Executive brief
OpenSSL's Certificate Management Protocol (CMP) implementation contains a flaw in how it validates server responses. An attacker controlling a CMP server endpoint can craft a malicious certificate with a specially formatted name that crashes CMP clients validating responses. This causes denial of service for applications relying on OpenSSL for secure certificate management.
Technical details
The vulnerability is a format string (CWE-134) in ossl_cmp_msg_check_update(), where a peer-supplied X.509 distinguished name is converted via X509_NAME_oneline() and passed directly as the format argument to ERR_raise_data(). Percent characters survive the conversion, allowing an attacker to inject format specifiers like "%s%n" into the DN (e.g., "CN=%s%n"). The format string reaches BIO_vsnprintf() without variadic arguments, causing crashes via invalid memory dereferences. This path is triggered when clients configure expected senders or pin server certificates—the normal configuration for CMP clients. Attack vector is network (malicious or man-in-the-middle CMP endpoint), and the reliable impact is denial of service; no memory corruption or RCE is possible since the attacker controls the format string but not the arguments.
Affected products
- OpenSSL OpenSSL <UNKNOWN>
Timeline
- 2026-08-25: disclosed
- 2026-08-25: advisory