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 26.2.5.21, 27.3.4.12, 28.5.0.1, and 29.0.1 corresponding to public_key from 0.22 before 1.15.1.7, 1.17.1.3, 1.20.3.1, and 1.21.1.
"}],"value":"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.\n\nIn 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.\n\nAny 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.\n\nThis issue affects OTP from OTP 17.0 before OTP 26.2.5.21, 27.3.4.12, 28.5.0.1, and 29.0.1 corresponding to public_key from 0.22 before 1.15.1.7, 1.17.1.3, 1.20.3.1, and 1.21.1."}],"impacts":[{"capecId":"CAPEC-475","descriptions":[{"lang":"en","value":"CAPEC-475 Signature Spoofing by Improper Validation"}]}],"metrics":[{"cvssV4_0":{"attackComplexity":"LOW","attackRequirements":"PRESENT","attackVector":"NETWORK","baseScore":7,"baseSeverity":"HIGH","privilegesRequired":"NONE","subAvailabilityImpact":"NONE","subConfidentialityImpact":"HIGH","subIntegrityImpact":"HIGH","userInteraction":"NONE","vectorString":"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","version":"4.0","vulnAvailabilityImpact":"NONE","vulnConfidentialityImpact":"LOW","vulnIntegrityImpact":"LOW"},"format":"CVSS","scenarios":[{"lang":"en","value":"GENERAL"}]}],"problemTypes":[{"descriptions":[{"cweId":"CWE-295","description":"CWE-295 Improper Certificate Validation","lang":"en","type":"CWE"},{"cweId":"CWE-296","description":"CWE-296 Improper Following of a Certificate's Chain of Trust","lang":"en","type":"CWE"}]}],"providerMetadata":{"orgId":"6b3ad84c-e1a6-4bf7-a703-f496b71e49db","shortName":"EEF","dateUpdated":"2026-07-01T04:45:34.928Z"},"references":[{"tags":["vendor-advisory","related"],"url":"https://github.com/erlang/otp/security/advisories/GHSA-c99q-jmpx-v8qq"},{"tags":["related"],"url":"https://cna.erlef.org/cves/CVE-2026-42789.html"},{"tags":["related"],"url":"https://osv.dev/vulnerability/EEF-CVE-2026-42789"},{"tags":["x_version-scheme"],"url":"https://www.erlang.org/doc/system/versions.html#order-of-versions"},{"tags":["patch"],"url":"https://github.com/erlang/otp/commit/471cd2f664300a95353c467873800bbe706005db"},{"tags":["patch"],"url":"https://github.com/erlang/otp/commit/59c8d824386b2eb1614ff9340624843ef6aca0fd"}],"source":{"discovery":"EXTERNAL"},"title":"Non-CA certificate accepted as intermediate issuer in public_key path validation","workarounds":[{"lang":"en","supportingMedia":[{"base64":false,"type":"text/html","value":"The verify_fun option in the ssl or public_key application can be used to ensure that path validation rejects chains where an intermediate certificate does not have basicConstraints cA:true."}],"value":"The verify_fun option in the ssl or public_key application can be used to ensure that path validation rejects chains where an intermediate certificate does not have basicConstraints cA:true."}],"x_generator":{"engine":"cvelib 1.8.0"}},"adp":[{"metrics":[{"other":{"type":"ssvc","content":{"timestamp":"2026-05-27T15:41:47.903975Z","id":"CVE-2026-42789","options":[{"Exploitation":"none"},{"Automatable":"no"},{"Technical Impact":"partial"}],"role":"CISA Coordinator","version":"2.0.3"}}}],"title":"CISA ADP Vulnrichment","providerMetadata":{"orgId":"134c704f-9b21-4f2e-91b3-4a467353bcc0","shortName":"CISA-ADP","dateUpdated":"2026-05-27T15:43:46.333Z"}},{"affected":[{"cpes":["cpe:/a:redhat:openstack:16.2"],"defaultStatus":"affected","product":"Red Hat OpenStack Platform 16.2","vendor":"Red Hat"},{"cpes":["cpe:/a:redhat:openstack:17.1"],"defaultStatus":"affected","product":"Red Hat OpenStack Platform 17.1","vendor":"Red Hat"},{"cpes":["cpe:/a:redhat:openstack:18.0"],"defaultStatus":"affected","product":"Red Hat OpenStack Platform 18.0","vendor":"Red Hat"}],"datePublic":"2026-05-27T12:23:06.355Z","descriptions":[{"lang":"en","value":"A flaw was found in Erlang OTP's public_key module. This vulnerability (CWE-295), related to improper certificate validation, allows a non-Certificate Authority (CA) certificate to be accepted as an intermediate issuer. A remote attacker, holding an end-entity certificate issued by a trusted CA, can exploit this by forging leaf certificates for arbitrary identities. This can lead to compromised server identity verification on the client side and client certificate verification on mutual Transport Layer Security (mTLS) servers, undermining the trust in certificate-based authentication."}],"metrics":[{"other":{"content":{"namespace":"https://access.redhat.com/security/updates/classification/","value":"Important"},"type":"Red Hat severity rating"}},{"cvssV3_1":{"attackComplexity":"HIGH","attackVector":"NETWORK","availabilityImpact":"NONE","baseScore":8,"baseSeverity":"HIGH","confidentialityImpact":"HIGH","integrityImpact":"HIGH","privilegesRequired":"NONE","scope":"CHANGED","userInteraction":"REQUIRED","vectorString":"CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:H/I:H/A:N","version":"3.1"},"format":"CVSS"}],"problemTypes":[{"descriptions":[{"cweId":"CWE-295","description":"Improper Certificate Validation","lang":"en","type":"CWE"}]}],"references":[{"tags":["vdb-entry","x_refsource_REDHAT"],"url":"https://access.redhat.com/security/cve/CVE-2026-42789"},{"name":"RHBZ#2482093","tags":["issue-tracking","x_refsource_REDHAT"],"url":"https://bugzilla.redhat.com/show_bug.cgi?id=2482093"},{"tags":["x_sadp-csaf-vex"],"url":"https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-42789.json"}],"timeline":[{"lang":"en","time":"2026-05-27T15:10:36.175Z","value":"Reported to Red Hat."},{"lang":"en","time":"2026-05-27T12:23:06.355Z","value":"Made public."}],"title":"erlang: Erlang OTP public_key: Certificate chain forgery via improper trust chain validation","workarounds":[{"lang":"en","value":"Mitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base, or stability."}],"x_adpType":"supplier","x_generator":{"engine":"sadp-cli 1.0.0"},"providerMetadata":{"orgId":"0b0ca135-0b70-47e7-9f44-1890c2a1c46c","shortName":"redhat-SADP","dateUpdated":"2026-06-30T12:08:38.614Z"}}]}}