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 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.

The version of Erlang/OTP installed on the remote host is 17.0 prior to 26.2.5.21, 27.0 prior to 27.3.4.12, 28.0 prior to 28.5.0.1, or 29.0 prior to 29.0.1. It is, therefore, affected by a vulnerability:

  - 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. (CVE-2026-42789)

Note that Nessus has not tested for this issue but has instead relied only on the application's self-reported version number.

The Linux/Unix host has one or more packages installed that are impacted by a vulnerability without a vendor supplied patch available.

  - 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 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. (CVE-2026-42789)

Note that Nessus relies on the presence of the package as reported by the vendor.

The version of FreeBSD installed on the remote host is prior to tested version. It is, therefore, affected by a vulnerability as referenced in the 9357a450-5a54-11f1-b886-4c526214c986 advisory.

    https://github.com/erlang/otp/security/advisories/GHSA-c99q-jmpx-v8qq reports:
    Erlang/OTP's public_key application contains a               path-validation flaw where non-CA certificates lacking               keyUsage extensions can be accepted as intermediate issuers.
              An attacker with an end-entity certificate issued by a               trusted CA can exploit this to forge arbitrary leaf               certificates, allowing public_key:pkix_path_validation/3 to               validate fraudulent certificate chains and potentially               compromise systems relying on SSL/TLS validation.

Tenable has extracted the preceding description block directly from the FreeBSD security advisory.

Note that Nessus has not tested for this issue but has instead relied only on the application's self-reported version number.

ID	Name	Product	Family	Severity
317739	Erlang/OTP 17.0 < 26.2.5.21 / 27.0 < 27.3.4.12 / 28.0 < 28.5.0.1 / 29.0 < 29.0.1 Certificate Chain Forgery (CVE-2026-42789)	Nessus	Misc.	high
317644	Linux Distros Unpatched Vulnerability : CVE-2026-42789	Nessus	Misc.	high
317388	FreeBSD : Erlang/OTP -- public_key accepts non-CA certificate as intermediate issuer (9357a450-5a54-11f1-b886-4c526214c986)	Nessus	FreeBSD Local Security Checks	high

Detections

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CVE-2026-42789

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Tenable Plugins

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high

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