CVE-2026-63073: Untrusted Sender DN Used as Format String in CMP Response Validation

Vulnerability summary

CVE-2026-63073 is a critical (CVSS 9.8) vulnerability in openssl. EPSS estimates a 1.16% chance of exploitation in the next 30 days, higher than 67.1% of scored vulnerabilities.

Timeline

1. 25-08-2026
   PublishedVulnerability first disclosed.
2. 31-08-2026
   Last modifiedVulnerability information updated.

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. Since the attacker controls the format string but none of the variadic arguments, such specifiers as %s and %n dereference or write through unrelated stack contents and crash the client. The reliable consequence is a denial of service, when the response comes from a malicious or intercepted CMP endpoint. There is no controlled memory write, arbitrary-address read, or reliable path to remote code execution. FIPS impact: no No FIPS modules are affected by this issue, as the CMP protocol implementation is outside the OpenSSL FIPS module boundary.

• CWE-134Use of Externally-Controlled Format StringThe product uses a function that accepts a format string as an argument, but the format string originates from an external source.