Thumbor has path traversal via post-validation URL decoding bypass in file_loader

Exploitation

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

CVSS 4.0

8.7 · CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N · score source

EPSS

• CVE-2026-53502: 0.52% estimated exploitation activity in the next 30 days; percentile 44.39 (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.

The fileloader performs unquote() on the file path AFTER the abspath() + startswith() security check. An attacker can use percent-encoded path traversal sequences (%2e%2e for ..) that pass the security check as literal directory names, but are then decoded to actual .. traversal sequences. Affected code thumbor/loaders/fileloader.py lines 28-49: The watermark and frame filters pass their URL parameters directly to the loader without re-encoding (unlike the main image URL flow which applies quote() in imaging.py line 37). PoC Flow: 1. %252e%252e arrives at Tornado, decoded to %2e%2e 2. Watermark filter passes %2e%2e/%2e%2e/.../etc/passwd to fileloader 3. join(ROOT, '%2e%2e/...') = ROOT/%2e%2e/... 4. abspath() sees %2e%2e as literal dir name (no dots to resolve) 5. startswith(ROOT) = True (passes check) 6. exists() returns False (literal %2e%2e dir doesn't exist) 7. unquote() converts %2e%2e to .. 8. OS resolves .. → reads files outside ROOT Prerequisites - LOADER =…

Published by the source

31 Jul 2026, 19:03 UTC

Last modified by the source

31 Jul 2026, 19:03 UTC

Source last checked

26 Oct 2026, 13:50 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-cj54-hpcc-gj6h

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# Patch Next advisory brief: CVE-2026-53502

Advisory: Thumbor has path traversal via post-validation URL decoding bypass in file_loader
IDs: CVE-2026-53502, GHSA-cj54-hpcc-gj6h
Record: https://patchnext.com/advisories/cve-2026-53502 (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 8.7)
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-cj54-hpcc-gj6h (GitHub-reviewed advisory GHSA-cj54-hpcc-gj6h lists affected ranges: PyPI thumbor >=0 <=7.7.7)

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