# CVE-2026-42791: OCSP responder certificate validity period not checked in public_key

Status: published. Published: 2026-05-27. Updated: 2026-11-11.

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, OTP 28.5.0.1, and OTP 29.0.1, corresponding to public_key from 1.16 before 1.17.1.3, 1.20.3.1, and 1.21.1.

## Remediation

- workaround: * For TLS clients using the ssl application, disable OCSP stapling by setting {stapling, no_staple} in the client options, or switch to CRL-based revocation checking with {crl_check, true}.
* For applications calling public_key:pkix_ocsp_validate/5 directly, validate the responder certificate's validity period in application code before calling the function. (source: CNA: EEF)
- fix: The reporting source identifies this reference as a patch. Check the advisory for affected versions and supported release branches. (source: CNA: EEF) — https://github.com/erlang/otp/commit/7995f1fdaee3da569bb810358ce0f546471d169b
- fix: The reporting source identifies this reference as a patch. Check the advisory for affected versions and supported release branches. (source: CNA: EEF) — https://github.com/erlang/otp/commit/b3870e02405c709a872b01ba6086065620cdfe76

## Affected products and versions

- Erlang / OTP (otp): ≥ 27.0; < * (affected, otp); unaffected at 27.3.4.12; unaffected at 28.5.0.1; unaffected at 29.0.1. Source: CNA: EEF
- Erlang / OTP (public_key): ≥ 1.16; < * (affected, otp); unaffected at 1.17.1.3; unaffected at 1.20.3.1; unaffected at 1.21.1. Source: CNA: EEF
- Erlang / OTP (erlang/otp): ≥ 2b1a742c651b90f8a7a1fb2ddde73f29915ea376; < * (affected, git); unaffected at 7995f1fdaee3da569bb810358ce0f546471d169b; unaffected at b3870e02405c709a872b01ba6086065620cdfe76. Source: CNA: EEF

## Severity and prioritization

- CVSS 4.0: 6.3 (medium); 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; source: CNA: EEF

- CISA KEV: No listing in collected sources

- EPSS: 0.32% as of 2026-11-10; percentile 24.76%

- SSVC: exploitation none; automatable no; technical impact partial; source ADP: CISA-ADP

- Weaknesses: CWE-295, CWE-672

## Exploit evidence

No exploit evidence in collected sources. This is not proof of no exploitation.

## References

- https://github.com/erlang/otp/security/advisories/GHSA-cjxj-wj6x-3fff (CNA: EEF; vendor-advisory, related)

- https://cna.erlef.org/cves/CVE-2026-42791.html (CNA: EEF; related)

- https://osv.dev/vulnerability/EEF-CVE-2026-42791 (CNA: EEF; related)

- https://www.erlang.org/doc/system/versions.html#order-of-versions (CNA: EEF; x_version-scheme)

- https://github.com/erlang/otp/commit/2b1a742c651b90f8a7a1fb2ddde73f29915ea376 (CNA: EEF; related)

- https://github.com/erlang/otp/commit/7995f1fdaee3da569bb810358ce0f546471d169b (CNA: EEF; patch)

- https://github.com/erlang/otp/commit/b3870e02405c709a872b01ba6086065620cdfe76 (CNA: EEF; patch)

## Provenance

- CNA: EEF: https://www.cve.org/CVERecord?id=CVE-2026-42791; retrieved 2026-11-11; source updated 2026-11-11

- FIRST EPSS: https://api.first.org/data/v1/epss?cve=CVE-2026-42791; retrieved 2026-11-11; source updated Not reported

Revision: b9482b87fa0903ac51eb567c249f1a0652399ce544223d7bd8a62a7b10709538

Canonical: https://y2.dev/cve/CVE-2026-42791

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