confirm it post-dates 35a819fa / 3.9.0. For statically linked builds, verify against your build manifest, not the system package list.
Reproduce safely. If you keep a corpus of MFER test files, parse them with the rebuilt binary in a sandbox and confirm oversized 0x85 tags are rejected cleanly instead of crashing.
Review crash history. Unexplained segfaults or aborts in any libbiosig-backed tool while opening MFER files are worth investigating as possible exploitation or failed attempts. Pull core dumps and audit logs around those events.
Audit ingestion logs. For server-side parsers, review which MFER files were processed, from which sources, and whether any worker crashed or behaved abnormally.

Frequently asked questions

Is CVE-2025-54494 actively exploited?

No. There is no CISA KEV entry for this CVE, and the CISA SSVC assessment records Exploitation as none. No public exploit or Metasploit module is referenced in the Talos advisory. It is still rated automatable with total technical impact, so patch it on a Critical severity timeline rather than waiting for in-the-wild confirmation.

What is the CVSS severity of CVE-2025-54494?

CVSS 3.1 base score 9.8, Critical. The vector is AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H: network reachable, low complexity, no privileges, no user interaction, with high confidentiality, integrity, and availability impact.

How do I fix it if I bundled libbiosig myself?

Update your vendored source to a tree that bounds the tag 133 read at biosig.c line 9205, recompile the library, and rebuild every application or firmware image that statically links it. A patched shared library on disk does not protect a binary that baked in the old object at build time.

What input triggers the bug?

A specially crafted MFER file. When libbiosig parses an MFER tag value of 133 (0x85), it reads a file-controlled length into a fixed stack buffer with no bounds check at line 9205 of biosig.c, overflowing the stack and allowing arbitrary code execution. Any workflow that opens untrusted MFER files is exposed.

Where can I read the official advisory?

The primary source is Cisco Talos report TALOS-2025-2234 at https://talosintelligence.com/vulnerability_reports/TALOS-2025-2234. The NVD entry mirrors the same metrics.

References

Official vendor advisory: https://talosintelligence.com/vulnerability_reports/TALOS-2025-2234
NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-54494
CISA KEV catalog (CVE-2025-54494 is not listed): https://www.cisa.gov/known-exploited-vulnerabilities-catalog
The Biosig Project: https://biosig.sourceforge.net/

Written by Sai Kiran Pandrala on 2026-05-25. Grounded in Cisco Talos report TALOS-2025-2234 and the CVE record for CVE-2025-54494. CVSS, CWE, affected versions, and the vulnerable code path are quoted directly from the advisory.

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