Case-Sensitive CN Bypass in Botan X.509 Name Constraints

Executive Summary

This vulnerability exists in the Botan C++ cryptography library prior to version 3.11.0. When processing an X.509 certificate path with name constraints that restrict allowable DNS names, Botan checked the Common Name (CN) in the end-entity certificate if no subject alternative name was defined. However, this check was case-sensitive and did not properly handle mixed-case CN values. As a result, a certificate with a CN like "Sub.EVIL.COM" could bypass DNS name constraints that were meant to exclude domains such as "evil.com".

Useful 0 Not Useful 0

Impact Analysis

This vulnerability can allow an attacker to bypass DNS name constraints in certificate validation by using mixed-case characters in the CN field of a certificate. This means that a certificate that should have been rejected due to domain restrictions might be accepted, potentially allowing unauthorized or malicious certificates to be trusted. This can lead to man-in-the-middle attacks or other security breaches where an attacker impersonates a trusted entity.

Useful 0 Not Useful 0

Mitigation Strategies

To mitigate this vulnerability, upgrade the Botan cryptography library to version 3.11.0 or later, where the issue has been patched.

Useful 0 Not Useful 0

Detection Guidance

This vulnerability affects Botan versions prior to 3.11.0 when processing X.509 certificates with name constraints and no subject alternative name. To detect if your system is vulnerable, you need to determine if Botan is used and its version is older than 3.11.0.

You can check the Botan library version installed on your system by running commands such as:

• For Linux systems, use: `dpkg -l | grep botan` or `rpm -qa | grep botan`
• If Botan is built from source or included in an application, check the application documentation or use: `strings /path/to/library | grep Botan` to find version information.

Additionally, to detect certificates that might exploit this vulnerability, you can inspect certificates for a common name (CN) with mixed case that bypasses DNS name constraints, but this requires custom scripting and detailed certificate analysis.

Useful 0 Not Useful 0