Thumbor has HMAC validation bypass via multiple .replace() calls when removing URL signature

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

8.2 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:L

EPSS

• CVE-2026-53501: 0.35% estimated exploitation activity in the next 30 days; percentile 28.93 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

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: The issue is that .replace() removes every occurrence of the substring in the URL, not just the first one. Because the signature itself appears in the URL, an…

Published by the source

31 Jul 2026, 18:51 UTC

Last modified by the source

30 Sep 2026, 18:44 UTC

Source last checked

26 Oct 2026, 13:54 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-mw3h-qjxj-6xg9

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# Patch Next advisory brief: CVE-2026-53501

Advisory: Thumbor has HMAC validation bypass via multiple .replace() calls when removing URL signature
IDs: CVE-2026-53501, GHSA-mw3h-qjxj-6xg9
Record: https://patchnext.com/advisories/cve-2026-53501 (revision 1, source checked 26 Oct 2026, 13:54 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 8.2)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: thumbor 7.8.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI thumbor: affected <= 7.7.7; fixed in 7.8.0

## Suggested inspection (read-only)
- `python -m pip show thumbor`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on thumbor.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-mw3h-qjxj-6xg9 (GitHub-reviewed advisory GHSA-mw3h-qjxj-6xg9 lists affected ranges: PyPI thumbor >=0 <=7.7.7)

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