CVE-2026-53500: 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.

• github.com/thumbor/thumbor/security/advisories/GHSA-6x26-6r6f-m537 (opens in a new tab)
• github.com/thumbor/thumbor/commit/68876715350c6c8f49c324e5515e64908830aed7 (opens in a new tab)
• github.com/thumbor/thumbor (opens in a new tab)
• github.com/thumbor/thumbor/releases/tag/7.8.0 (opens in a new tab)

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