Custom cacerts or cacertfile do not restrict trust to the pinned CA in hackney

Overview

Improper Certificate Validation vulnerability in hackney_ssl in benoitc hackney allows an on-path attacker to impersonate a server pinned to a private certificate authority via a certificate chain that ends in a publicly trusted root. hackney adds {partial_chain, fun hackney_ssl:partial_chain/1} to every verify_peer configuration. The callback returns any certificate in the peer chain whose public key matches a root in the bundled certifi store as the trust anchor, and OTP honours that callback over the configured cacerts. hackney_ssl:merge_ssl_opts/3 kept the callback when the caller replaced the trust store with cacerts or cacertfile in ssl_options, so a connection pinned to a private CA still accepted any chain that carried a publicly trusted root.

The hostname check still applies, so the attacker needs a publicly trusted certificate for the pinned hostname.

This issue affects hackney: from 1.24.0 before 4.9.0.

Affected

• hackney >= 1.24.0 < 4.9.0
• benoitc/hackney >= d8a9afcf5c3849904274e2eba1df56c63d1a1a0c < 915eb5737e9c489d8fc22eec8cdae30e9b027be4

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.

Workarounds

Pass your own {partial_chain, Fun} in ssl_options alongside cacerts or cacertfile. A caller-supplied partial_chain replaces hackney's in the merge, and a callback that returns unknown_ca makes OTP validate against the configured cacerts only.