CVE-2026-63073

/// Technical description

Issue summary: OpenSSL CMP response validation passed an unexpected response

sender distinguished name directly as the format string to `ERR_raise_data()`.

Impact summary: A malicious or intercepted CMP endpoint can crash a CMP client

that enforces an expected sender or uses a pinned server certificate whose

subject becomes the default expected sender.

CWE: CWE-134 (Use of Externally-Controlled Format String)

Description: When validating a received CMP message, ossl_cmp_msg_check_update()

converts the peer-supplied sender distinguished name with X509_NAME_oneline()

and passes it directly as the format argument to ERR_raise_data(). Percent

characters survive the conversion, so a sender DN such as "CN=%s%n" reaches

BIO_vsnprintf() as an attacker-controlled format string with no matching variadic

arguments. This path is only reached when the caller configures an expected

sender or pins a server certificate, which is the normal configuration for a

CMP client validating server responses.

/// Remediation

Check the references section for vendor advisories, patches, and mitigation guidance. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.

/// References