CVE-2025-9820 Overview
CVE-2025-9820 is a stack-based buffer overflow [CWE-121] in the GnuTLS 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 expects, it writes past the buffer boundary. This memory corruption can crash applications linked against GnuTLS or, under specific conditions, enable local code execution. Applications relying on GnuTLS for TLS operations and smart card or hardware security module (HSM) integration are exposed to denial of service and potential local privilege escalation.
Critical Impact
A local attacker can trigger a stack buffer overflow in gnutls_pkcs11_token_init() by supplying an oversized PKCS#11 token label, crashing the process or potentially executing arbitrary code within the calling application's context.
Affected Products
- GnuTLS library versions vulnerable per GNUTLS-SA-2025-11-18
- Red Hat Enterprise Linux distributions (multiple RHSA advisories issued)
- Siemens products referenced in advisory SSA-032379
Discovery Timeline
- 2025-11-18 - GnuTLS security advisory GNUTLS-SA-2025-11-18 published
- 2025-11-20 - Public disclosure on OpenWall OSS-Security mailing list
- 2026-01-26 - CVE-2025-9820 published to NVD
- 2026-06-30 - Last updated in NVD database
Technical Details for CVE-2025-9820
Vulnerability Analysis
The vulnerability is a classic stack-based buffer overflow classified under [CWE-121]. The gnutls_pkcs11_token_init() function allocates a fixed-size buffer on the stack to hold the token label during PKCS#11 token initialization. The routine fails to validate that the caller-supplied label length fits within this buffer before copying data into it.
When the label exceeds the buffer size, adjacent stack memory is overwritten. This corrupts saved return addresses, frame pointers, and local variables. The immediate consequence is a process crash, resulting in denial of service. Depending on compiler-inserted mitigations such as stack canaries and address space layout randomization (ASLR), a skilled attacker may leverage the overflow to hijack control flow and execute code with the privileges of the calling application.
Root Cause
The root cause is missing bounds validation on the token label input before writing into a fixed-size stack buffer. GnuTLS commit 1d56f96f6ab5034d677136b9d50b5a75dff0faf5 addresses the flaw by enforcing length checks on the label parameter prior to the memory write operation.
Attack Vector
The attack vector is local. An attacker requires the ability to invoke gnutls_pkcs11_token_init() with attacker-controlled input, typically by triggering an application workflow that initializes a PKCS#11 token with a maliciously crafted label. No authentication or user interaction is required for the underlying function call. The impact is limited to availability, though code execution scenarios cannot be excluded on systems lacking modern stack protection mitigations.
For technical details, see the GnuTLS Security Advisory GNUTLS-SA-2025-11-18 and the upstream patch commit.
Detection Methods for CVE-2025-9820
Indicators of Compromise
- Unexpected crashes or segmentation faults in processes linked against libgnutls, particularly during PKCS#11 or smart card operations.
- Core dumps referencing gnutls_pkcs11_token_init in the call stack.
- Stack canary violation messages (stack smashing detected) in system logs from GnuTLS-linked binaries.
Detection Strategies
- Inventory installed GnuTLS package versions across Linux systems and compare against fixed versions listed in vendor advisories.
- Enable core dump collection and inspect crashes in TLS-enabled applications that consume PKCS#11 tokens.
- Correlate abnormal process termination events with recent PKCS#11 API activity through centralized log aggregation.
Monitoring Recommendations
- Monitor dmesg and audit logs for repeated segmentation faults tied to GnuTLS-linked services.
- Track patch compliance status via package management telemetry to confirm remediation across the fleet.
- Alert on unusual invocations of PKCS#11 initialization routines from non-standard binaries or user accounts.
How to Mitigate CVE-2025-9820
Immediate Actions Required
- Apply the patched GnuTLS packages from your Linux distribution vendor as soon as they are available.
- For Red Hat Enterprise Linux, install updates referenced in the applicable advisories such as RHSA-2026:3477 and related errata.
- Restart all services and applications that dynamically link against libgnutls to load the patched library.
Patch Information
The upstream fix is committed to GnuTLS as commit 1d56f96f6ab5034d677136b9d50b5a75dff0faf5. Refer to GnuTLS Issue #1732 for tracking. Red Hat has released multiple advisories including 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 customers should consult SSA-032379.
Workarounds
- Restrict local access to systems that expose PKCS#11 token initialization workflows to trusted users only.
- Disable PKCS#11 support in GnuTLS-linked applications where token operations are not required.
- Ensure stack protection mitigations such as stack canaries, ASLR, and non-executable stack are enabled to reduce exploitability while patches are deployed.
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
