{"id":"CVE-2026-53505","aliases":["GHSA-phj3-59pf-cp83","PYSEC-2026-3621"],"url":"https://o3.security/vulnerability/CVE-2026-53505","summary":"Thumbor proportion filter allows unbounded post-transform resize leading to remote DoS","details":"### Summary\nThumbor'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.\n\n### Details\n- Filter implementation: `thumbor/filters/proportion.py`\n  - `value` is parsed as a float (`BaseFilter.DecimalNumber`) with no maximum.\n  - The filter computes `new_width = source_width * value` and `new_height = source_height * value` and then calls `engine.resize(new_width, new_height)`.\n- 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`).\n\nDocumentation states the `percentage` argument should be `0.0 to 1.0` (`docs/proportion.rst`), but the implementation does not enforce this constraint.\n\n### PoC\n#### Preconditions\n- The `proportion` filter is enabled (it is enabled by default via `BUILTIN_FILTERS`).\n- Either:\n  - `/unsafe/` URLs are allowed (`ALLOW_UNSAFE_URL=True`, common default in some deployments), OR\n  - `/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).\n\n#### Example request 1 (signed URL)\nThe following request was used to reproduce the issue and causes severe resource exhaustion:\n\n`http://<host>:<port>/<url-sign>/100x100/filters:proportion(10000)/example.jpg`\n\n#### Example request 2 (/unsafe/)\nIf `/unsafe/` is enabled:\n\n`http://<host>:<port>/unsafe/100x100/filters:proportion(10000)/example.jpg`\n\n### Impact\n- Remote Denial of Service via CPU and/or memory exhaustion (and potentially process crash / OOM kill).\n- Exploitability depends on deployment:\n  - If `/unsafe/` is enabled: unauthenticated remote DoS.\n  - 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.\n\n### Suggested remediation\n- 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.","published":"2026-07-31T19:00:22.755Z","modified":"2026-10-28T02:31:14.926233948Z","cvss":{"score":7.5,"severity":"HIGH","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"},"epss":{"score":0.00335,"percentile":0.26763,"asOf":"2026-09-17"},"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"PyPI","name":"thumbor","fixedVersion":"7.8.0"}],"fix":{"url":"https://github.com/thumbor/thumbor/commit/2c716119de986cfc68c7071af52a98187e006023","label":"thumbor/thumbor@2c71611"},"references":[{"type":"WEB","url":"https://github.com/thumbor/thumbor/releases/tag/7.8.0"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/53xxx/CVE-2026-53505.json"},{"type":"ADVISORY","url":"https://github.com/thumbor/thumbor/security/advisories/GHSA-phj3-59pf-cp83"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-53505"},{"type":"FIX","url":"https://github.com/thumbor/thumbor/commit/2c716119de986cfc68c7071af52a98187e006023"},{"type":"PACKAGE","url":"https://github.com/thumbor/thumbor"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-10-28T02:31:14.926233948Z"}}