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Not in CISA KEV
HIGH severity

CVE-2026-53505 — thumbor

HIGHFix: thumbor/thumbor@2c71611

CVE-2026-53505 is a high-severity (CVSS 7.5) Uncontrolled Resource Consumption vulnerability in thumbor. A fix is available for thumbor — see the affected versions and patch details below.

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

Also known asGHSA-phj3-59pf-cp83PYSEC-2026-3621
Published
Updated
Affected
1 pkg
Patched
1 / 1
Exploits
None indexed
Exploitation data as of Oct 29, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

Proof-of-concept exploit code exists

  • CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
  • CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.

Exploitation and automatability from CISA’s SSVC triage for CVE-2026-53505.

EPSS Exploitation Probability

via FIRST.org ↗
0.6%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs49th percentile — riskier than 49% of all scored CVEsHighest risk
0.00%0.37%0.74%1.11%0.3%0.3%0.6%0.6%Aug 26Oct 26Oct 26

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

1 pkg affected
🐍thumbor

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.

Description

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 new_width = source_width * value and new_height = source_height * value and then calls engine.resize(new_width, new_height).
  • Execution phase: proportion runs in the default POST_TRANSFORM 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., MAX_WIDTH/MAX_HEIGHT 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 by default via BUILTIN_FILTERS).
  • Either:
    • /unsafe/ URLs are allowed (ALLOW_UNSAFE_URL=True, common default in some deployments), OR
    • /unsafe/ is disabled, and the attacker has a valid signed URL (i.e., the attacker is an authorized user/partner, or otherwise can obtain signed URLs issued by a trusted signing service).

Example request 1 (signed URL)

The following request was used to reproduce the issue and causes severe resource exhaustion:

http://<host>:<port>/<url-sign>/100x100/filters:proportion(10000)/example.jpg

Example request 2 (/unsafe/)

If /unsafe/ is enabled:

http://<host>:<port>/unsafe/100x100/filters:proportion(10000)/example.jpg

Impact

  • Remote Denial of Service via CPU and/or memory exhaustion (and potentially process crash / OOM kill).
  • Exploitability depends on deployment:
    • If /unsafe/ is enabled: unauthenticated remote DoS.
    • If /unsafe/ is disabled: the attacker needs a valid signed URL (i.e., the attacker can legitimately request signed URLs, or has access to signed URLs issued for other users/partners). If signed URLs are not exposed to untrusted parties, exploitability is reduced but the risk still applies to any party who can generate/use signed URLs.

Suggested remediation

  • Enforce a strict bound on the proportion parameter (e.g., 0.0 < value <= 1.0 as documented), or define a safe maximum based on intended semantics.

Affected Packages

1 total 1 fixed
EcosystemPackageVulnerable rangeFix
🐍PyPIthumborall versions7.8.0pip install --upgrade 'thumbor==7.8.0'

Detection & mitigation playbook

Open-source dependency

  1. Detect

    Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for thumbor, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

  2. Fix

    Update thumbor to 7.8.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-53505 is resolved across your whole dependency graph.

  3. Workarounds

    Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.

Frequently Asked Questions

### 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 `new_width = source_width * value` and `new_height = source_height * value` and then calls `engine.resize(new_width, new_height)`. - Execution phase: `proportion` runs in the default P
O3 Security · Impact-Aware SCA

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CVE-2026-53505: thumbor DoS — Fixed in 7.8.0