# CVE-2026-42790

## Summary

- **CVE ID:** CVE-2026-42790
- **Severity:** HIGH
- **CVSS Score:** 7.6 (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)
- **CWE:** CWE-295, CWE-297
- **Published:** May 27, 2026
- **Last Modified:** Aug 14, 2026

## 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 26.2.5.21, 27.3.4.12, 28.5.0.1, and 29.0.1 corresponding to public_key from 1.4 before 1.15.1.7, 1.17.1.3, 1.20.3.1, and 1.21.1.

## Affected Products

- Erlang — OTP (1.4)
- Erlang — OTP (19.3)
- Erlang — OTP (b0c245e8132bb13171e277b1af59c0cec00c9459)

## References

- [CNA](https://github.com/erlang/otp/security/advisories/GHSA-22cw-4ph4-6447)
- [CNA](https://cna.erlef.org/cves/CVE-2026-42790.html)
- [CNA](https://osv.dev/vulnerability/EEF-CVE-2026-42790)
- [CNA](https://www.erlang.org/doc/system/versions.html#order-of-versions)
- [CNA](https://github.com/erlang/otp/commit/0769050c69d73762672b0db1347b6993a5b31759)
- [CNA](https://github.com/erlang/otp/commit/fb67c6d1836f51105a96d8b769e71e4215a79457)
- [CNA](https://github.com/erlang/otp/commit/21abed64eb2026b5f82f432709e4e932f9be389a)
- [redhat-SADP](https://access.redhat.com/security/cve/CVE-2026-42790)
- [redhat-SADP](https://bugzilla.redhat.com/show_bug.cgi?id=2482286)
- [redhat-SADP](https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-42790.json)
- [redhat-SADP](https://access.redhat.com/errata/RHSA-2026:39809)
- [redhat-SADP](https://access.redhat.com/errata/RHSA-2026:54757)

## Exploitation Prediction (EPSS)

- **EPSS Score:** 0.34%
- **EPSS Percentile:** 26.9

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_Exported from OnDuty AI Vulnerability Intelligence on 2026-09-11._