CVE-2026-63073 - Vulnerability Details

- Untrusted Sender DN Used as Format String in CMP Response Validation

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.

Published: 2026-08-25

Score: 9.8 Critical

EPSS: < 1% Very Low

KEV: No

Impact:

Action:

Analysis

Impact

The flaw occurs when a CMP client accepts a response with a sender distinguished name that is passed as an uncontrolled format string to ERR_raise_data(). An attacker can craft a DN containing percent sequences such as %s and %n. Because no matching arguments are supplied, BIO_vsnprintf() dereferences or writes to unrelated stack memory, causing the client to crash. This is a classic externally controlled format string weakness (CWE‑134) that leads to denial of service with no possibility of remote code execution or data exposure.

Affected Systems

Affected installations include any OpenSSL build that implements CMP client functionality and is configured to enforce an expected sender DN or to use certificate pinning. Specific version numbers are not provided in the CVE data; the defect has been fixed in the OpenSSL source code merged in several commits (e.g., commit 0cc20b322639919aa423e90799d9a57c3b4b76ca). All projects using OpenSSL CMP components should verify that they run a version that includes those commits.

Risk and Exploitability

The exploit requires control of a CMP endpoint that sends a crafted DN. The attack path is straightforward: the client validates the sender DN, the bug is triggered, and the client crashes, resulting in a denial of service. No privileged escalation or data leak occurs. The CVSS score of 9.8 indicates critical severity, and the EPSS score of <1% indicates a low but non‑zero probability of exploitation. The vulnerability is not listed in the CISA KEV catalog, suggesting a heightened risk in environments that use CMP validation.

Default status is the baseline for the product, each version can override it (e.g. patched versions marked unaffected).

Vendor Product Default status Versions

OpenSSL

unaffected

Version	Status	Constraints
`4.0.0`	affected	< 4.0.2
`3.6.0`	affected	< 3.6.4
`3.5.0`	affected	< 3.5.8
`3.4.0`	affected	< 3.4.7

Configuration 1 [-]

OR	cpe:2.3:a:openssl:openssl::::::::
	cpe:2.3:a:openssl:openssl::::::::
	cpe:2.3:a:openssl:openssl::::::::
	cpe:2.3:a:openssl:openssl::::::::

No data.

Vendor Product Confidence Versions

Openssl

100%

Version	Status	Scheme	Platform
`[4.0.0,4.0.2)`	affected	semver	—
`[3.6.0,3.6.4)`	affected	semver	—
`[3.5.0,3.5.8)`	affected	semver	—
`[3.4.0,3.4.7)`	affected	semver	—

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Remediation

No vendor fix or workaround currently provided.

OpenCVE Recommended Actions

Upgrade OpenSSL to a version that contains the fixed CMP response validation commits (e.g., after the merges shown in the GitHub references).
If an immediate patch is not yet available, modify the client configuration to disable expected sender validation or certificate pinning for CMP communications, thereby preventing the sender DN from being treated as a format string.
Apply robust input validation to any user‑supplied or peer‑supplied distinguished names before passing them to formatting functions, ensuring no percent‑encoded specifiers reach underlying printf‑style calls.

Generated by OpenCVE AI on August 31, 2026 at 21:51 UTC.

Tracking

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Advisories

Source	ID	Title
Debian DSA	DSA-6465-1	openssl security update
Ubuntu USN	USN-8678-1	OpenSSL vulnerabilities

No CVSS v4.0

Attack Vector Network

Attack Complexity Low

Privileges Required None

Scope Unchanged

Confidentiality Impact High

Integrity Impact High

Availability Impact High

User Interaction None

No CVSS v3.0

No CVSS v2

This CVE is not in the KEV list.

The EPSS score is 0.00929.

Exploitation none

Automatable yes

Technical Impact total

References

Link	Providers
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://nvd.nist.gov/vuln/detail/CVE-2026-63073
https://openssl-library.org/news/secadv/20260825.txt
https://www.cve.org/CVERecord?id=CVE-2026-63073

History

Fri, 11 Sep 2026 23:45:00 +0000

Type	Values Removed	Values Added
CPEs		cpe:2.3:a:openssl:openssl::::::::

Mon, 31 Aug 2026 20:30:00 +0000

Type	Values Removed	Values Added
Metrics	cvssV3_1 `{'score': 5.9, 'vector': 'CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H'}`	ssvc `{'options': {'Automatable': 'yes', 'Exploitation': 'none', 'Technical Impact': 'total'}, 'version': '2.0.3'}` cvssV3_1 `{'score': 9.8, 'vector': 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H'}`

Wed, 26 Aug 2026 00:15:00 +0000

Type	Values Removed	Values Added
References		https://nvd.nist.gov/vuln/detail/CVE-2026-63073 https://www.cve.org/CVERecord?id=CVE-2026-63073
Metrics	threat_severity `None`	cvssV3_1 `{'score': 5.9, 'vector': 'CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H'}` threat_severity `Low`

Tue, 25 Aug 2026 15:00:00 +0000

Type	Values Removed	Values Added
First Time appeared		Openssl Openssl openssl
Vendors & Products		Openssl Openssl openssl

Tue, 25 Aug 2026 13:15:00 +0000

Type	Values Removed	Values Added
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.
Title		Untrusted Sender DN Used as Format String in CMP Response Validation
Weaknesses		CWE-134
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

Subscriptions

Openssl Openssl

MITRE

Status: PUBLISHED

Assigner: openssl

Published: 2026-08-25T12:59:43.356Z

Updated: 2026-08-31T19:12:54.136Z

Reserved: 2026-07-15T13:10:26.188Z

Link: CVE-2026-63073

Vulnrichment

Updated: 2026-08-31T19:12:32.854Z

NVD

Status : Analyzed

Published: 2026-08-25T13:19:26.147

Modified: 2026-09-11T21:16:45.633

Link: CVE-2026-63073

Redhat

Severity : Low

Publid Date: 2026-08-25T00:00:00Z

Links: CVE-2026-63073 - Bugzilla

OpenCVE Enrichment

Updated: 2026-08-31T22:00:06Z

Weaknesses

CWE-134
Use of Externally-Controlled Format String

Impact

Affected Systems

Risk and Exploitability

Tracking

Attack Vector Network

Attack Complexity Low

Privileges Required None

Scope Unchanged

Confidentiality Impact High

Integrity Impact High

Availability Impact High

User Interaction None

Exploitation none

Automatable yes

Technical Impact total

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