CVE-2025-13151: libtasn1 Buffer Overflow Vulnerability

CVE-2025-13151 Overview

A stack-based buffer overflow vulnerability has been identified in libtasn1 version v4.20.0. The vulnerability exists in the asn1_expand_octet_string function, which fails to properly validate the size of input data before processing, resulting in a buffer overflow condition. This flaw can be exploited remotely over the network without requiring authentication or user interaction.

Critical Impact

Remote attackers can exploit this buffer overflow vulnerability to cause denial of service conditions by crashing applications that rely on the vulnerable libtasn1 library for ASN.1 parsing operations.

Affected Products

• libtasn1 version v4.20.0
• Applications and libraries that depend on libtasn1 for ASN.1 parsing (including GnuTLS)
• Systems using vulnerable libtasn1 builds for certificate processing

Discovery Timeline

• 2026-01-07 - CVE-2025-13151 published to NVD
• 2026-01-08 - Last updated in NVD database

Technical Details for CVE-2025-13151

Vulnerability Analysis

This vulnerability is classified as a stack-based buffer overflow (CWE-121), a dangerous memory corruption issue that occurs when data is written beyond the boundaries of a fixed-length buffer allocated on the stack. In the context of libtasn1, the asn1_expand_octet_string function processes ASN.1 encoded data structures but lacks proper bounds checking on the input data size.

When processing specially crafted ASN.1 data, the function copies input data into a stack-allocated buffer without first verifying that the input length does not exceed the buffer's capacity. This oversight allows an attacker to overflow the buffer, potentially overwriting adjacent stack memory including return addresses and saved registers.

The vulnerability is particularly concerning because libtasn1 is widely used as a dependency for cryptographic libraries like GnuTLS, which handle TLS/SSL certificate parsing. Any application processing untrusted ASN.1 data through the vulnerable function is at risk.

Root Cause

The root cause of this vulnerability is insufficient input validation in the asn1_expand_octet_string function. The function does not properly check the size of incoming octet string data against the destination buffer's allocated size before performing memory copy operations. This missing boundary validation allows oversized input to corrupt stack memory.

Attack Vector

This vulnerability can be exploited remotely over the network. An attacker can craft malicious ASN.1 encoded data containing an oversized octet string and deliver it to any application that parses the data using the vulnerable libtasn1 library. Attack scenarios include:

• Sending malformed certificates or certificate requests to TLS-enabled services
• Providing crafted ASN.1 data to applications that process untrusted input
• Targeting any network service that relies on libtasn1 for ASN.1 parsing

The attack requires no authentication and no user interaction, making it highly accessible to remote attackers. The primary confirmed impact is denial of service through application crashes, though memory corruption vulnerabilities of this nature may potentially lead to more severe consequences depending on the target application's memory layout and security mitigations in place.

Detection Methods for CVE-2025-13151

Indicators of Compromise

• Unexpected crashes in applications using libtasn1 for ASN.1 parsing
• Segmentation faults or stack corruption errors in GnuTLS or dependent services
• Abnormal service terminations correlated with TLS handshake or certificate processing activities

Detection Strategies

• Monitor for application crashes with stack-related error signatures in services using libtasn1
• Deploy memory safety monitoring tools to detect buffer overflow attempts
• Use intrusion detection systems with signatures for malformed ASN.1 data patterns
• Implement crash analysis to identify exploitation attempts targeting asn1_expand_octet_string

Monitoring Recommendations

• Enable core dump analysis for applications using libtasn1 to capture crash details
• Monitor system logs for repeated crashes in TLS-enabled services
• Implement anomaly detection for unusual certificate or ASN.1 data processing patterns
• Track network traffic for unusually large or malformed ASN.1 structures in TLS handshakes

How to Mitigate CVE-2025-13151

Immediate Actions Required

• Update libtasn1 to the patched version as soon as it becomes available from the vendor
• Review the GitLab Merge Request #121 for patch details
• Identify all applications and services in your environment that depend on libtasn1
• Consider restricting network access to vulnerable services until patching is complete
• Monitor the OpenWall OSS Security Discussion for additional remediation guidance

Patch Information

The libtasn1 maintainers have addressed this vulnerability through Merge Request #121 in the official GitLab repository. Organizations should update to the patched version of libtasn1 once released. The fix implements proper bounds checking in the asn1_expand_octet_string function to prevent buffer overflow conditions.

For the latest source code and updates, refer to the official libtasn1 repository.

Workarounds

• Limit network exposure of services that process untrusted ASN.1 data until patching is complete
• Implement network-level filtering to block known malicious ASN.1 patterns where possible
• Consider using application-level sandboxing or containerization to limit the impact of potential crashes
• Deploy web application firewalls or intrusion prevention systems with deep packet inspection capabilities for TLS traffic

# Check installed libtasn1 version
pkg-config --modversion libtasn1

# Verify library location and version
ldconfig -p | grep libtasn1

# For systems using apt package manager, check installed version
dpkg -l | grep libtasn1