OpenSSL up to 3.4.6/3.5.7/3.6.3/4.0.1 CMP ossl_cmp_msg_check_update format format string

Summaryinfo

A vulnerability was found in OpenSSL up to 3.4.6/3.5.7/3.6.3/4.0.1 and classified as problematic. Affected is the function ossl_cmp_msg_check_update of the component CMP. Such manipulation of the argument format leads to format string. This vulnerability is uniquely identified as CVE-2026-63073. The attack can be launched remotely. No exploit exists. It is suggested to upgrade the affected component.

Detailsinfo

A vulnerability was found in OpenSSL up to 3.4.6/3.5.7/3.6.3/4.0.1 and classified as problematic. Affected by this issue is the function ossl_cmp_msg_check_update of the component CMP. The manipulation of the argument format with an unknown input leads to a format string vulnerability. Using CWE to declare the problem leads to CWE-134. The product uses a function that accepts a format string as an argument, but the format string originates from an external source. Impacted is availability. CVE summarizes:

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.

The advisory is available at openssl-library.org. This vulnerability is handled as CVE-2026-63073 since 07/15/2026. The exploitation is known to be easy. The attack may be launched remotely. Technical details are known, but there is no available exploit. The structure of the vulnerability defines a possible price range of USD $5k-$25k at the moment (estimation calculated on 09/13/2026).

Upgrading to version 3.4.7, 3.5.8, 3.6.4 or 4.0.2 eliminates this vulnerability.

The vulnerability is also documented in the vulnerability database at CERT Bund (WID-SEC-2026-3034). If you want to get best quality of vulnerability data, you may have to visit VulDB.

Affected

• FreeBSD Project FreeBSD OS

Productinfo

Type

• Network Encryption Software

Name

• OpenSSL

Version

• 3.4.0
• 3.4.1
• 3.4.2
• 3.4.3
• 3.4.4
• 3.4.5
• 3.4.6
• 3.5.0
• 3.5.1
• 3.5.2
• 3.5.3
• 3.5.4
• 3.5.5
• 3.5.6
• 3.5.7
• 3.6.0
• 3.6.1
• 3.6.2
• 3.6.3
• 4.0.0
• 4.0.1

License

• open-source

Website

• Product: https://www.openssl.org/

CPE 2.3info

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CPE 2.2info

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Entryinfo

If you want to get best quality of vulnerability data, you may have to visit VulDB.

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