OpenSSL CMP Client DoS via External Format String in ERR_raise_data
CVE-2026-63073 Published on August 25, 2026
Untrusted Sender DN Used as Format String in CMP Response Validation
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.
Weakness Type
Use of Externally-Controlled Format String
The software uses a function that accepts a format string as an argument, but the format string originates from an external source.
Products Associated with CVE-2026-63073
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Affected Versions
OpenSSL:- Version 4.0.0 and below 4.0.2 is affected.
- Version 3.6.0 and below 3.6.4 is affected.
- Version 3.5.0 and below 3.5.8 is affected.
- Version 3.4.0 and below 3.4.7 is affected.