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Fencer Vulns
high 8.0

CVE-2026-42789 json

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

published 2026-05-27 5 mo ago modified 2026-10-26 14 d ago also EUVD-2026-32272

cvss
8.0
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.4%
EPSS 0.4% chance of exploitation in 30 days, percentile 32%
public exploits
0
none known

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/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.

Patch / fix

  • Erlang OTP 1.17.1.7, 1.20.3.6, 1.21.8, 27.3.4.19, 28.5.0.8, 29.1.2
  • Red Hat Red Hat OpenStack Platform 16.2 0:23.3.4.18-2.el8ost
  • Red Hat Red Hat OpenStack Services on OpenShift 18.0 0:26.2.5.21-2.el9ost
  • erlang Debian 13 1:27.3.4.1+dfsg-1+deb13u3
  • erlang Debian 14 1:27.3.4.12+dfsg-1

debian advisories DEBIAN-CVE-2026-42789

  • erlang Ubuntu 22.04 LTS 1:24.2.1+dfsg-1ubuntu0.8
  • erlang Ubuntu 24.04 LTS 1:25.3.2.8+dfsg-1ubuntu4.8
  • erlang Ubuntu 26.04 LTS 1:27.3.4.6+dfsg-1ubuntu0.2
  • erlang Ubuntu Pro 14.04 LTS 1:16.b.3-dfsg-1ubuntu2.2+esm3
  • erlang Ubuntu Pro 16.04 LTS 1:18.3-dfsg-1ubuntu3.1+esm4
  • erlang Ubuntu Pro 18.04 LTS 1:20.2.2+dfsg-1ubuntu2+esm4
  • erlang Ubuntu Pro 20.04 LTS 1:22.2.7+dfsg-1ubuntu0.5+esm3

ubuntu advisories UBUNTU-CVE-2026-42789

Ubuntu security data © Canonical Ltd., from https://github.com/canonical/ubuntu-security-notices via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

  • erlang27 hummingbird 1 27.3.4.17-1.hum1
  • erlang27-et hummingbird 1 27.3.4.17-1.hum1
  • erlang27-wx hummingbird 1 27.3.4.17-1.hum1
  • erlang27-ftp hummingbird 1 27.3.4.17-1.hum1
  • erlang27-ssh hummingbird 1 27.3.4.17-1.hum1
  • erlang27-ssl hummingbird 1 27.3.4.17-1.hum1
  • erlang27-asn1 hummingbird 1 27.3.4.17-1.hum1
  • erlang27-edoc hummingbird 1 27.3.4.17-1.hum1

red hat advisories RHSA-2026:63160RHSA-2026:54757RHSA-2026:39809

  • erlang SLES for SAP 15 SP6, SLES for SAP 15 SP5, SLES for SAP 15 SP4 +8 more 23.3.4.19-150300.3.39.1
  • erlang26 SLES for SAP 15 SP6, SLES 15 SP6-LTSS, SLE Module for Server Applications 15 SP7 26.2.1-150300.7.28.1
  • erlang SLES for SAP applications 16.0, SLES 16.0 27.1.3-160000.5.1

suse advisories SUSE-SU-2026:3645-1SUSE-SU-2026:3579-1SUSE-SU-2026:22082-1

  • erlang openSUSE Tumbleweed 28.5.0.4-1.1
  • erlang openSUSE Leap 16.0 27.1.3-160000.5.1
  • erlang27 openSUSE Tumbleweed 27.1.3-2.1

opensuse advisories openSUSE-SU-2026:11559-1openSUSE-SU-2026:20907-1openSUSE-SU-2026:10947-1

  • erlang Azure Linux 3.0 26.2.5.21-1

azure linux advisories AZL-89172

Azure Linux vulnerability data, Copyright (c) Microsoft Corporation, from https://github.com/microsoft/AzureLinuxVulnerabilityData via OSV, under the MIT licence; changed: normalised to fixed package versions per release. MIT

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2026-42789 updated cvss 8.0

erlang: Erlang OTP public_key: Certificate chain forgery via improper trust chain validation

  • Red Hat OpenStack Platform 17.1 erlang, erlang-ssl, erlang-asn1, erlang-erts, erlang-sasl, erlang-snmp, +14 more
    affected: workaround, no fix available
  • Red Hat Hardened Images erlang27 27.3.4.17-1.hum1, erlang27-et 27.3.4.17-1.hum1, erlang27-wx 27.3.4.17-1.hum1, erlang27-ftp 27.3.4.17-1.hum1, erlang27-ssh 27.3.4.17-1.hum1, erlang27-ssl 27.3.4.17-1.hum1, +33 more
    fixed: vendor fix RHSA-2026:63160
  • Red Hat OpenStack Platform 16.2 erlang 23.3.4.18-2.el8ost, erlang-ssl 23.3.4.18-2.el8ost, erlang-asn1 23.3.4.18-2.el8ost, erlang-erts 23.3.4.18-2.el8ost, erlang-hipe 23.3.4.18-2.el8ost, erlang-sasl 23.3.4.18-2.el8ost, +15 more
    fixed: vendor fix RHSA-2026:54757
  • Red Hat OpenStack Services on OpenShift 18.0 erlang 26.2.5.21-2.el9ost, erlang-ssl 26.2.5.21-2.el9ost, erlang-asn1 26.2.5.21-2.el9ost, erlang-erts 26.2.5.21-2.el9ost, erlang-sasl 26.2.5.21-2.el9ost, erlang-snmp 26.2.5.21-2.el9ost, +14 more
    fixed: vendor fix RHSA-2026:39809
  • Red Hat OpenStack Platform 16.2 etcd 3.3.23-22.el8ost, python-pyasn1 0.4.6-4.el8ost, python3-pyasn1 0.4.6-4.el8ost, collectd-sensubility 0.2.1-1.1.el8ost, python3-pyasn1-modules 0.4.6-4.el8ost, puppet-git 0.5.0-2.20250728140843.4e4498e.el8ost, +60 more
    not affected: vulnerable code not present

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

CISA SSVC

Exploitation
none
Automatable
no
Technical impact
partial
Assessed
5 mo ago

Exploit prediction

0.38% chance of exploitation in the next 30 days, higher than 32% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
CNA 4.0 7.0 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
CISA ADP 3.1 8.0 CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:H/I:H/A:N
CNA 4.0 7.0 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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
NVD 3.1 4.8 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N
CNA 3.1 8.0 CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:H/I:H/A:N

Affected products

  • Erlang OTP
  • Red Hat Red Hat OpenStack Platform 16.2
  • Red Hat Red Hat OpenStack Services on OpenShift 18.0
  • Red Hat Red Hat Hardened Images
  • Red Hat Red Hat OpenStack Platform 17.1

Classification

Weakness
  • CWE-295 Improper Certificate Validation
  • CWE-296 Improper Following of a Certificate's Chain of Trust

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

CPEs and fixed versions use Anchore's correction of NVD's data: Added CPE configurations because not yet analyzed by NVD.