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Name Constraints and Subject CommonName Fallback in TLS hostname Verification

High
IngelaAndin published GHSA-22cw-4ph4-6447 May 27, 2026

Package

OTP

Affected versions

>= 19.3

Patched versions

29.0.1, 28.5.0.1, 27.3.4.12, 26.2.5.21
public_key (OTP)
>= 1.4
1.21.1, 1.20.3.1, 1.17.1.3, 1.15.1.7

Description

The combination of supporting RFC 6125 legacy fall back to ' Subject CommonName' for hostname matching by TLS clients and RFC 5280 path validation when 'Subject Alternative Name' is missing or has no domain name can in combination create a bypass possibility of domain name constraints.

Instead the public_key function public_key:pkix_verify_hostname/3 should adhere to RFC 9525 that is TLS hostname check should fail if no 'Subject Alternative Name' (SAN) extension is present in the certificate.
That is, previous fallback behavior of checking 'Subject CommonName' when 'Subject Alternative Name' is missing or has no domain name is disallowed.

Also according to RFC 5280 Subject Alternative Name 'MAY' adhere to certificate name constraints extension while the subject name 'MUST' adhere to it (Sse RFC 2119 for interpretation of 'MAY' and 'MUST').
'public_key' will verify that both adheres to the constraint, although this was treated as one error which was not clearly documented and could perhaps result in someone using a 'verify_fun' to accept this error rendering both errors optional, when only the 'MAY' error can be acceptable to ignore.
Now two different errors will be generated to avoid this possible misconfiguration.

Impact

This impacts the TLS clients using Erlang/OTP 'ssl' application, as well as possible other TLS client implementations relying on the 'public_key' function. The worst case scenario is if an attacker can control a DNS-constrained sub-CA key (legitimate delegation or compromise) and is in a position to intercept network traffic allowing a Man in the Middle Attack to forge a certificate issued for a different domain.

The severity of this CVE can be mitigated if the system is not running on a public network or possible alternative validation
technics are also applied.

Workarounds

The 'ssl' 'verify_fun' option can be used ensure that the path validation will fail if the end entity certificate is missing the SAN-extension or has no domain name.

Do not use a 'verify_fun' that accepts the 'name_not_permitted' error

Affected/Unaffected Versions

A version larger than or equal to one of the listed patched versions is unaffected; otherwise, a version that satisfies an expression listed under affected versions is affected, and if it does not, it is unaffected.

In the case of this vulnerability, ssl application part of OTP versions prior to OTP 20.0 does not implement the hostname check, and hence should not be used for that reason, unless it is implemented using a custom verify_fun.

Severity

High

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v4 base metrics

Exploitability Metrics
Attack Vector Network
Attack Complexity High
Attack Requirements Present
Privileges Required None
User interaction Passive
Vulnerable System Impact Metrics
Confidentiality High
Integrity High
Availability None
Subsequent System Impact Metrics
Confidentiality None
Integrity None
Availability None

CVSS v4 base metrics

Exploitability Metrics
Attack Vector: This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity: This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements: This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required: This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction: This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:P/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N

CVE ID

CVE-2026-42790

Weaknesses

Improper Certificate Validation

The product does not validate, or incorrectly validates, a certificate. Learn more on MITRE.

Improper Validation of Certificate with Host Mismatch

The product communicates with a host that provides a certificate, but the product does not properly ensure that the certificate is actually associated with that host. Learn more on MITRE.

Credits