CVE-2026-53505: Thumbor Denial of Service Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-phj3-59pf-cp83",
  "modified" : "2026-07-31T19:00:44Z",
  "published" : "2026-07-31T19:00:44Z",
  "aliases" : [ "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.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "thumbor"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "7.8.0"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 7.7.7"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/thumbor/thumbor/security/advisories/GHSA-phj3-59pf-cp83"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/thumbor/thumbor/commit/2c716119de986cfc68c7071af52a98187e006023"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/thumbor/thumbor"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/thumbor/thumbor/releases/tag/7.8.0"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-400" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-07-31T19:00:44Z",
    "nvd_published_at" : null
  }
}