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

• Severity: critical (CVSS 9.8 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 1.2% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-08-25 · updated 2026-09-11
• Weakness: CWE-134

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.

Affected products

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

How to fix

Upgrade to a fixed version: OpenSSL 3.4.7, 3.5.8, 3.6.4, 4.0.2. Check the vendor advisory for mitigations.

In the news

• ZDI-26-789: OpenSSL CMP Use of Externally-Controlled Format String Information Disclosure Vulnerability

References

• https://github.com/openssl/openssl/commit/0cc20b322639919aa423e90799d9a57c3b4b76ca
• https://github.com/openssl/openssl/commit/6a0acc072b4d37a7cac1252a29c1ce1f00c5ec29
• https://github.com/openssl/openssl/commit/7eb2e3ec9d1d4f35c8022fccd4b03398b3f33e21
• https://github.com/openssl/openssl/commit/a7e5a6eea8fd3ccca6b6fbba031a5fbf8a3d93b4
• https://openssl-library.org/news/secadv/20260825.txt

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