CVE-2026-42791: erlang\/otp Cert Validation Flaw — Fix

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.

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CVSS v3.1 Score

3.7

LOW

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N

EPSS — Exploit Prediction

0.0031

Probability of exploitation

0.22%

Percentile rank

EPSS estimates the probability that this vulnerability will be exploited in the wild within the next 30 days. A higher score means more likely to be exploited.

Weakness Type (CWE)

CWE-295 CWE-295

CWE-672 CWE-672

Affected Products

Vendor	Product	Versions
erlang	erlang\/otp	27.0 — 27.3.4.12
erlang	erlang\/otp	28.0 — 28.5.0.1
erlang	erlang\/otp	29.0 — 29.0.1

References

Advisories & Patches

https://github.com/erlang/otp/commit/7995f1fdaee3da569bb810358ce0f546471d169b https://github.com/erlang/otp/commit/b3870e02405c709a872b01ba6086065620cdfe76 https://github.com/erlang/otp/security/advisories/GHSA-cjxj-wj6x-3fff

Other References

https://cna.erlef.org/cves/CVE-2026-42791.html https://osv.dev/vulnerability/EEF-CVE-2026-42791 https://www.erlang.org/doc/system/versions.html#order-of-versions

Frequently Asked Questions

What is CVE-2026-42791? +

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. It has a CVSS v3.1 base score of 3.7 (LOW).

How severe is CVE-2026-42791? +

CVE-2026-42791 has a CVSS v3.1 score of 3.7 out of 10, rated LOW. This is a low-severity vulnerability with limited impact. The EPSS score is 0.0031, placing it in the 0th percentile for exploitation probability.

What products are affected by CVE-2026-42791? +

CVE-2026-42791 affects products from erlang, specifically: erlang\/otp. Check the affected products table above for specific version ranges.

How do I check if I'm vulnerable to CVE-2026-42791? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

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CVE-2026-42791