--- id: CVE-2026-42789 title: >- 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… summary: >- 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… severity: medium cvss: 4.8 cvssVector: 'CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N' cwe: - CWE-295 - CWE-296 vendor: erlang product: erlang/otp affected: - 'erlang/otp >= 17.0, < 26.2.5.21' - 'erlang/otp >= 27.0, < 27.3.4.12' - 'erlang/otp >= 28.0, < 28.5.0.1' - 'erlang/otp >= 29.0, < 29.0.1' patched: - erlang/otp 29.0.1 published: '2026-05-27' updated: '2026-09-18' sourceUpdated: '2026-09-18T13:18:22.333' source: NVD sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-42789' references: - url: 'https://cna.erlef.org/cves/CVE-2026-42789.html' label: 6b3ad84c-e1a6-4bf7-a703-f496b71e49db - url: >- https://github.com/erlang/otp/commit/471cd2f664300a95353c467873800bbe706005db label: 6b3ad84c-e1a6-4bf7-a703-f496b71e49db - url: >- https://github.com/erlang/otp/commit/59c8d824386b2eb1614ff9340624843ef6aca0fd label: 6b3ad84c-e1a6-4bf7-a703-f496b71e49db - url: 'https://github.com/erlang/otp/security/advisories/GHSA-c99q-jmpx-v8qq' label: 6b3ad84c-e1a6-4bf7-a703-f496b71e49db - url: 'https://osv.dev/vulnerability/EEF-CVE-2026-42789' label: 6b3ad84c-e1a6-4bf7-a703-f496b71e49db - url: 'https://www.erlang.org/doc/system/versions.html#order-of-versions' label: 6b3ad84c-e1a6-4bf7-a703-f496b71e49db - url: 'https://access.redhat.com/errata/RHSA-2026:39809' label: 0b0ca135-0b70-47e7-9f44-1890c2a1c46c - url: 'https://access.redhat.com/errata/RHSA-2026:54757' label: 0b0ca135-0b70-47e7-9f44-1890c2a1c46c - url: 'https://access.redhat.com/security/cve/CVE-2026-42789' label: 0b0ca135-0b70-47e7-9f44-1890c2a1c46c - url: 'https://bugzilla.redhat.com/show_bug.cgi?id=2482093' label: 0b0ca135-0b70-47e7-9f44-1890c2a1c46c - url: >- https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-42789.json label: 0b0ca135-0b70-47e7-9f44-1890c2a1c46c - url: 'https://www.cve.org/CVERecord?id=CVE-2026-42789' - url: 'https://nvd.nist.gov/vuln/detail/CVE-2026-42789' - url: 'https://access.redhat.com/errata/RHSA-2026:63160' tags: - nvd - csaf - vex - red-hat - cve.org - score-dispute epss: 0.0033 epssPercentile: 0.26309 ssvc: exploitation: none automatable: 'no' technicalImpact: partial timestamp: '2026-05-27T15:41:47.903975Z' scores: nvd: 4.8 vendor: 8 cna: 7 ingestedAt: '2026-07-16T02:48:54.973Z' --- ## Overview 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 29.0.1, OTP 28.5.0.1, OTP 27.3.4.12 and OTP 26.2.5.21, corresponding to public_key from 0.22 before 1.21.1, 1.20.3.1, 1.17.1.3 and 1.15.1.7. ## Affected - `erlang/otp >= 17.0, < 26.2.5.21` - `erlang/otp >= 27.0, < 27.3.4.12` - `erlang/otp >= 28.0, < 28.5.0.1` - `erlang/otp >= 29.0, < 29.0.1` ## Remediation Upgrade past the affected range: - `erlang/otp 29.0.1` ## Vendor advisories - **RHSA-2026:54757** · Red Hat · fixed in: Red Hat OpenStack Platform 16.2 · released 2026-08-13 · [advisory](https://access.redhat.com/errata/RHSA-2026:54757) - **RHSA-2026:39809** · Red Hat · fixed in: Red Hat OpenStack Services on OpenShift 18.0 · released 2026-07-15 · [advisory](https://access.redhat.com/errata/RHSA-2026:39809) - **Red Hat VEX** · Important · affected: Red Hat OpenStack Platform 17.1 · no fix planned: Red Hat OpenStack Platform 17.1 · updated 2026-09-21 · [vex](https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-42789.json) - **RHSA-2026:63160** · Red Hat · fixed in: Red Hat Hardened Images · released 2026-09-03 · [advisory](https://access.redhat.com/errata/RHSA-2026:63160)