CVE-2026-42789: CWE-295 Improper Certificate Validation in Erlang OTP
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.
AI Analysis
Technical Summary
The vulnerability (CWE-295) exists in Erlang OTP's public_key module, specifically in the pubkey_cert:validate_extensions/7 function. It fails to properly reject certificates with basicConstraints cA:false and no keyUsage extension when used as intermediate issuers in a certificate chain. This flaw allows an attacker holding an end-entity certificate from a trusted CA to forge leaf certificates by exploiting the improper chain validation in public_key:pkix_path_validation/3. The issue compromises server and client identity verification in TLS and mutual TLS connections. Red Hat has issued advisories and patches addressing this vulnerability, particularly for Red Hat OpenStack Services on OpenShift 18.0.
Potential Impact
This vulnerability allows an attacker with an existing end-entity certificate issued by a trusted CA to forge leaf certificates for arbitrary identities. This undermines the trust model of TLS and mTLS connections by compromising server and client identity verification. Applications relying on Erlang OTP's default certificate verifier may accept forged certificates, potentially leading to spoofing and man-in-the-middle attacks. The impact is rated as Important (high severity) by Red Hat, with a CVSS score of 7.0, indicating significant risk to confidentiality and integrity.
Mitigation Recommendations
A patch is available and should be applied to affected systems. Red Hat has released updated Erlang packages addressing this vulnerability, particularly for Red Hat OpenStack Services on OpenShift 18.0. Users should update to the fixed versions as detailed in the Red Hat advisory (RHSA-2026:39809). No effective mitigations other than applying the official patch are currently available or meet Red Hat's criteria for widespread deployment and stability.
CVE-2026-42789: CWE-295 Improper Certificate Validation in Erlang OTP
Description
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.
CVSS v4.0
Score 7.0high
Affected software
Erlang
OTP
Erlang
OTP
pkg:github/erlang/otpcpe:2.3:a:erlang:erlang\/otp:*:*:*:*:*:*:*:*Run on your own infrastructure? Check whether these packages are installed with threat-finder — our free open-source scanner.
Weaknesses
AI-Powered Analysis
Machine-generated threat intelligence
Technical Analysis
The vulnerability (CWE-295) exists in Erlang OTP's public_key module, specifically in the pubkey_cert:validate_extensions/7 function. It fails to properly reject certificates with basicConstraints cA:false and no keyUsage extension when used as intermediate issuers in a certificate chain. This flaw allows an attacker holding an end-entity certificate from a trusted CA to forge leaf certificates by exploiting the improper chain validation in public_key:pkix_path_validation/3. The issue compromises server and client identity verification in TLS and mutual TLS connections. Red Hat has issued advisories and patches addressing this vulnerability, particularly for Red Hat OpenStack Services on OpenShift 18.0.
Potential Impact
This vulnerability allows an attacker with an existing end-entity certificate issued by a trusted CA to forge leaf certificates for arbitrary identities. This undermines the trust model of TLS and mTLS connections by compromising server and client identity verification. Applications relying on Erlang OTP's default certificate verifier may accept forged certificates, potentially leading to spoofing and man-in-the-middle attacks. The impact is rated as Important (high severity) by Red Hat, with a CVSS score of 7.0, indicating significant risk to confidentiality and integrity.
Mitigation Recommendations
A patch is available and should be applied to affected systems. Red Hat has released updated Erlang packages addressing this vulnerability, particularly for Red Hat OpenStack Services on OpenShift 18.0. Users should update to the fixed versions as detailed in the Red Hat advisory (RHSA-2026:39809). No effective mitigations other than applying the official patch are currently available or meet Red Hat's criteria for widespread deployment and stability.
Technical Details
- Data Version
- 5.2
- Assigner Short Name
- EEF
- Date Reserved
- 2026-04-29T18:06:33.251Z
- Cvss Version
- 4.0
- State
- PUBLISHED
- Is Cloud Service
- true
- Vendor Advisory Urls
- [{"url":"https://access.redhat.com/security/cve/CVE-2026-42789","vendor":"Red Hat"}]
Threat ID: 6a16f9d5e29bf47b50c0e93d
Added to database: 05/27/2026, 14:04:05 UTC
Last enriched: 08/14/2026, 13:09:07 UTC
Last updated: 09/11/2026, 19:31:55 UTC
Views: 102
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