Thumbor convolution filter allows divide-by-zero in C extension 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-2026-53503: 0.75% estimated exploitation activity in the next 30 days; percentile 55.02 (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:convolution(<matrix, <columns, <shouldnormalize) filter passes the user-controlled <columns value to a C extension (thumbor/ext/filters/convolution.c) where it is used as a divisor (for % and /) without validating columns 0. When columns=0, the C code triggers undefined behavior; on x8664 this reliably results in a fatal divide-by-zero trap (SIGFPE) and crashes the Thumbor process (confirmed on Linux x8664 and macOS Intel x8664), causing a remote denial of service. Details Root cause The Python filter accepts columns=0, and the native C extension uses columnscount as a divisor without validating it. 1) Python filter entry point allows columns=0 (thumbor/filters/convolution.py): BaseFilter.PositiveNumber matches "0" (thumbor/filters/init.py): 2) C extension divides/modulos by columnscount without a zero check (thumbor/ext/filters/convolution.c): PoC Test environment - Linux x8664 Preconditions - The convolution filter is enabled (it is enabled by default via…

Published by the source

31 Jul 2026, 18:54 UTC

Last modified by the source

31 Jul 2026, 18:54 UTC

Source last checked

26 Oct 2026, 13:50 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-cqjp-jf4r-h5q9

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# Patch Next advisory brief: CVE-2026-53503

Advisory: Thumbor convolution filter allows divide-by-zero in C extension leading to remote DoS
IDs: CVE-2026-53503, GHSA-cqjp-jf4r-h5q9
Record: https://patchnext.com/advisories/cve-2026-53503 (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-cqjp-jf4r-h5q9 (GitHub-reviewed advisory GHSA-cqjp-jf4r-h5q9 lists affected ranges: PyPI thumbor >=0 <=7.7.7)

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