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# XSS in a JavaScript URL with Limited Characters
XSS in a JavaScript URL with Limited Characters notes for PortSwigger Web Security Academy labs, including exploitation workflow, payload testing, Burp Suite validation, and remediation context.
### Reflected XSS in a JavaScript URL with Some Characters Blocked
#### Objective
Using a reflected XSS vulnerability in a `JavaScript:` URL to show `alert` containing the `1337` string (the `1337` string must be present somewhere in the message).
The application reflects our entry in a JavaScript URL. At first glance the challenge seems simple, but the application filters certain characters to prevent XSS attacks. We must find a way to escape the JavaScript context despite these restrictions and trigger `alert(...)` containing `1337`.
#### Observed Source Code Analysis
In the source code you can find the following link:
{% code overflow="wrap" %}
```javascript
Back to Blog
```
{% endcode %}
After decoding URL you get:
{% code overflow="wrap" %}
```javascript
Back to Blog
```
{% endcode %}
Click Back to Blog executes this JavaScript URL. By intercepting the request, we see that a call is made to `/analytics`. The `postId` parameter is taken from the URL (`/post?postId=...`) and reflected in the body of the analytics query.
#### First Attempt — Parsing Break
By injecting a single apostrophe the parsing is broken (ID becomes invalid):
```javascript
2'},{x:'
```
The apostrophe breaks the structure, which shows that the value is inserted in a context where quotas have an impact.
#### Fuzzing Special Characters
We quickly tested which special characters produce a valid response (with `wfuzz` and the `special-chars.txt` list). Example command used:
{% code overflow="wrap" %}
```bash
wfuzz -c -w /usr/share/SecLists/Fuzzing/special-chars.txt 'https://0a530041036b803fbb5cd40a009500a8.web-security-academy.net/post?postId=2FUZZ%27},{x:%27'
```
{% endcode %}
The results show that `&` and `#` produce a 200 code — they are accepted by the parser and allow to insert sequences that properly close the structure.
Observed interpretation:
```javascript
post?postId=2&
```
#### Construction of Escape Vector
By properly closing the value and inserting code, you can obtain:
```javascript
post?postId=2&'},{x:''
```
Here `x` remains empty and syntax is accepted.
Then we try to add a `alert` directly:
```javascript
post?postId=2&'},alert(1),{x:''
```
But `()` parentheses from `alert(1)` are removed by the filter — so you have to bypass the obligation to use `()`.
#### Contouring without Parentheses
One escapes the use of parentheses and uses more creative JavaScript expressions to invoke `alert` with `1337` in output. The final payload used is:
```javascript
2&'},x=x=>{throw/**/onerror=alert,1337},toString=x,window%2b'',{x:'
```
---
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