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# Web Shell via Race Condition
Web Shell via Race Condition notes for PortSwigger Web Security Academy labs, including exploitation workflow, payload testing, Burp Suite validation, and remediation context.
### Web Shell Upload via Race Condition
**Lab objective**
This lab features a dēupload image feature with apparently robust validation. / However, it is possible to bypass this validation by using a **run condition** during processing of the file.
The goal is to upload a web shell PHP, then use it to read the sensitive file:/ `/home/carlos/secret`.
**Context**
* Authentication possible with: `wiener:peter`
* Only **JPG** and **PNG** files are allowed
* Server response takes about **1 to 2 seconds**, suggesting exploitable asynchronous processing
**Initial attempt**
Try to upload a classic PHP file:
```php
?php
system($_GET['cmd']);
?>
```
Outcome:
* Server refusal
* Message indicating that only JPG and PNG files are accepted
{% hint style="danger" %}
Sorry, only JPG & PNG files are allowed Sorry, there was an error uploading your file.
{% endhint %}
**Key observation**
A **perceptible time period** exists between:
1. When the file is written on the disk
2. When validation is complete and file potentially deleted
This time period opens the door to a race condition.
**Operation of the race condition**
Launch an attack with **Intruder** (or an equivalent tool)
* Repetitive and fast delivery of the download request
* Payload null or repetitive, loop run
While the server processes uploads:
* Continuously refresh the URL of the assumed file
```bash
/files/avatars/cmd.php?cmd=whoami
```
At some point, the file is accessible **before** validation invalidates
* PHP code is then interpreted by the server
**Web Shell Exploitation**
Once executed, you can read the secret file:
```bash
/files/avatars/cmd.php?cmd=cat /home/carlos/secret
```
---
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