CVE-2026-63073
Received Received - Intake

Format String Vulnerability in OpenSSL CMP Client

Publication date: 2026-08-25

Last updated on: 2026-08-25

Assigner: OpenSSL Software Foundation

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.

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Meta Information

Published
2026-08-25
Last Modified
2026-08-25
Generated
2026-08-25
AI Q&A
2026-08-25
EPSS Evaluated
N/A
NVD
CVE-2026-63073
EUVD
EUVD-2026-65483

Affected Vendors & Products

Showing 1 associated CPE
Vendor Product Version / Range
openssl openssl *

Helpful Resources

Exploitability

CWE
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KEV
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CWE ID Description
CWE-134 The product uses a function that accepts a format string as an argument, but the format string originates from an external source.

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Executive Summary

This vulnerability involves OpenSSL's Certificate Management Protocol (CMP) response validation. When validating a CMP message, the sender's distinguished name is converted to a string and passed directly as a format string to ERR_raise_data(). If the sender's name contains percent characters (e.g., CN=%s%n), it becomes an attacker-controlled format string. This can cause the program to crash by dereferencing unrelated stack contents, leading to a denial of service.

Detection Guidance

Detecting this vulnerability requires checking for vulnerable OpenSSL versions and monitoring for crashes in CMP clients. Inspect OpenSSL installations with commands like 'openssl version' or 'dpkg -l | grep openssl' to identify versions before the patched release. Monitor system logs for crashes in applications using CMP, particularly those with format string errors.

Impact Analysis

The primary impact is a denial of service. If an attacker sends a malicious or intercepted CMP response with a crafted sender distinguished name, it can crash the CMP client. This affects systems where the client enforces an expected sender or uses a pinned server certificate, which is a common secure configuration.

Compliance Impact

This vulnerability primarily causes denial of service by crashing CMP clients through format string attacks. It does not directly lead to data breaches or unauthorized access, so its impact on compliance like GDPR or HIPAA is likely minimal unless service disruption affects regulated operations.

Mitigation Strategies

Update OpenSSL to the latest patched version to address the format string vulnerability in CMP response validation. Ensure CMP clients enforce strict sender validation and avoid using pinned server certificates with untrusted distinguished names.

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