Thumbor proportion filter allows unbounded post-transform resize leading to remote DoS

Exploitation

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

CVSS

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

EPSS

• CVE-2026-53505: 0.61% estimated exploitation activity in the next 30 days; percentile 49.21 (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.

Summary Thumbor's filters:proportion(<value) filter does not enforce an upper bound on <value and runs in the post-transform phase. An attacker can trigger extremely large resizes (CPU/memory exhaustion) and cause denial of service. Details - Filter implementation: thumbor/filters/proportion.py - value is parsed as a float (BaseFilter.DecimalNumber) with no maximum. - The filter computes newwidth = sourcewidth value and newheight = sourceheight value and then calls engine.resize(newwidth, newheight). - Execution phase: proportion runs in the default POSTTRANSFORM phase (after the main transform pipeline). This means it can effectively bypass request-level size clamping that happens earlier in the request lifecycle (e.g., MAXWIDTH/MAXHEIGHT applied to req.width/req.height). Documentation states the percentage argument should be 0.0 to 1.0 (docs/proportion.rst), but the implementation does not enforce this constraint. PoC Preconditions - The proportion filter is enabled (it is enabled…

Published by the source

31 Jul 2026, 19:00 UTC

Last modified by the source

31 Jul 2026, 19:00 UTC

Source last checked

26 Oct 2026, 13:50 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-phj3-59pf-cp83

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# Patch Next advisory brief: CVE-2026-53505

Advisory: Thumbor proportion filter allows unbounded post-transform resize leading to remote DoS
IDs: CVE-2026-53505, GHSA-phj3-59pf-cp83
Record: https://patchnext.com/advisories/cve-2026-53505 (revision 1, source checked 26 Oct 2026, 13:50 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.5)
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-phj3-59pf-cp83 (GitHub-reviewed advisory GHSA-phj3-59pf-cp83 lists affected ranges: PyPI thumbor >=0 <=7.7.7)

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