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CORS misconfiguration for credentials transfer

ID: js/cors-misconfiguration-for-credentials
Kind: path-problem
Security severity: 7.5
Severity: error
Precision: high
Tags:
   - security
   - external/cwe/cwe-346
   - external/cwe/cwe-639
   - external/cwe/cwe-942
Query suites:
   - javascript-code-scanning.qls
   - javascript-security-extended.qls
   - javascript-security-and-quality.qls

Click to see the query in the CodeQL repository

A server can send the "Access-Control-Allow-Credentials" CORS header to control when a browser may send user credentials in Cross-Origin HTTP requests.

When the Access-Control-Allow-Credentials header is "true", the Access-Control-Allow-Origin header must have a value different from "*" in order to make browsers accept the header. Therefore, to allow multiple origins for Cross-Origin requests with credentials, the server must dynamically compute the value of the "Access-Control-Allow-Origin" header. Computing this header value from information in the request to the server can therefore potentially allow an attacker to control the origins that the browser sends credentials to.

Recommendation

When the Access-Control-Allow-Credentials header value is "true", a dynamic computation of the Access-Control-Allow-Origin header must involve sanitization if it relies on user-controlled input.

Since the "null" origin is easy to obtain for an attacker, it is never safe to use "null" as the value of the Access-Control-Allow-Origin header when the Access-Control-Allow-Credentials header value is "true".

Example

In the example below, the server allows the browser to send user credentials in a Cross-Origin request. The request header origins controls the allowed origins for such a Cross-Origin request.

var https = require('https'),
    url = require('url');

var server = https.createServer(function(){});

server.on('request', function(req, res) {
    let origin = url.parse(req.url, true).query.origin;
     // BAD: attacker can choose the value of origin
    res.setHeader("Access-Control-Allow-Origin", origin);
    res.setHeader("Access-Control-Allow-Credentials", true);

    // ...
});

This is not secure, since an attacker can choose the value of the origin request header to make the browser send credentials to their own server. The use of a whitelist containing allowed origins for the Cross-Origin request fixes the issue:

var https = require('https'),
    url = require('url');

var server = https.createServer(function(){});

server.on('request', function(req, res) {
    let origin = url.parse(req.url, true).query.origin,
        whitelist = {
            "https://example.com": true,
            "https://subdomain.example.com": true,
            "https://example.com:1337": true
        };

    if (origin in whitelist) {
        // GOOD: the origin is in the whitelist
        res.setHeader("Access-Control-Allow-Origin", origin);
        res.setHeader("Access-Control-Allow-Credentials", true);
    }

    // ...
});

References

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