Improper Following of a Certificate's Chain of Trust vulnerability in Erlang OTP public_key (pubkey_cert module) allows a non-CA certificate to be accepted as an intermediate issuer, enabling certificate chain forgery.

In lib/public_key/src/pubkey_cert.erl, pubkey_cert:validate_extensions/7 contains two flaws that together allow a certificate with basicConstraints cA:false and no keyUsage extension to be used as an intermediate issuer in a chain passed to public_key:pkix_path_validation/3: the cA:false clause recurses into the remaining extensions without rejecting the certificate when it is in issuer position, and the keyUsage check only fires when the extension is present, so a certificate lacking keyUsage entirely bypasses the keyCertSign enforcement.

Any party holding an end-entity certificate with basicConstraints cA:false and no keyUsage extension, issued by any CA in the victim's trust store, can use that certificate's private key to sign forged leaf certificates for arbitrary identities. public_key:pkix_path_validation/3 accepts the resulting chain, and by extension every TLS or mTLS endpoint built on the OTP ssl application that relies on the default verifier is affected, including server identity verification on the client side and client certificate verification on mTLS servers.

This issue affects OTP from OTP 17.0 before OTP 26.2.5.21, 27.3.4.12, 28.5.0.1, and 29.0.1 corresponding to public_key from 0.22 before 1.15.1.7, 1.17.1.3, 1.20.3.1, and 1.21.1.

Attack Vector Network

Attack Complexity Low

Privileges Required None

Attack Requirements Present

User Interaction None

Vulnerable System Confidentiality Impact Low

Vulnerable System Integrity Impact Low

Vulnerable System Availability Impact None

Subsequent System Confidentiality Impact High

Subsequent System Integrity Impact High

Subsequent System Availability Impact None

No CVSS v3.1

No CVSS v3.0

No CVSS v2

This CVE is not in the KEV list.

No EPSS score available.

Exploitation none

Automatable no

Technical Impact partial

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

Vendor Product Default status Versions
Erlang OTP unknown
Version Status Constraints
0.22 affected < *
Erlang OTP unknown
Version Status Constraints
17.0 affected < *
84adefa331c4159d432d22840663c38f155cd4c1 affected < *

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Advisories

No advisories yet.

Fixes

Solution

No solution given by the vendor.

Workaround

The verify_fun option in the ssl or public_key application can be used to ensure that path validation rejects chains where an intermediate certificate does not have basicConstraints cA:true.

References
Link Providers
https://cna.erlef.org/cves/CVE-2026-42789.html cve-icon cve-icon
https://github.com/erlang/otp/commit/471cd2f664300a95353c467873800bbe706005db cve-icon cve-icon
https://github.com/erlang/otp/commit/59c8d824386b2eb1614ff9340624843ef6aca0fd cve-icon cve-icon
https://github.com/erlang/otp/security/advisories/GHSA-c99q-jmpx-v8qq cve-icon cve-icon
https://osv.dev/vulnerability/EEF-CVE-2026-42789 cve-icon cve-icon
https://www.erlang.org/doc/system/versions.html#order-of-versions cve-icon cve-icon
History

Wed, 27 May 2026 16:30:00 +0000

Type Values Removed Values Added
Metrics ssvc

{'options': {'Automatable': 'no', 'Exploitation': 'none', 'Technical Impact': 'partial'}, 'version': '2.0.3'}

Wed, 27 May 2026 14:15:00 +0000

Type Values Removed Values Added
Description Improper Following of a Certificate's Chain of Trust vulnerability in Erlang OTP public_key (pubkey_cert module) allows a non-CA certificate to be accepted as an intermediate issuer, enabling certificate chain forgery. In lib/public_key/src/pubkey_cert.erl, pubkey_cert:validate_extensions/7 contains two flaws that together allow a certificate with basicConstraints cA:false and no keyUsage extension to be used as an intermediate issuer in a chain passed to public_key:pkix_path_validation/3: the cA:false clause recurses into the remaining extensions without rejecting the certificate when it is in issuer position, and the keyUsage check only fires when the extension is present, so a certificate lacking keyUsage entirely bypasses the keyCertSign enforcement. Any party holding an end-entity certificate with basicConstraints cA:false and no keyUsage extension, issued by any CA in the victim's trust store, can use that certificate's private key to sign forged leaf certificates for arbitrary identities. public_key:pkix_path_validation/3 accepts the resulting chain, and by extension every TLS or mTLS endpoint built on the OTP ssl application that relies on the default verifier is affected, including server identity verification on the client side and client certificate verification on mTLS servers. This issue affects OTP from OTP 17.0 before OTP 26.2.5.21, 27.3.4.12, 28.5.0.1, and 29.0.1 corresponding to public_key from 0.22 before 1.15.1.7, 1.17.1.3, 1.20.3.1, and 1.21.1.
Title Non-CA certificate accepted as intermediate issuer in public_key path validation
First Time appeared Erlang
Erlang erlang\/otp
Weaknesses CWE-295
CWE-296
CPEs cpe:2.3:a:erlang:erlang\/otp:*:*:*:*:*:*:*:*
Vendors & Products Erlang
Erlang erlang\/otp
References
Metrics cvssV4_0

{'score': 7, 'vector': 'CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:L/VI:L/VA:N/SC:H/SI:H/SA:N'}

Projects

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cve-icon MITRE

Status: PUBLISHED

Assigner: EEF

Published:

Updated: 2026-05-27T15:46:57.832Z

Reserved: 2026-04-29T18:06:33.251Z

Link: CVE-2026-42789

cve-icon Vulnrichment

Updated: 2026-05-27T15:43:29.627Z

cve-icon NVD

Status : Received

Published: 2026-05-27T14:16:53.267

Modified: 2026-05-27T14:16:53.267

Link: CVE-2026-42789

cve-icon Redhat

No data.

cve-icon OpenCVE Enrichment

Updated: 2026-05-27T16:30:36Z

Weaknesses