CVE-2025-9820: GnuTLS Buffer Overflow Vulnerability

CVE-2025-9820 Overview

CVE-2025-9820 is a stack-based buffer overflow [CWE-121] in the GnuTLS cryptographic library. The flaw resides in the gnutls_pkcs11_token_init() function, which handles PKCS#11 token initialization. When the function processes a token label longer than the fixed-size stack buffer can hold, it writes past the buffer boundary. The result is a memory corruption condition that can crash applications linked against GnuTLS or, under specific conditions, enable local code execution. Linux distributions including Red Hat Enterprise Linux and industrial products from Siemens consume GnuTLS, broadening the attack surface across server, desktop, and operational technology environments.

Critical Impact

A local attacker supplying an oversized PKCS#11 token label can corrupt the stack of any application using gnutls_pkcs11_token_init(), leading to denial of service or potential local privilege escalation.

Affected Products

• GnuTLS library versions addressed by advisory GNUTLS-SA-2025-11-18
• Red Hat Enterprise Linux packages updated under RHSA-2026:3477, RHSA-2026:4188, RHSA-2026:4655, RHSA-2026:4943, RHSA-2026:5585, RHSA-2026:5606, RHSA-2026:7329, RHSA-2026:7477, and RHSA-2026:13812
• Siemens products referenced in advisory SSA-032379

Discovery Timeline

• 2026-01-26 - CVE-2025-9820 published to the National Vulnerability Database
• 2026-05-12 - Last updated in NVD database

Technical Details for CVE-2025-9820

Vulnerability Analysis

The vulnerability is a classic stack buffer overflow in GnuTLS, a TLS/SSL and PKCS#11 cryptographic library distributed across nearly every Linux distribution. The defect lives in gnutls_pkcs11_token_init(), the API used to initialize a cryptographic token (such as a smartcard or HSM) through the PKCS#11 interface. The function copies a caller-provided token label into a fixed-size on-stack buffer without validating that the label length fits the destination. When the label exceeds the buffer size, adjacent stack memory is overwritten, including saved frame pointers and return addresses.

Exploitation requires local access because the attacker must influence the token label passed to the API. The result is application instability at minimum, with the potential for arbitrary code execution if stack protections are weak or bypassable. The defect is tracked under [CWE-121: Stack-based Buffer Overflow].

Root Cause

The root cause is missing length validation. GnuTLS allocates a fixed-size buffer on the stack to hold the token label, then copies the caller-controlled string into it without enforcing the maximum size defined by the PKCS#11 specification (32 bytes for CK_TOKEN_INFO.label). Any label exceeding the buffer size overflows the stack frame.

Attack Vector

The attack requires local access. An attacker who can influence the label argument passed to gnutls_pkcs11_token_init() (for example, through a privileged tool that initializes tokens with attacker-controlled input, or through a setuid binary that consumes the library) can trigger the overflow. The fix is committed upstream at 1d56f96f6ab5034d677136b9d50b5a75dff0faf5 in the GnuTLS repository. See the GnuTLS Commit Log Entry for the patched bounds check.

Detection Methods for CVE-2025-9820

Indicators of Compromise

• Unexpected SIGSEGV or SIGABRT crashes from processes linked against libgnutls.so, especially those invoking PKCS#11 token initialization
• Core dumps showing corrupted stack frames inside gnutls_pkcs11_token_init() or callers such as p11tool
• Audit log entries showing token initialization attempts with abnormally long label strings (greater than 32 bytes)

Detection Strategies

• Inventory installed GnuTLS versions on all Linux hosts using rpm -q gnutls, dpkg -s libgnutls30, or equivalent package queries, and compare against fixed versions in the referenced Red Hat CVE Report CVE-2025-9820.
• Hunt for invocations of p11tool --initialize and other PKCS#11 initialization utilities with command-line arguments containing oversized label values.
• Correlate process crash telemetry with use of cryptographic libraries to surface exploitation attempts that produce abnormal terminations.

Monitoring Recommendations

• Enable kernel auditing for execve events on PKCS#11 utilities (p11tool, pkcs11-tool, gnutls-cli) and forward to a central log store for retention and search.
• Monitor systemd-coredump output and ABRT reports for repeated crashes in GnuTLS-linked binaries.
• Track package update compliance against the Red Hat and Siemens advisories listed in the references.

How to Mitigate CVE-2025-9820

Immediate Actions Required

• Apply vendor-supplied GnuTLS updates from your Linux distribution. Red Hat customers should consult the relevant RHSA errata, including RHSA-2026:3477 and later releases.
• Identify all applications that link libgnutls and restart them after upgrading so the patched library is loaded into memory.
• Restrict local access to systems that handle PKCS#11 tokens, particularly multi-user servers and developer workstations.

Patch Information

The upstream fix is documented in GnuTLS Security Advisory GNUTLS-SA-2025-11-18 and committed at 1d56f96f6ab5034d677136b9d50b5a75dff0faf5. Tracking is available in GnuTLS Issue #1732 and Red Hat Bug Report #2392528. Industrial operators should review the Siemens Security Advisory SSA-032379 for OT-specific updates.

Workarounds

• Avoid passing untrusted token labels to gnutls_pkcs11_token_init() in custom applications, and enforce a 32-byte maximum length in calling code per the PKCS#11 specification.
• Disable PKCS#11 provider modules that are not in active use to reduce attack surface until patches are deployed.
• Ensure stack protection compiler flags (-fstack-protector-strong) and address space layout randomization remain enabled on hosts running GnuTLS-linked services.

# Verify installed GnuTLS version and apply updates
rpm -q gnutls
sudo dnf update gnutls
# Confirm services using GnuTLS and restart them
sudo lsof | grep libgnutls | awk '{print $1}' | sort -u