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

Affected: thumbor

Thumbor is an open-source photo thumbnail service by globo.com. Prior to 7.8.0, 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. This issue is fixed in 7.8.0.

GHSA-mw3h-qjxj-6xg9 — Thumbor has HMAC validation bypass via multiple .replace() calls when removing URL signature

Affected: PyPI: thumbor

# HMAC validation bypass via multiple `.replace()` calls when removing URL signature

## Summary

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.

## Details

Thumbor signs URLs using **HMAC-SHA1** to prevent abuse such as loading arbitrary external images or invoking filters without authorization.

During request validation, Thumbor removes the signature from the request URL before recalculating the HMAC. The relevant code:

```python url_signature = self.context.request.hash if url_signature: signer = self.context.modules.url_signer( self.context.server.security_key )

try: quoted_hash = quote(self.context.request.hash) except KeyError: self._error(400, f"Invalid hash: {self.context.request.hash}") return

url_to_validate = url.replace( f"/{self.context.request.hash}/", "" ).replace(f"/{quoted_hash}/", "")

valid = signer.validate( unquote(url_signature).encode(), url_to_validate ) ```

The issue is that `.replace()` removes **every occurrence** of the substring in the…

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

GHSA-mw3h-qjxj-6xg9 — Thumbor has HMAC validation bypass via multiple .replace() calls when removing URL signature