GHSA-6x26-6r6f-m537

## Summary

The `ALLOWED_SOURCES` configuration is meant to restrict which hosts Thumbor's HTTP loader may fetch images from. Plain-string entries in that list (the overwhelming majority of real-world and documented configurations) are passed directly to `re.match()` without escaping. Because `.` is a regex wildcard, every dot in a domain name becomes a bypass vector: `s.glbimg.com` silently matches `sXglbimgYcom`, `sAglbimg.com`, and any other hostname that differs only at a dot position. This undermines the primary SSRF defence that `ALLOWED_SOURCES` is intended to provide.

## Affected component

`thumbor/loaders/http_loader.py` — `validate()`

## Proof of concept

```python import re from thumbor.config import Config from thumbor.context import Context from thumbor.loaders import http_loader as loader

config = Config() config.ALLOWED_SOURCES = ["s.glbimg.com"] # typical user config ctx = Context(None, config, None)

# These should be blocked — both return True due to the unescaped dot print(loader.validate(ctx, "http://sXglbimgYcom/secret.jpg")) # True ← bypass print(loader.validate(ctx, "http://sAglbimg.com/secret.jpg")) # True ← bypass

# Legitimate origin — correctly allowed print(loader.validate(ctx, "http://s.glbimg.com/logo.jpg")) # True ← correct ```

## Root cause

`thumbor/loaders/http_loader.py` (before fix):

```python for pattern in context.config.ALLOWED_SOURCES: if isinstance(pattern, Pattern): match = url else: pattern = f"^{pattern}$" # <-- dots not escaped, act as regex wildcard match = res.hostname

if re.match(pattern, match): return True ```

## Impact

An attacker who can influence the image source URL passed to Thumbor can fetch images from arbitrary hosts, bypassing the `ALLOWED_SOURCES` allowlist.

**Preconditions:**

- `ALLOWED_SOURCES` contains at least one plain-string entry (the common case; all official documentation examples use plain strings). - The attacker can supply or influence the image URL — true whenever `ALLOW_UNSAFE_URL = True` (the default), or when the application forwards user input to a signed URL endpoint.

## Fix

Apply `re.escape()` to plain-string patterns before compiling them, so every character is matched literally:

```python else: pattern = f"^{re.escape(pattern)}$" # dots and other metacharacters are now literal match = res.hostname ```

This is a one-call addition with no breaking change for correctly written configurations. Users who need real regular-expression behaviour should supply a compiled pattern (`re.compile(r"s\.glbimg\.com")`), which is already handled by the existing `isinstance(pattern, Pattern)` branch and is unaffected by this change.

The `ALLOWED_SOURCES` docstring in `config.py` was also updated to document the two-mode behaviour explicitly.