hex_core's default HTTP adapter doesn't verify TLS certificates on OTP 24 and 25

Summary

Improper Certificate Validation vulnerability in hexpm hex_core allows an attacker who can intercept HTTPS connections to impersonate hex.pm, repo.hex.pm or any other repository or API server to programs that use hex_core's default HTTP adapter on Erlang/OTP 24, or on OTP 25 when the operating system's CA certificates can't be loaded.

hex_core:default_config/0 selects the built-in hex_http_httpc adapter. For an HTTPS request whose adapter config has no ssl option, hex_http_httpc:http_options/2 builds options that verify the server certificate against public_key:cacerts_get/0. When that call raises, the adapter prints a warning and sends the request without ssl options, so httpc uses the defaults of the ssl application, which before OTP 26 don't verify the server certificate. public_key:cacerts_get/0 doesn't exist on OTP 24, so every HTTPS request falls back there, and on OTP 25 it raises when no CA store is found. hex_core versions before 0.11.0 never set ssl options, so they don't verify the server certificate on OTP 25 and earlier whether or not a CA store is found.

The fallback was intended and documented, but the documentation didn't make clear that it meant no certificate verification. Since hex_core 0.8.2 the README has warned that the built-in httpc adapter "has insecure default settings" and recommended other HTTP clients, and the adapter prints "using default ssl options which are insecure" on every request that falls back. hex_core 0.11.0 added certificate verification with the changelog entry "Use secure SSL defaults for OTP 25+". Neither the README nor the warning says that the server certificate isn't verified, the README doesn't say which OTP versions the warning applies to, and on OTP 25 without CA certificates the warning tells users to upgrade to OTP 25 or later, which they already run. This advisory is published so that users who took those notices as general advice know their HTTPS requests weren't authenticated.

This issue affects hex_core: from 0.1.0 before 0.20.0.

Details

1. hex_core:default_config/0 sets http_adapter to {hex_http_httpc, #{profile => default}}.
2. hex_http_httpc:request/5 and hex_http_httpc:request_to_file/6 build their httpc options with http_options/2.
3. For an https URI whose adapter config has no ssl entry in http_options, http_options/2 adds {ssl, [{verify, verify_peer}, {cacerts, public_key:cacerts_get()}, ...]} inside a try.
4. When public_key:cacerts_get/0 raises (undef on OTP 24; an error on OTP 25 when no CA store is found), the catch prints a warning with io:format/1 and returns the options without ssl.
5. httpc then uses the ssl application's defaults, which are verify_none before OTP 26, and completes the handshake with any certificate.
6. hex_core before 0.11.0 (e57b4fb) had no step 3: it never passed ssl options, so step 5 applied on every OTP version before 26.

Proof of concept

1. Compile src/hex_http.erl and src/hex_http_httpc.erl from hex_core main 4fbc5d1 in the erlang:24 Docker image and run hex_http_httpc:request(get, <<"https://self-signed.badssl.com/">>, #{}, undefined, #{profile => default}) after starting inets and ssl.
2. The adapter prints [hex_http_httpc] using default ssl options which are insecure. and returns {ok, {200, Headers, Body}} from the server with the self-signed certificate.
3. In the erlang:25 image with an empty tmpfs mounted on /etc/ssl/certs, public_key:cacerts_get/0 raises and the request also returns {ok, {200, ...}}.
4. In the erlang:25 image with the CA store present (150 certificates), the same request fails with {error, {failed_connect, ...}} after a Bad Certificate alert.

Impact

An attacker on the network path between a program using hex_core's default adapter and a Hex repository or API server completes the TLS handshake with any certificate. It receives the API keys, OAuth tokens and repository keys the program sends, and can alter API requests and responses. Registry signatures, which hex_core verifies by default, still prevent changes to registry contents. Hex and rebar3 aren't affected because they configure their own HTTP adapters.

Workarounds

Pass ssl options that verify the server certificate in the adapter config, for example {hex_http_httpc, #{http_options => [{ssl, SslOpts}]}}, use another HTTP adapter, or run on OTP 26 or later.

Configurations

Affects programs that use the hex_http_httpc adapter, the default in hex_core:default_config/0, without an ssl option in the adapter's http_options, on OTP 24, or on OTP 25 when public_key:cacerts_get/0 fails. hex_core versions before 0.11.0 are affected on OTP 25 and earlier whether or not the CA certificates load. On OTP 26 and later the ssl defaults verify the server certificate. Hex and rebar3 vendor hex_core but configure their own HTTP adapters, so they aren't affected.

Solutions

Upgrade hex_core to 0.20.0 or later. On OTP 24, or on OTP 25 without the operating system's CA certificates, HTTPS requests through hex_http_httpc then fail with {error, {cacerts_unavailable, Reason}} unless the adapter config has ssl options.