CVE-2026-42789: Non-CA certificate accepted as intermediate issuer in public_key path validation
Improper Following of a Certificate's Chain of Trust vulnerability in Erlang OTP publickey (pubkeycert module) allows a non-CA certificate to be accepted as an intermediate issuer, enabling certificate chain forgery.
In lib/publickey/src/pubkeycert.erl, pubkeycert:validateextensions/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 publickey:pkixpathvalidation/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. publickey:pkixpathvalidation/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 publickey from 0.22 before 1.15.1.7, 1.17.1.3, 1.20.3.1, and 1.21.1.
Other sources
Improper Following of a Certificate's Chain of Trust vulnerability in Erlang OTP publickey (pubkeycert module) allows a non-CA certificate to be accepted as an intermediate issuer, enabling certificate chain forgery.
In lib/publickey/src/pubkeycert.erl, pubkeycert:validateextensions/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 publickey:pkixpathvalidation/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. publickey:pkixpathvalidation/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 publickey from 0.22 before 1.17.1.7, 1.20.3.6, and 1.21.8. Whether OTP before OTP 17.0, corresponding to publickey before 0.22, is affected is unknown.
— MITRE
Non-CA certificate accepted as intermediate issuer in publickey path validation
— Microsoft
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
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ubuntu/erlangto a version that resolves this vulnerability.Fixed in 1:16.b.3-dfsg-1ubuntu2.2+esm3 - Upgrade
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ubuntu/erlangto a version that resolves this vulnerability.Fixed in 1:18.3-dfsg-1ubuntu3.1+esm4 - Upgrade
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ubuntu/erlangto a version that resolves this vulnerability.Fixed in 1:20.2.2+dfsg-1ubuntu2+esm4 - Upgrade
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ubuntu/erlangto a version that resolves this vulnerability.Fixed in 1:22.2.7+dfsg-1ubuntu0.5+esm3 - Upgrade
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ubuntu/erlangto a version that resolves this vulnerability.Fixed in 1:24.2.1+dfsg-1ubuntu0.8 - Upgrade
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ubuntu/erlangto a version that resolves this vulnerability.Fixed in 1:25.3.2.8+dfsg-1ubuntu4.8 - Upgrade
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ubuntu/erlangto a version that resolves this vulnerability.Fixed in 1:27.3.4.6+dfsg-1ubuntu0.2 - Upgrade
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Erlang/OTPto a version that resolves this vulnerability.Fixed in 26.2.5.21 - Upgrade
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Erlang/OTPto a version that resolves this vulnerability.Fixed in 27.3.4.12 - Upgrade
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Erlang/OTPto a version that resolves this vulnerability.Fixed in 28.5.0.1 - Upgrade
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Erlang/OTPto a version that resolves this vulnerability.Fixed in 29.0.1 - Upgrade
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Erlang/OTP public_keyto a version that resolves this vulnerability.Fixed in 1.15.1.7 - Upgrade
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Erlang/OTP public_keyto a version that resolves this vulnerability.Fixed in 1.17.1.3 - Upgrade
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Erlang/OTP public_keyto a version that resolves this vulnerability.Fixed in 1.20.3.1 - Upgrade
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Erlang/OTP public_keyto a version that resolves this vulnerability.Fixed in 1.21.1 - Upgrade
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Erlang/OTPto a version that resolves this vulnerability.Fixed in 27.3.4.19 - Upgrade
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Erlang/OTPto a version that resolves this vulnerability.Fixed in 28.5.0.8 - Upgrade
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Erlang/OTPto a version that resolves this vulnerability.Fixed in 29.1.2 - Upgrade
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Erlang/OTP public_keyto a version that resolves this vulnerability.Fixed in 1.17.1.7 - Upgrade
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Erlang/OTP public_keyto a version that resolves this vulnerability.Fixed in 1.20.3.6 - Upgrade
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Erlang/OTP public_keyto a version that resolves this vulnerability.Fixed in 1.21.8
Event History
Frequently Asked Questions
What is the severity of CVE-2026-42789?
CVE-2026-42789 has a severity rating of high, with a CVSS score of 7.
How do I fix CVE-2026-42789?
To fix CVE-2026-42789, apply the available patches provided by the Erlang OTP team.
What is the impact of CVE-2026-42789?
CVE-2026-42789 allows a non-CA certificate to be accepted as an intermediate issuer, potentially enabling certificate chain forgery.
Which software is affected by CVE-2026-42789?
CVE-2026-42789 affects the Erlang/OTP public_key module, specifically in the pubkey_cert module.
When was CVE-2026-42789 published?
CVE-2026-42789 was published on May 27, 2026.