CVE-2025-9820 – A stack buffer overflow vulnerability in GnuTLS... (How to Fix)

Q: What is the severity of CVE-2025-9820?

CVE-2025-9820 has a CVSS score of 4.0 (MEDIUM). A stack buffer overflow vulnerability in GnuTLS's PKCS#11 token initialization function allows writing past buffer boundaries when processing overly long token labels. This can crash applications or potentially enable code execution, affecting any system or application using vulnerable GnuTLS versions for cryptographic operations.

📋 TL;DR

A stack buffer overflow vulnerability in GnuTLS's PKCS#11 token initialization function allows writing past buffer boundaries when processing overly long token labels. This can crash applications or potentially enable code execution, affecting any system or application using vulnerable GnuTLS versions for cryptographic operations.

💻 Affected Systems

Products:

GnuTLS

Versions: Versions prior to the fix commit 1d56f96f6ab5034d677136b9d50b5a75dff0faf5

Operating Systems: Linux, Unix-like systems using GnuTLS

Default Config Vulnerable: ⚠️ Yes

Notes: Only affects systems using PKCS#11 token functionality; many deployments may not use this feature.

⚠️ Manual Verification Required

This CVE does not have specific version information in our database, so automatic vulnerability detection cannot determine if your system is affected.

Why? The CVE database entry doesn't specify which versions are vulnerable (no version ranges provided by the vendor/NVD).

🔒 Custom verification scripts are available for registered users. Sign up free to download automated test scripts.

Recommended Actions:

Review the CVE details at NVD
Check vendor security advisories for your specific version
Test if the vulnerability is exploitable in your environment
Consider updating to the latest version as a precaution

⚠️ Risk & Real-World Impact

🔴

Worst Case

Local privilege escalation leading to full system compromise through arbitrary code execution.

🟠

Likely Case

Denial of service through application crashes when processing malformed PKCS#11 tokens.

🟢

If Mitigated

Limited impact with proper input validation and memory protections enabled.

🌐 Internet-Facing: LOW - Requires local access or specific PKCS#11 token interaction.

🏢 Internal Only: MEDIUM - Local attackers could exploit if they can trigger token initialization.

🎯 Exploit Status

Public PoC: ✅ No

Weaponized: UNKNOWN

Unauthenticated Exploit: ✅ No

Complexity: MEDIUM

Exploitation requires triggering the vulnerable function with specific input; may require local access.

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: Fixed in commit 1d56f96f6ab5034d677136b9d50b5a75dff0faf5

Vendor Advisory: https://www.gnutls.org/security-new.html#GNUTLS-SA-2025-11-18

Restart Required: Yes

Instructions:

1. Update GnuTLS to patched version from official repositories. 2. Restart affected services using GnuTLS. 3. Verify fix with version check.

🔧 Temporary Workarounds

Disable PKCS#11 token usage

linux

Prevent use of vulnerable function by disabling PKCS#11 token support if not required.

Configure applications to not use PKCS#11 tokens
Remove or disable PKCS#11 modules

🧯 If You Can't Patch

Implement strict input validation for PKCS#11 token labels in application code.
Apply system hardening measures like ASLR and stack protection to reduce exploit impact.

🔍 How to Verify

Check if Vulnerable:

Check GnuTLS version and compare with patched version; examine if PKCS#11 functionality is enabled.

Check Version:

gnutls-cli --version | head -1

Verify Fix Applied:

Verify GnuTLS version is updated beyond the fix commit; test PKCS#11 token initialization with long labels.

📡 Detection & Monitoring

Log Indicators:

Application crashes related to GnuTLS or PKCS#11 operations
Segmentation faults in processes using GnuTLS

Network Indicators:

None - local vulnerability

SIEM Query:

Process:gnutls AND (EventID:1000 OR Signal:SIGSEGV)

📊 Metadata

CVE ID: CVE-2025-9820

CVSS v3 Score: 4.0 (MEDIUM)

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

CWE: CWE-121

EPSS Score: 0.01% exploit probability (1.1th percentile)

Published: January 26, 2026

Last Updated: March 2, 2026

🔗 References

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