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

Status: published. Published: 2026-10-12. Updated: 2026-11-11.

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.

## Remediation

- fix: Upgrade Hex to 2.5.2 or later, for example with mix local.hex. With cacerts_path (HEX_CACERTS_PATH) set, Hex then only accepts server certificates that chain to a CA in that file, and it doesn't trust copies of CA certificates sent by the server. (source: CNA: EEF)
- workaround: There's no Hex configuration that stops the bundled CAs from being trusted. (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/hexpm/hex/commit/5f5a2136c7cf5991b6e8a30d645ca9f8e74b0785
- 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/hexpm/hex/commit/6232a6498015e559743bec4f4fbec9138837cec8

## Affected products and versions

- hexpm / hex (hex): ≥ 0.20.2; < 2.5.2 (affected, semver). Source: CNA: EEF
- hexpm / hex (hexpm/hex): ≥ c9b26a46928bfcd546773571e09c1d05f4cd9fb7; < * (affected, git); unaffected at 5f5a2136c7cf5991b6e8a30d645ca9f8e74b0785; unaffected at 6232a6498015e559743bec4f4fbec9138837cec8. Source: CNA: EEF

## Severity and prioritization

- CVSS 4.0: 5.9 (medium); CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:A/VC:H/VI:L/VA:N/SC:N/SI:N/SA:N; source: CNA: EEF

- CISA KEV: No listing in collected sources

- EPSS: 0.21% as of 2026-11-10; percentile 10.85%

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

- Weaknesses: CWE-295

## Exploit evidence

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

## References

- https://github.com/hexpm/hex/security/advisories/GHSA-rw8q-hg7m-cxgr (CNA: EEF; related, vendor-advisory)

- https://cna.erlef.org/cves/CVE-2026-103503.html (CNA: EEF; related)

- https://osv.dev/vulnerability/EEF-CVE-2026-103503 (CNA: EEF; related)

- https://github.com/hexpm/hex/commit/c9b26a46928bfcd546773571e09c1d05f4cd9fb7 (CNA: EEF; related)

- https://github.com/hexpm/hex/commit/5f5a2136c7cf5991b6e8a30d645ca9f8e74b0785 (CNA: EEF; patch)

- https://github.com/hexpm/hex/commit/6232a6498015e559743bec4f4fbec9138837cec8 (CNA: EEF; patch)

## Provenance

- CNA: EEF: https://www.cve.org/CVERecord?id=CVE-2026-103503; retrieved 2026-11-11; source updated 2026-11-11

- FIRST EPSS: https://api.first.org/data/v1/epss?cve=CVE-2026-103503; retrieved 2026-11-11; source updated Not reported

Revision: a26bede0c059a51c4830fc63ccda5ea0fadc72ab7782e3ecc9b6f76bb5b81ba6

Canonical: https://y2.dev/cve/CVE-2026-103503

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