CVE-2026-42789
Non-CA certificate accepted as intermediate issuer in public_key path validation
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.
INFO
Published Date :
May 27, 2026, 2:16 p.m.
Last Modified :
June 5, 2026, 5:16 p.m.
Remotely Exploit :
Yes !
Source :
6b3ad84c-e1a6-4bf7-a703-f496b71e49db
CVSS Scores
| Score | Version | Severity | Vector | Exploitability Score | Impact Score | Source |
|---|---|---|---|---|---|---|
| CVSS 3.1 | MEDIUM | [email protected] | ||||
| CVSS 4.0 | HIGH | 6b3ad84c-e1a6-4bf7-a703-f496b71e49db | ||||
| CVSS 4.0 | HIGH | 6b3ad84c-e1a6-4bf7-a703-f496b71e49db |
Solution
- Update Erlang OTP to a version with the fix.
- Verify the patch addresses certificate chain validation.
- Review public_key_cert.erl for validation logic.
- Re-test TLS/mTLS endpoints.
References to Advisories, Solutions, and Tools
Here, you will find a curated list of external links that provide in-depth
information, practical solutions, and valuable tools related to
CVE-2026-42789.
| URL | Resource |
|---|---|
| https://cna.erlef.org/cves/CVE-2026-42789.html | Third Party Advisory |
| https://github.com/erlang/otp/commit/471cd2f664300a95353c467873800bbe706005db | Patch |
| https://github.com/erlang/otp/commit/59c8d824386b2eb1614ff9340624843ef6aca0fd | Patch |
| https://github.com/erlang/otp/security/advisories/GHSA-c99q-jmpx-v8qq | Vendor Advisory |
| https://osv.dev/vulnerability/EEF-CVE-2026-42789 | Third Party Advisory |
| https://www.erlang.org/doc/system/versions.html#order-of-versions | Product |
CWE - Common Weakness Enumeration
While CVE identifies
specific instances of vulnerabilities, CWE categorizes the common flaws or
weaknesses that can lead to vulnerabilities. CVE-2026-42789 is
associated with the following CWEs:
Common Attack Pattern Enumeration and Classification (CAPEC)
Common Attack Pattern Enumeration and Classification
(CAPEC)
stores attack patterns, which are descriptions of the common attributes and
approaches employed by adversaries to exploit the CVE-2026-42789
weaknesses.
We scan GitHub repositories to detect new proof-of-concept exploits. Following list is a collection of public exploits and proof-of-concepts, which have been published on GitHub (sorted by the most recently updated).
Results are limited to the first 15 repositories due to potential performance issues.
The following list is the news that have been mention
CVE-2026-42789 vulnerability anywhere in the article.
The following table lists the changes that have been made to the
CVE-2026-42789 vulnerability over time.
Vulnerability history details can be useful for understanding the evolution of a vulnerability, and for identifying the most recent changes that may impact the vulnerability's severity, exploitability, or other characteristics.
-
Initial Analysis by [email protected]
Jun. 05, 2026
Action Type Old Value New Value Added CVSS V3.1 AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N Added CPE Configuration OR *cpe:2.3:a:erlang:erlang/otp:*:*:*:*:*:*:*:* versions from (including) 27.0 up to (excluding) 27.3.4.12 *cpe:2.3:a:erlang:erlang/otp:*:*:*:*:*:*:*:* versions from (including) 28.0 up to (excluding) 28.5.0.1 *cpe:2.3:a:erlang:erlang/otp:*:*:*:*:*:*:*:* versions from (including) 29.0 up to (excluding) 29.0.1 *cpe:2.3:a:erlang:erlang/otp:*:*:*:*:*:*:*:* versions from (including) 17.0 up to (excluding) 26.2.5.21 Added Reference Type EEF: https://cna.erlef.org/cves/CVE-2026-42789.html Types: Third Party Advisory Added Reference Type EEF: https://github.com/erlang/otp/commit/471cd2f664300a95353c467873800bbe706005db Types: Patch Added Reference Type EEF: https://github.com/erlang/otp/commit/59c8d824386b2eb1614ff9340624843ef6aca0fd Types: Patch Added Reference Type EEF: https://github.com/erlang/otp/security/advisories/GHSA-c99q-jmpx-v8qq Types: Vendor Advisory Added Reference Type EEF: https://osv.dev/vulnerability/EEF-CVE-2026-42789 Types: Third Party Advisory Added Reference Type EEF: https://www.erlang.org/doc/system/versions.html#order-of-versions Types: Product -
New CVE Received by 6b3ad84c-e1a6-4bf7-a703-f496b71e49db
May. 27, 2026
Action Type Old Value New Value Added 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. Added CVSS V4.0 AV:N/AC:L/AT:P/PR:N/UI:N/VC:L/VI:L/VA:N/SC:H/SI:H/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X Added CWE CWE-295 Added CWE CWE-296 Added Reference https://cna.erlef.org/cves/CVE-2026-42789.html Added Reference https://github.com/erlang/otp/commit/471cd2f664300a95353c467873800bbe706005db Added Reference https://github.com/erlang/otp/commit/59c8d824386b2eb1614ff9340624843ef6aca0fd Added Reference https://github.com/erlang/otp/security/advisories/GHSA-c99q-jmpx-v8qq Added Reference https://osv.dev/vulnerability/EEF-CVE-2026-42789 Added Reference https://www.erlang.org/doc/system/versions.html#order-of-versions