> For the complete documentation index, see [llms.txt](https://hacking-notes.jord4n.pro/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://hacking-notes.jord4n.pro/web/xss/xss-with-csp-bypass.md). # XSS with CSP and Bypass Technique XSS with CSP and Bypass Technique notes for PortSwigger Web Security Academy labs, including exploitation workflow, payload testing, Burp Suite validation, and remediation context. ### Reflected XSS Protected by CSP, with CSP Bypass #### Lab Description Cross-site reflected scripting protected by a Content Security Policy (CSP). The goal is to execute `alert()` by bypassing the CSP. The intended solution only works on Chrome. #### Objective Find a thoughtful XSS vector that bypasses the CSP and calls the `alert()` function. Observations * By injecting a `` — the tag is visible but blocked by the CSP. ```javascript ' ```

CSP allows only scripts from the same domain (`'self'`).

* The CSP report indicates that a token is expected (the `token` parameter appears related to the behaviour of the CSP).

* By adding `&token=test` to the URL, this parameter is interpreted by the application. ``` &token=test ```

It is possible to inject a CSP directive directly via the `token` parameter, for example: ``` &token=;script-src-elem 'unsafe-inline' ``` the Directive is taken into account

After forcing the addition of `script-src-elem 'unsafe-inline'`, the following injection allows JavaScript execution: ```javascript &token=;script-src-elem 'unsafe-inline' ```

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