Gotenberg is an API for converting document formats. Prior to version 8.29.0, the fix introduced for CVE-2024-21527 can be bypassed using mixed-case or uppercase URL schemes. This issue has been patched in version 8.29.0.

Affected: Go: github.com/gotenberg/gotenberg/v8, Go: github.com/gotenberg/gotenberg/v7

### Impact

The fix introduced in version 8.1.0 for GHSA-rh2x-ccvw-q7r3 (CVE-2024-21527) can be bypassed using mixed-case or uppercase URL schemes.

The default `--chromium-deny-list` value is `^file:(?!//\/tmp/).*`. This regex is anchored to lowercase `file:` at the start. However, per RFC 3986 Section 3.1, URI schemes are case-insensitive. Chromium normalizes the scheme to lowercase before navigation, so a URL like `FILE:///etc/passwd` or `File:///etc/passwd` bypasses the deny-list check but still gets resolved by Chromium as `file:///etc/passwd`.

The root cause is in `pkg/gotenberg/filter.go` — the `FilterDeadline` function compiles the deny-list regex with `regexp2.MustCompile(denied.String(), 0)`, where `0` means no flags (case-sensitive). Since the regex pattern itself doesn't include a `(?i)` flag, matching is strictly case-sensitive.

This affects both the URL endpoint and HTML conversion (via iframes, link tags, etc.).

### Steps to Reproduce

1. Start Gotenberg with default settings:

```bash docker run --rm -p 3000:3000 gotenberg/gotenberg:8.26.0 gotenberg ```

2. Read `/etc/passwd` via the URL endpoint using an uppercase scheme:

```bash curl -X POST 'http://localhost:3000/forms/chromium/convert/url' \ --form 'url=FILE:///etc/passwd' -o output.pdf ```

3. Open `output.pdf` — it contains the contents of `/etc/passwd`.

4. Alternatively, create an `index.html`:

```html <iframe src="FILE:///etc/passwd" width="100%" height="100%"></iframe> ```

Then convert it:…