CVE-2026-63073: OpenSSL

Critical severity, CVSS 9.8. EPSS: 1.2% chance of exploitation in the next 30 days.

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.

Affected products

• OpenSSL OpenSSL: from 3.4.0, before 3.4.7 (fixed in 3.4.7); from 3.5.0, before 3.5.8 (fixed in 3.5.8); from 3.6.0, before 3.6.4 (fixed in 3.6.4); from 4.0.0, before 4.0.2 (fixed in 4.0.2)

Published 2026-08-25. Last modified 2026-09-11.