{"id":"PYSEC-2026-3619","summary":"Thumbor convolution filter allows divide-by-zero in C extension leading to remote DoS","details":"### Summary\nThumbor's `filters:convolution(\u003cmatrix\u003e, \u003ccolumns\u003e, \u003cshould_normalize\u003e)` filter passes the user-controlled `\u003ccolumns\u003e` value to a C extension (`thumbor/ext/filters/_convolution.c`) where it is used as a divisor (for `%` and `/`) without validating `columns \u003e 0`. When `columns=0`, the C code triggers undefined behavior; on x86_64 this reliably results in a fatal divide-by-zero trap (SIGFPE) and crashes the Thumbor process (confirmed on Linux x86_64 and macOS Intel x86_64), causing a remote denial of service.\n\n### Details\n#### Root cause\nThe Python filter accepts `columns=0`, and the native C extension uses `columns_count` as a divisor without validating it.\n\n1) Python filter entry point allows `columns=0` (`thumbor/filters/convolution.py`):\n```python\n@filter_method(\n    r\"(?:[-]?[\\d]+\\.?[\\d]*[;])*(?:[-]?[\\d]+\\.?[\\d]*)\",\n    BaseFilter.PositiveNumber,  # accepts 0\n    BaseFilter.Boolean,\n)\nasync def convolution(self, matrix, columns, should_normalize=True):\n    ...\n    imgdata = _convolution.apply(..., matrix, columns, should_normalize)\n```\n`BaseFilter.PositiveNumber` matches `\"0\"` (`thumbor/filters/__init__.py`):\n```python\nclass BaseFilter:\n    PositiveNumber = {\"regex\": r\"[\\d]+\", \"parse\": int}  # matches \"0\"\n    PositiveNonZeroNumber = {\"regex\": r\"[\\d]*[1-9][\\d]*\", \"parse\": int}\n```\n\n2) C extension divides/modulos by `columns_count` without a zero check (`thumbor/ext/filters/_convolution.c`):\n```c\nkernel_size = PyTuple_Size(kernel_tuple);\nif ((kernel_size % columns_count != 0) ||\n    (kernel_size % 2 == 0) ||\n    ((kernel_size / columns_count) % 2) == 0) {\n    // TODO: error, not a valid kernel\n    return NULL;\n}\n```\n\n\n### PoC\n#### Test environment\n- Linux x86_64\n\n#### Preconditions\n- The `convolution` filter is enabled (it is enabled by default via `BUILTIN_FILTERS`).\n- Either:\n  - `/unsafe/` URLs are allowed (`ALLOW_UNSAFE_URL=True`), OR\n  - `/unsafe/` is disabled, and the attacker has a valid signed URL (i.e., the attacker is an authorized user/partner, or can obtain signed URLs from a trusted signing service).\n\n#### Example request (signed URL)\n`http://\u003chost\u003e:\u003cport\u003e/\u003curl-sign\u003e/400x400/filters:convolution(1;2;1;2;4;2;1;2;1,0,true)/example.jpg`\n\n#### Example request (/unsafe/)\n`http://\u003chost\u003e:\u003cport\u003e/unsafe/400x400/filters:convolution(1;2;1;2;4;2;1;2;1,0,true)/example.jpg`\n\n\n### Impact\n- Remote Denial of Service via process crash (SIGFPE) on x86_64 (confirmed on Linux x86_64 and macOS Intel x86_64).\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 is an authorized user/partner, or can obtain signed URLs from a trusted signing service)\n\n\n### Suggested remediation\n- In the C extension (`thumbor/ext/filters/_convolution.c`):\n  - Reject `columns_count \u003c= 0` before any `%` or `/`.\n- In the Python filter (`thumbor/filters/convolution.py`):\n  - Require `columns` to be non-zero (e.g., use `BaseFilter.PositiveNonZeroNumber`).","aliases":["CVE-2026-53503","GHSA-cqjp-jf4r-h5q9"],"modified":"2026-08-04T14:30:17.900502764Z","published":"2026-08-04T11:34:46.338544Z","references":[{"type":"WEB","url":"https://github.com/thumbor/thumbor/security/advisories/GHSA-cqjp-jf4r-h5q9"},{"type":"WEB","url":"https://github.com/thumbor/thumbor/commit/447e192e3fb92e64c12e5354a56a4a7133f69d73"},{"type":"PACKAGE","url":"https://github.com/thumbor/thumbor"},{"type":"WEB","url":"https://github.com/thumbor/thumbor/releases/tag/7.8.0"},{"type":"PACKAGE","url":"https://pypi.org/project/thumbor"},{"type":"ADVISORY","url":"https://github.com/advisories/GHSA-cqjp-jf4r-h5q9"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-53503"}],"affected":[{"package":{"name":"thumbor","ecosystem":"PyPI","purl":"pkg:pypi/thumbor"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"7.8.0"}]}],"versions":["4.1.3","4.10.0","4.10.1","4.10.2","4.10.3","4.11.0","4.11.1","4.12.0","4.12.1","4.12.2","4.4.1","4.5.3","4.5.4","4.6.0","4.7.0","4.7.1","4.8.0","4.8.1","4.8.2","4.8.3","4.8.4","4.8.5","4.8.6","4.9.0","4.9.1","5.0.0","5.0.0rc1","5.0.0rc2","5.0.1","5.0.2","5.0.3","5.0.4","5.0.5","5.0.6","5.1.0","5.2.0","5.2.1","6.0.0","6.0.0b1","6.0.0b2","6.0.0b3","6.0.0b4","6.0.0b5","6.0.1","6.0.2","6.1.0","6.1.1","6.1.2","6.1.3","6.1.4","6.1.5","6.2.0","6.2.1","6.3.0","6.3.1","6.3.2","6.4.0","6.4.1","6.4.2","6.4.3","6.5.0","6.5.1","6.5.2","6.6.0","6.6.1","6.7.0","6.7.1","6.7.2","6.7.3","6.7.4","6.7.5","6.7.6","7.0.0","7.0.0a1","7.0.0a2","7.0.0a3","7.0.0a4","7.0.0a5","7.0.0b1","7.0.1","7.0.10","7.0.11","7.0.12","7.0.2","7.0.3","7.0.5","7.0.6","7.0.7","7.0.8","7.0.9","7.1.0","7.1.1","7.1.2","7.2.0","7.2.1","7.3.0","7.3.1","7.3.2","7.4.0","7.4.1","7.4.2","7.4.3","7.4.4","7.4.5","7.4.6","7.4.7","7.5.0","7.5.1","7.5.2","7.6.0","7.7.0","7.7.1","7.7.2","7.7.3","7.7.4","7.7.5","7.7.6","7.7.7"],"database_specific":{"source":"https://github.com/pypa/advisory-database/blob/main/vulns/thumbor/PYSEC-2026-3619.yaml"}}],"schema_version":"1.8.0","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"}]}