CVE-2026-42790

Vulnerability Description

Improper Certificate Validation vulnerability in Erlang OTP public_key (pubkey_cert and public_key modules) allows a DNS nameConstraints bypass via subject CommonName fallback in TLS hostname verification. Two flaws combine to allow a subordinate CA whose DNS nameConstraints are restricted (e.g. permitted;DNS:allowed.example.com) to issue a leaf certificate that an OTP TLS client accepts as a valid identity for an out-of-scope hostname (e.g. victim.example.com): First, pubkey_cert:validate_names/6 in lib/public_key/src/pubkey_cert.erl only checks SAN DNS entries against nameConstraints. Per RFC 5280, a permitted DNS subtree only restricts certificates that contain a DNS-typed name. A leaf with no subjectAltName therefore trivially satisfies any permitted;DNS:... constraint regardless of its subject commonName. Second, public_key:pkix_verify_hostname/3 in lib/public_key/src/public_key.erl falls back to the subject commonName when no subjectAltName is present, extracting id-at-commonName attributes as presented IDs and matching them against the reference hostname. The strict pkix_verify_hostname_match_fun(https) matcher does not suppress this fallback. The result is that path validation accepts a CN-only leaf under a DNS-constrained intermediate (no SAN means the nameConstraints are not triggered), and hostname verification then accepts it via the CN fallback. The bypass is reachable from stock ssl:connect with verify_peer, a trusted CA, SNI, and the canonical strict https hostname matcher. This issue affects OTP from OTP 19.3 before OTP 29.0.1, OTP 28.5.0.1, OTP 27.3.4.12 and OTP 26.2.5.21, corresponding to public_key from 1.4 before 1.21.1, 1.20.3.1, 1.17.1.3 and 1.15.1.7.

CVSS Score

Affected Products

Related Weaknesses (CWE)

References

• https://cna.erlef.org/cves/CVE-2026-42790.htmlThird Party Advisory
• https://github.com/erlang/otp/commit/0769050c69d73762672b0db1347b6993a5b31759Patch
• https://github.com/erlang/otp/commit/21abed64eb2026b5f82f432709e4e932f9be389aPatch
• https://github.com/erlang/otp/commit/fb67c6d1836f51105a96d8b769e71e4215a79457Patch
• https://github.com/erlang/otp/security/advisories/GHSA-22cw-4ph4-6447Vendor Advisory
• https://osv.dev/vulnerability/EEF-CVE-2026-42790MitigationThird Party Advisory
• https://www.erlang.org/doc/system/versions.html#order-of-versionsProduct
• https://access.redhat.com/errata/RHSA-2026:39809
• https://access.redhat.com/errata/RHSA-2026:54757
• https://access.redhat.com/security/cve/CVE-2026-42790
• https://bugzilla.redhat.com/show_bug.cgi?id=2482286
• https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-42790.json