CVE-2026-103503: Hex trusts its bundled CA certificates when cacerts_path is set

Severity: medium (CVSS 5.9 v4.0)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.2% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-10-12 · updated 2026-10-16
Weakness: CWE-295

Description

Improper Certificate Validation vulnerability in hexpm hex allows an attacker who can intercept HTTPS connections, and holds a certificate for a Hex host issued by a CA in Hex's bundled CA store, to impersonate that host to users who set cacerts_path to a CA store that leaves that CA out. cacerts_path (HEX_CACERTS_PATH) is documented as the path to the CA certificate store, used instead of the CA bundle that ships with Hex. Hex.HTTP.SSL.ssl_opts/1 passes the configured store as the cacerts option, but builds the partial_chain callback from the bundled store whether cacerts_path is set or not. Hex.HTTP.SSL.partial_chain/2 accepts a server chain when any certificate in it has the same public key as a bundled CA certificate, so a chain that leads to a bundled CA passes verification although the configured store doesn't contain that CA. Hostname verification still applies. This issue affects hex: from 0.20.2 before 2.5.2.

Affected products

hexpm hex: from 0.20.2 before 2.5.2 — fixed in 2.5.2

How to fix

Upgrade to a fixed version: hexpm hex 2.5.2. Check the vendor advisory for mitigations.

References

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