Summary
Thumbor proportion filter allows unbounded post-transform resize leading to remote DoS
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.pyvalueis parsed as a float (BaseFilter.DecimalNumber) with no maximum.- The filter computes
new_width = source_width * valueandnew_height = source_height * valueand then callsengine.resize(new_width, new_height).
- Execution phase:
proportionruns 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_HEIGHTapplied toreq.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
proportionfilter is enabled (it is enabled by default viaBUILTIN_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
Suggested remediation
- Enforce a strict bound on the
proportionparameter (e.g.,0.0 < value <= 1.0as documented), or define a safe maximum based on intended semantics.
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.
- If
Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service. Typical impact: denial of service.
CVE-2026-53505 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (7.8.0); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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Frequently Asked Questions
- What is CVE-2026-53505? CVE-2026-53505 is a high-severity uncontrolled resource consumption vulnerability in thumbor (pip), affecting versions <= 7.7.7. It is fixed in 7.8.0. Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service.
- How severe is CVE-2026-53505? CVE-2026-53505 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of thumbor are affected by CVE-2026-53505? thumbor (pip) versions <= 7.7.7 is affected.
- Is there a fix for CVE-2026-53505? Yes. CVE-2026-53505 is fixed in 7.8.0. Upgrade to this version or later.
- Is CVE-2026-53505 exploitable, and should I be worried? Whether CVE-2026-53505 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-53505 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-53505? Upgrade
thumborto 7.8.0 or later.