GHSA-cj54-hpcc-gj6h

The file_loader performs `unquote()` on the file path AFTER the `abspath() + startswith()` security check. An attacker can use percent-encoded path traversal sequences (`%2e%2e` for `..`) that pass the security check as literal directory names, but are then decoded to actual `..` traversal sequences.

## Affected code

`thumbor/loaders/file_loader.py` lines 28-49: ```python async def load(context, path): file_path = join( context.config.FILE_LOADER_ROOT_PATH.rstrip("/"), path.lstrip("/") ) file_path = abspath(file_path) inside_root_path = file_path.startswith( # Security check abspath(context.config.FILE_LOADER_ROOT_PATH) ) result = LoaderResult() if not inside_root_path: result.error = LoaderResult.ERROR_NOT_FOUND result.successful = False return result

if not exists(file_path): file_path = unquote(file_path) # unquote AFTER check!

if exists(file_path) and isfile(file_path): with open(file_path, "rb") as source_file: ... ```

The watermark and frame filters pass their URL parameters directly to the loader without re-encoding (unlike the main image URL flow which applies `quote()` in `imaging.py` line 37).

## PoC

```bash # Read /etc/passwd via watermark filter path traversal # %252e is double-encoded: Tornado decodes to %2e, filter passes to file_loader, # file_loader unquote decodes %2e to . curl 'http://thumbor-host:8888/unsafe/filters:watermark(%252e%252e/%252e%252e/%252e%252e/%252e%252e/etc/passwd,0,0,100)/some-valid-image.jpg' ```

Flow: 1. `%252e%252e` arrives at Tornado, decoded to `%2e%2e` 2. Watermark filter passes `%2e%2e/%2e%2e/.../etc/passwd` to file_loader 3. `join(ROOT, '%2e%2e/...')` = `ROOT/%2e%2e/...` 4. `abspath()` sees `%2e%2e` as literal dir name (no dots to resolve) 5. `startswith(ROOT)` = True (passes check) 6. `exists()` returns False (literal `%2e%2e` dir doesn't exist) 7. `unquote()` converts `%2e%2e` to `..` 8. OS resolves `..` → reads files outside ROOT

## Prerequisites

- `LOADER = thumbor.loaders.file_loader` (or `file_loader_http_fallback`) - `ALLOW_UNSAFE_URL = True` (this is the default) - watermark/frame filters are enabled by default (in BUILTIN_FILTERS)

## Impact

Arbitrary file read on the thumbor server. When the file is a valid image format, its content is returned in the response overlaid as a watermark. Can be used to read configuration files, secrets, private keys, etc.