CVE-2026-53501: Thumbor has HMAC validation bypass via multiple .replace() calls when removing URL signature

Thumbor’s HMAC validation can be bypassed due to the use of Python’s .replace() when removing the signature from the URL before validation. Since .replace() removes all occurrences of the substring, an attacker can insert the same signature multiple times in the URL and manipulate the final URL used for validation.

This allows crafting URLs where the validated string differs from the actual requested resource, enabling loading images from unintended domains or paths.