68 lines
2.1 KiB
Plaintext
68 lines
2.1 KiB
Plaintext
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=== How to fix it in Express.js
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The following code is vulnerable to cross-site scripting because it returns an HTML response that contains unsanitized user input.
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If you do not intend to send HTML code to clients, the vulnerability can be fixed by specifying the type of data returned in the response.
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For example, you can use the `JsonResponse` class to safely return JSON messages.
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[cols="a"]
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|===
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| ==== Noncompliant Code Example
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[source,javascript]
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----
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function (req, res) {
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json = JSON.stringify({ "data": req.query.input });
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res.send(tainted); // Noncompliant
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};
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----
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| ==== Compliant Solution
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[source,javascript]
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----
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function (req, res) {
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res.json({ "data": req.query.input });
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};
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----
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|===
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It is also possible to set the content-type header manually using the `content_type` parameter when creating an `HttpResponse` object.
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[cols="a"]
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|===
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| ==== Noncompliant Code Example
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[source,javascript]
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----
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function (req, res) {
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res.send(req.query.input); // Noncompliant
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};
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----
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| ==== Compliant Solution
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[source,javascript]
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----
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function (req, res) {
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res.set('Content-Type', 'text/plain');
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res.send(req.query.input);
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};
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----
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|===
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=== How does this work?
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In case the response consists of HTML code, it is highly recommended to use a template engine like https://ejs.co/[ejs] to generate it.
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This template engine separates the view from the business logic and automatically encodes the output of variables, drastically reducing the risk of cross-site scripting vulnerabilities.
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If you do not intend to send HTML code to clients, the vulnerability can be resolved by telling them what data they are receiving with the `content-type` HTTP header.
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This header tells the browser that the response does not contain HTML code and should not be parsed and interpreted as HTML.
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Thus, the HTTP response is not vulnerable to reflected Cross-Site Scripting.
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For example, setting the content-type header to `text/plain` allows to safely reflect user input since browsers will not try to parse and execute the response.
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=== Pitfalls
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include::../../common/pitfalls/content-types.adoc[]
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include::../../common/pitfalls/validation.adoc[]
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