CVE-2021-4156 – An out-of-bounds read vulnerability in libsndfi... (How to Fix)

Q: What is the severity of CVE-2021-4156?

CVE-2021-4156 has a CVSS score of 7.1 (HIGH). An out-of-bounds read vulnerability in libsndfile's FLAC codec allows attackers to crash applications or potentially leak memory information by submitting specially crafted FLAC files. This affects any application using vulnerable versions of libsndfile to process FLAC audio files. Users who open untrusted FLAC files are at risk.

📋 TL;DR

An out-of-bounds read vulnerability in libsndfile's FLAC codec allows attackers to crash applications or potentially leak memory information by submitting specially crafted FLAC files. This affects any application using vulnerable versions of libsndfile to process FLAC audio files. Users who open untrusted FLAC files are at risk.

💻 Affected Systems

Products:

libsndfile
Applications using libsndfile (audio editors, media players, processing tools)

Versions: libsndfile versions before 1.1.0

Operating Systems: Linux, Windows, macOS - any OS running vulnerable libsndfile

Default Config Vulnerable: ⚠️ Yes

Notes: Any application linking against vulnerable libsndfile versions and processing FLAC files is affected. The vulnerability is in the FLAC codec implementation within libsndfile.

📦 What is this software?

⚠️ Risk & Real-World Impact

🔴

Worst Case

Memory information disclosure leading to further exploitation, potential remote code execution in combination with other vulnerabilities, or denial of service through application crashes.

🟠

Likely Case

Application crash (denial of service) when processing malicious FLAC files, potentially disrupting audio processing services or applications.

🟢

If Mitigated

No impact if patched versions are used or if FLAC file processing is restricted to trusted sources.

🌐 Internet-Facing: MEDIUM - Applications accepting user-uploaded FLAC files could be exploited, but requires user interaction or file upload capability.

🏢 Internal Only: LOW - Requires users to open malicious files, typically through social engineering or compromised internal systems.

🎯 Exploit Status

Public PoC: ⚠️ Yes

Weaponized: LIKELY

Unauthenticated Exploit: ⚠️ Yes

Complexity: LOW

Exploitation requires user interaction to open a malicious FLAC file or an application that automatically processes uploaded FLAC files. Proof-of-concept details are available in public references.

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: libsndfile 1.1.0 and later

Vendor Advisory: https://github.com/libsndfile/libsndfile/security/advisories

Restart Required: Yes

Instructions:

1. Update libsndfile to version 1.1.0 or later using your system package manager. 2. For Linux: Use 'sudo apt update && sudo apt upgrade libsndfile1' (Debian/Ubuntu) or 'sudo yum update libsndfile' (RHEL/CentOS). 3. For Windows/macOS: Download and install the latest version from the official repository. 4. Rebuild any applications statically linked against libsndfile. 5. Restart affected applications or services.

🔧 Temporary Workarounds

Disable FLAC processing

all

Configure applications to avoid processing FLAC files if not required

Application-specific configuration - consult documentation

File type restrictions

linux

Implement file upload restrictions to block FLAC files in web applications

Web server configuration (e.g., .htaccess, nginx rules) to reject .flac uploads

🧯 If You Can't Patch

Implement strict file upload validation and sanitization for FLAC files
Use application sandboxing or containerization to limit impact of potential crashes

🔍 How to Verify

Check if Vulnerable:

Check libsndfile version: 'sndfile-info --version' or 'dpkg -l | grep libsndfile' or 'rpm -q libsndfile'. If version is below 1.1.0, system is vulnerable.

Check Version:

sndfile-info --version 2>/dev/null || dpkg -l libsndfile* 2>/dev/null || rpm -q libsndfile 2>/dev/null || echo 'Check package manager for libsndfile version'

Verify Fix Applied:

Confirm libsndfile version is 1.1.0 or later using version check commands. Test with known safe FLAC files to ensure functionality.

📡 Detection & Monitoring

Log Indicators:

Application crashes or segmentation faults when processing FLAC files
Unexpected memory access errors in application logs

Network Indicators:

Unusual FLAC file uploads to web applications
Multiple failed file processing attempts

SIEM Query:

source="application.logs" AND ("segmentation fault" OR "out of bounds" OR "libsndfile") AND (".flac" OR "FLAC")

📊 Metadata

CVE ID: CVE-2021-4156

CVSS v3 Score: 7.1 (HIGH)

CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:N/A:H

CWE: CWE-125

Published: March 23, 2022

Last Updated: December 11, 2025

🔗 References

https://bugzilla.redhat.com/show_bug.cgi?id=2027690 Exploit,Issue Tracking,Third Party Advisory
https://github.com/libsndfile/libsndfile/issues/731 Exploit,Issue Tracking,Third Party Advisory
https://github.com/libsndfile/libsndfile/pull/732/commits/4c30646abf7834e406f7e2429c70bc254e18beab Patch,Third Party Advisory
https://lists.debian.org/debian-lts-announce/2022/06/msg00020.html Mailing List,Third Party Advisory
https://lists.debian.org/debian-lts-announce/2022/09/msg00036.html Mailing List,Third Party Advisory
https://security.gentoo.org/glsa/202309-11
https://bugzilla.redhat.com/show_bug.cgi?id=2027690 Exploit,Issue Tracking,Third Party Advisory
https://github.com/libsndfile/libsndfile/issues/731 Exploit,Issue Tracking,Third Party Advisory
https://github.com/libsndfile/libsndfile/pull/732/commits/4c30646abf7834e406f7e2429c70bc254e18beab Patch,Third Party Advisory
https://lists.debian.org/debian-lts-announce/2022/06/msg00020.html Mailing List,Third Party Advisory
https://lists.debian.org/debian-lts-announce/2022/09/msg00036.html Mailing List,Third Party Advisory
https://lists.debian.org/debian-lts-announce/2025/12/msg00013.html
https://security.gentoo.org/glsa/202309-11

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