Thumbor treats ALLOWED_SOURCES string patterns as unescaped regex, allowing hostname bypass via wildcard dot
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.
