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# Bypass with Null Byte and Extension Validation
Bypass with Null Byte and Extension Validation notes for PortSwigger Web Security Academy labs, including exploitation workflow, payload testing, Burp Suite validation, and remediation context.
### File Path Traversal, Validation of File Extension with Null Byte Bypass
This lab presents a vulnerability of path traversal when displaying the product images. / The application checks that the filename provided ends well with the expected extension.
The objective is to read the contents of the **/etc/passwd** file.
#### **Analysis**
If the path is provided directly
```bash
../../../../etc/passwd
```
The application returns an error because it requires the file to have the extension **.jpg**.
In older versions of PHP (before **5.3.4**), it was possible to use a zero **byte (%00)** to bypass this validation: / the PHP engine stopped reading the file name at the null byte, while the application continued to see a valid extension.
#### **Payload**
Add a null byte before the extension:
```bash
%00.jpg
```
The browser still interprets ".jpg", but PHP stops at " /etc/passwd", which allows you to read the sensitive file.
```bash
../../../../etc/passwd%00.jpg
```
---
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