Hex trusts its bundled CA certificates when cacerts_path is set

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.

1. Hex.HTTP.SSL.get_ca_certs/0 returns the certificates from cacerts_path when it's set, and ssl_opts/1 passes them as cacerts.
2. ssl_opts/1 sets partial_chain to &partial_chain(Certs.cacerts(), &1), which always uses the bundled store.
3. When the server's chain doesn't lead to a certificate in cacerts, OTP calls partial_chain with the chain. partial_chain/2 returns {:trusted_ca, der} for the first certificate whose public key matches a bundled CA certificate, and OTP validates the chain with that certificate as the trust anchor.
4. Chain validation and the hostname check from customize_hostname_check/1 still run, so the attacker needs a certificate for the requested host that chains to a bundled CA.

1. Create a PEM file that contains only an unrelated test CA certificate and set cacerts_path to it.
2. Open a TLS connection to hex.pm with the options from Hex.HTTP.SSL.ssl_opts("https://hex.pm"). The handshake succeeds, and the accepted chain leads to ISRG Root X2 cross-signed by ISRG Root X1, neither of which is in the PEM file.
3. Remove the partial_chain option and connect again. The handshake fails with unknown_ca.

There's no Hex configuration that stops the bundled CAs from being trusted.

Affects users who set cacerts_path or HEX_CACERTS_PATH to a CA store that leaves out some of the CAs bundled with Hex. Users who don't set it trust the bundled store already and aren't affected.

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.