> 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/validating-the-start-of-the-path-directory-traversal.md).

# Validating the Start of the Path Directory Traversal

Validating the Start of the Path Directory Traversal notes for PortSwigger Web Security Academy labs, including exploitation workflow, payload testing, Burp Suite validation, and remediation context.

### File Path Traversal, Validation of Start of Path

The application transmits the entire file path via a query parameter and only checks that this path **starts** with the expected folder. This validation is insufficient: by adding after the allowed prefix, it is possible to access arbitrary files.

#### **Analysis**

The filename parameter directly includes an absolute path, for example:

```bash
/var/www/images
```

<figure><img src="https://131707722-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2Fl4EVJpcOrsgjSGsLI7p2%2Fuploads%2Fgit-blob-0ecb977074c246375d014d04cfb0b6980b01da53%2Fimage%20(254).png?alt=media" alt=""><figcaption></figcaption></figure>

If one tries a classic crossing:

```bash
../../../../../etc/passwd
```

application blocks it.

But if we provide **the expected path**, followed by `../` sequences to get out, validation fails to detect escape:

As the path starts well with `/var/www/images/`, the check is satisfied, but the final resolution actually points to `/etc/passwd`.

```bash
/var/www/images/../../../../../etc/passwd
```

<figure><img src="https://131707722-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2Fl4EVJpcOrsgjSGsLI7p2%2Fuploads%2Fgit-blob-0d1dc24319c0bcc94915926664e561591ad27409%2Fimage%20(255).png?alt=media" alt=""><figcaption></figcaption></figure>


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation by asking a question.

Perform an HTTP GET request on the following URL with the `ask` and `goal` query parameters:

```
GET https://hacking-notes.jord4n.pro/web/path-traversal/validating-the-start-of-the-path-directory-traversal.md?ask=<question>&goal=<user_goal>
```

`ask` is the immediate question: it should be specific, self-contained, and written in natural language.
`goal` is what the user is ultimately trying to achieve, the reason they need the answer. Sharing it helps GitBook give you a better, more relevant answer. A goal is most helpful when it describes the outcome the user wants rather than restating the question. For example, with `ask=how do I create an API token`, a goal like `automate deployments from our CI pipeline` lets GitBook tailor the answer to that use case.

The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.