> 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/request-smuggling/client-side-desync.md).
# Client-Side Desync
Client-Side Desync notes for PortSwigger Web Security Academy labs, including exploitation workflow, payload testing, Burp Suite validation, and remediation context.
### Client-side Desync
This lab is vulnerable to client-side desynchronization attacks because the server ignores the Content-Length header for some endpoints. This weakness allows the victim's browser to disclose its session cookie. / Lab objective:
1. Identify a client-side desynchronization vector with Burp, then check that it is reproducible in the browser.
2. Find an application element to inject or store text.
3. Combine both to force the victim's browser to send a series of inter-domain requests that disclose his cookie.
4. Use this cookie to access the victim's account.
#### **Analysis of Server Behavior**
By sending a query with a deliberately swollen Content-Length, the server ignores and instead treats the following content as a new query:
```http
POST / HTTP/1.1
Host: 0a69005a041eb75c828761ef00630000.h1-web-security-academy.net
Content-Type: application/x-www-form-urlencoded
Content-Length: 100
GET /error HTTP/1.1
Test: hello
```
This reaction confirms the existence of client-side desynchronization.
#### **Demonstrates Desync**
By configuring two requests in Burp (one simulating the client, the other attacking) and sending them in sequence, the error response to the attacker is sent to the legitimate client.
This shows that the server unsynchronizes HTTP streams.
#### **Exfiltration of the Victim's Cookie**
To force the victim's browser to disclose its session cookie, we target the comment feature, which allows you to store text in the application.
Send a comment query with a swollen Content-Length:
```http
POST /en/post/comment HTTP/1.1
Host: 0a69005a041eb75c828761ef00630000.h1-web-security-academy.net
Content-Type: application/x-www-form-urlencoded
Content-Length: 118
csrf=5eEtViEnAhAKWZz68JwV8leC2rJYFIAf&postId=1&comment=tst&name=tst&email=tst%40test.com&website=http%3A%2F%2Ftest.com
```
```http
csrf=5eEtViEnAhAKWZz68JwV8leC2rJYFIAf&postId=1&name=tst&email=tst%40test.com&website=http%3A%2F%2Ftest.com&comment=tst
```
By increasing this field (e.g. to 500), the published comment then reveals the session cookie of the connected account.
#### **Automation via a JavaScript Script**
To turn the attack into a exploit usable by the victim, a smuggled query encapsulated in a script is built:
```javascript
```
This payload results in the automatic sending of the smuggled request by the victim's browser.
The stolen session cookie then appears in the comments section.
---
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