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URL redirection from remote source

ID: cs/web/unvalidated-url-redirection Kind: path-problem Security severity: 6.1 Severity: error Precision: high Tags: - security - external/cwe/cwe-601 Query suites: - csharp-code-scanning.qls - csharp-security-extended.qls - csharp-security-and-quality.qls

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Directly incorporating user input into a URL redirect request without validating the input can facilitate phishing attacks. In these attacks, unsuspecting users can be redirected to a malicious site that looks very similar to the real site they intend to visit, but which is controlled by the attacker.

Recommendation

To guard against untrusted URL redirection, it is advisable to avoid putting user input directly into a redirect URL. Instead, maintain a list of authorized redirects on the server; then choose from that list based on the user input provided.

If this is not possible, then the user input should be validated in some other way, for example, by verifying that the target URL is on the same host as the current page.

Example

The following example shows an HTTP request parameter being used directly in a URL redirect without validating the input, which facilitates phishing attacks:

usingSystem;usingSystem.Web;publicclassUnvalidatedUrlHandler:IHttpHandler{publicvoidProcessRequest(HttpContextctx){// BAD: a request parameter is incorporated without validation into a URL redirectctx.Response.Redirect(ctx.Request.QueryString["page"]);}}

One way to remedy the problem is to validate the user input against a known fixed string before doing the redirection:

usingSystem;usingSystem.Web;usingSystem.Collections.Generic;publicclassUnvalidatedUrlHandler:IHttpHandler{privateList<string>VALID_REDIRECTS=newList<string>{"http://cwe.mitre.org/data/definitions/601.html","http://cwe.mitre.org/data/definitions/79.html"};publicvoidProcessRequest(HttpContextctx){if(VALID_REDIRECTS.Contains(ctx.Request.QueryString["page"])){// GOOD: the request parameter is validated against a known list of stringsctx.Response.Redirect(ctx.Request.QueryString["page"]);}}}

Alternatively, we can check that the target URL does not redirect to a different host by checking that the URL is either relative or on a known good host:

usingSystem;usingSystem.Web;publicclassUnvalidatedUrlHandler:IHttpHandler{publicvoidProcessRequest(HttpContextctx){varurlString=ctx.Request.QueryString["page"];varurl=newUri(urlString,UriKind.RelativeOrAbsolute);varurl=newUri(redirectUrl,UriKind.RelativeOrAbsolute);if(!url.IsAbsoluteUri){// GOOD: The redirect is to a relative URLctx.Response.Redirect(url.ToString());}if(url.Host=="example.org"){// GOOD: The redirect is to a known hostctx.Response.Redirect(url.ToString());}}}

Note that as written, the above code will allow redirects to URLs on example.com, which is harmless but perhaps not intended. You can substitute your own domain (if known) for example.com to prevent this.

References

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