CVE-2026-42789Non-CA certificate accepted as intermediate issuer in public_key path validation

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 27.3.4.19, OTP 28.5.0.8, and OTP 29.1.2, corresponding to public_key from 0.22 before 1.17.1.7, 1.20.3.6, and 1.21.8. Whether OTP before OTP 17.0, corresponding to public_key before 0.22, is affected is unknown.

Data: CVE.org, NVD, CISA KEV, FIRST EPSS, Ubuntu. Published as the sources state it, in English.

Affected products and versions

References and patches

• GitHub Advisory relatedvendor-advisoryVendor Advisory
• EEF CNA record for CVE-2026-42789 relatedThird Party Advisory
• OSV record EEF-CVE-2026-42789 relatedThird Party Advisory
• Erlang/OTP version ordering Product
• Fix commit 0a05b7c in erlang/otp patch
• Fix commit 75ca4c0 in erlang/otp patch
• Fix commit 82af606 in erlang/otp patch
• https://access.redhat.com/errata/RHSA-2026:39809
• https://access.redhat.com/errata/RHSA-2026:54757
• https://access.redhat.com/security/cve/CVE-2026-42789
• https://bugzilla.redhat.com/show_bug.cgi?id=2482093
• https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-42789.json