CVE-2026-63073: OpenSSL CMP response validation format string

CVE-2026-63073 · Severity: critical · CVSS 9.8 · Published 2026-08-25

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.

Technologies: OpenSSL. Vendors: OpenSSL.

Quick answers

Is CVE-2026-63073 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-63073 being exploited in the wild.
How severe is CVE-2026-63073?
Critical severity, with a CVSS score of 9.8, published 25 August 2026.
Which products and versions are affected?
OpenSSL <UNKNOWN>.
Is there a fix for CVE-2026-63073?
No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.
How is CVE-2026-63073 attacked?
It is attacked over the network, in a software library.
Am I affected by CVE-2026-63073?
Check whether you run OpenSSL at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

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

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