# CVE-2026-42791

## Summary

- **CVE ID:** CVE-2026-42791
- **Severity:** MEDIUM
- **CVSS Score:** 6.3 (CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:L/SI:L/SA:N)
- **CWE:** CWE-295, CWE-672
- **Published:** May 27, 2026
- **Last Modified:** Jul 24, 2026

## Description

Improper Certificate Validation vulnerability in Erlang OTP public_key (pubkey_ocsp module) allows forged OCSP responses signed with an expired responder certificate to be accepted as valid.

OCSP response verification in pubkey_ocsp:verify_response/5 and pubkey_ocsp:is_authorized_responder/3 in lib/public_key/src/pubkey_ocsp.erl does not check the validity period (notBefore/notAfter) of the OCSP responder certificate. An attacker who has obtained the private key of an expired CA-designated OCSP responder certificate can forge OCSP responses that Erlang/OTP accepts as valid.

This affects TLS clients using OCSP stapling via the ssl application: a malicious or compromised server can present a revoked TLS certificate together with a forged OCSP response signed by an expired responder key, and the client will accept the revoked certificate as valid. It also affects applications calling public_key:pkix_ocsp_validate/5 directly, where the impact depends on the use case — server-side client certificate validation using this API may allow authentication bypass with a revoked client certificate.

This issue affects OTP from OTP 27.0 before OTP 27.3.4.12, 28.5.0.1, and 29.0.1 corresponding to public_key from 1.16 before 1.17.1.3, 1.20.3.1, and 1.21.1.

## Affected Products

- Erlang — OTP (1.16)
- Erlang — OTP (27.0)
- Erlang — OTP (2b1a742c651b90f8a7a1fb2ddde73f29915ea376)

## References

- [CNA](https://github.com/erlang/otp/security/advisories/GHSA-cjxj-wj6x-3fff)
- [CNA](https://cna.erlef.org/cves/CVE-2026-42791.html)
- [CNA](https://osv.dev/vulnerability/EEF-CVE-2026-42791)
- [CNA](https://www.erlang.org/doc/system/versions.html#order-of-versions)
- [CNA](https://github.com/erlang/otp/commit/7995f1fdaee3da569bb810358ce0f546471d169b)
- [CNA](https://github.com/erlang/otp/commit/b3870e02405c709a872b01ba6086065620cdfe76)

## Exploitation Prediction (EPSS)

- **EPSS Score:** 0.32%
- **EPSS Percentile:** 24.3

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_Exported from OnDuty AI Vulnerability Intelligence on 2026-09-10._