Crypto Foundation Cracked: One-Byte Overflow in GNU libtasn1 (CVE-2025-13151)

A potentially dangerous vulnerability has been uncovered in GNU libtasn1, a foundational software library used by countless applications to handle secure communications and digital signatures. The flaw, tracked as CVE-2025-13151 with a CVSS score of 7.5, is a stack-based buffer overflow that could lead to memory corruption in security-sensitive contexts.

The library is a critical component in the cryptographic supply chain, implementing the complex rules for parsing ASN.1 data structures—the same format used for X.509 digital certificates and SSL/TLS protocols.

The vulnerability lies deep within the asn1_expand_octet_string function in the file decoding.c. According to the vulnerability note, the issue stems from “unsafe string concatenation” where the code constructs a local stack buffer without proper bounds checking.

In a classic programming oversight, the developers used “unbounded string manipulation functions (strcpy and strcat)” to combine two names with a separator dot.

“In the worst-case scenario, both source strings may be at their maximum allowed length,” the report explains. “When concatenated together with an additional separator character (‘.’) and a terminating null byte, the destination buffer is undersized by one byte”.

This seemingly minor miscalculation causes the final null terminator to “overflow the allocated stack buffer by a single byte”.

While a one-byte overflow might sound trivial, in the world of cryptography, precision is everything. “One-byte stack overflows have historically led to subtle memory corruption issues and may cause unexpected behavior, including crashes, during cryptographic operations such as signature verification or certificate parsing”.

However, there are mitigating factors. Triggering the flaw requires an attacker to feed the library “malformed ASN.1 data,” effectively breaking the assumption that data has already been validated by the main application. Furthermore, modern defenses like “stack canaries” and _FORTIFY_SOURCE may limit the potential for exploitation.

The vulnerability was disclosed by Benny Zelster from Microsoft Research. A patch has been proposed to the GNU libtasn1 project to fix the unsafe string handling.

Developers and integrators are urged to “evaluate the patch and apply appropriate mitigations, such as using bounded string operations” to eliminate this hidden risk in their secure applications.

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