Weakness Type (CWE)

CWE-295 — CWE-295 Improper Certificate Validation

CAPEC

CVSS 4.0 Score

7

HIGH

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

Vulnerability 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 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.

Affected

pkg:otp/public_key

Module Source File Routine
pubkey_cert src/pubkey_cert.erl pubkey_cert:validate_extensions/7
Status Type Version Changes / Fixed in
affected otp ⓘ 0.22
  • unaffected at 1.15.1.7
  • unaffected at 1.17.1.3
  • unaffected at 1.20.3.1
  • unaffected at 1.21.1

pkg:github/erlang/otp

Module Source File Routine
pubkey_cert lib/public_key/src/pubkey_cert.erl pubkey_cert:validate_extensions/7
Status Type Version Changes / Fixed in
affected otp ⓘ 17.0
affected git ⓘ 84adefa331

Workarounds

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.

References

Credits

  • Finder: John Downey
  • Remediation developer: Ingela Anderton Andin
  • Analyst: Jakub Witczak

CVE record as JSON:  GET /cves/CVE-2026-42789.json
OSV record as JSON:  GET /osv/EEF-CVE-2026-42789.json