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CVE-2026-42789

Non-CA certificate accepted as intermediate issuer in public_key path validation

High7.0EPSS 0.38%
Published
Updated
Reserved
Assigned by
EEF

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.

Scores

CVSS scores of CVE-2026-42789, by who scored it
Scored byCVSSScoreVector
EEFCNA (assigner)4.0High7.0CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:L/VI:L/VA:N/SC:H/SI:H/SA:N
redhat-SADPADP (enrichment)3.1High8.0CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:H/I:H/A:N
NVDNVD · Modified4.0High7.0CVSS:4.0/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
EPSS (exploitation in 30 days)
0.38% · top 68% of all CVEs (6 Nov 2026)
CISA SSVC
Exploitation: none · Automatable: no · Technical impact: partial
Loading the EPSS history…

The list uses the assigner's score when there is one, then CISA's, then NVD's: High 7.0.

Weakness

Affected products

  • Erlang · OTP

    • affected 17.0 – < *

  • Erlang · OTP

    • affected 0.22 – < *

  • Erlang · OTP

    • affected 84adefa331c4159d432d22840663c38f155cd4c1 – < *
    • unaffected 0a05b7ca33fb0b3f0a6cb9ae34252bb3a74032b8 – < *
    • unaffected 75ca4c0dc2bbdb286ac3c3fe89cb4b4d2b4dc333 – < *
    • unaffected 82af606b5437c4de1276dbcc8855a47e88231226 – < *

What to do

Workaround: The verify_fun option in the ssl or public_key application can be used to ensure that path validation rejects chains where an intermediate certificate does not have basicConstraints cA:true.

References (12)

Sources

The same CVE in the primary databases and aggregators, to check against.

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Record last updated .