### Summary Thumbor's `filters:convolution(<matrix>, <columns>, <should_normalize>)` filter passes the user-controlled `<columns>` value to a C extension (`thumbor/ext/filters/_convolution.c`) where it is used as a divisor (for `%` and `/`) without validating `columns > 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. ### Details #### Root cause The Python filter accepts `columns=0`, and the native C extension uses `columns_count` as a divisor without validating it. 1) Python filter entry point allows `columns=0` (`thumbor/filters/convolution.py`): ```python @filter_method( r"(?:[-]?[\d]+\.?[\d]*[;])*(?:[-]?[\d]+\.?[\d]*)", BaseFilter.PositiveNumber, # accepts 0 BaseFilter.Boolean, ) async def convolution(self, matrix, columns, should_normalize=True): ... imgdata = _convolution.apply(..., matrix, columns, should_normalize) ``` `BaseFilter.PositiveNumber` matches `"0"` (`thumbor/filters/__init__.py`): ```python class BaseFilter: PositiveNumber = {"regex": r"[\d]+", "parse": int} # matches "0" PositiveNonZeroNumber = {"regex": r"[\d]*[1-9][\d]*", "parse": int} ``` 2) C extension divides/modulos by `columns_count` without a zero check (`thumbor/ext/filters/_convolution.c`): ```c kernel_size = PyTuple_Size(kernel_tuple); if ((kernel_size % columns_count != 0) || (kernel_size % 2 == 0) || ((kernel_size / columns_count) % 2) == 0) { // TODO: error, not a valid kernel return NULL; } ``` ### PoC #### Test environment - Linux x86_64 #### Preconditions - The `convolution` filter is enabled (it is enabled by default via `BUILTIN_FILTERS`). - Either: - `/unsafe/` URLs are allowed (`ALLOW_UNSAFE_URL=True`), OR - `/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). #### Example request (signed URL) `http://<host>:<port>/<url-sign>/400x400/filters:convolution(1;2;1;2;4;2;1;2;1,0,true)/example.jpg` #### Example request (/unsafe/) `http://<host>:<port>/unsafe/400x400/filters:convolution(1;2;1;2;4;2;1;2;1,0,true)/example.jpg` ### Impact - Remote Denial of Service via process crash (SIGFPE) on x86_64 (confirmed on Linux x86_64 and macOS Intel x86_64). - 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 is an authorized user/partner, or can obtain signed URLs from a trusted signing service) ### Suggested remediation - In the C extension (`thumbor/ext/filters/_convolution.c`): - Reject `columns_count <= 0` before any `%` or `/`. - In the Python filter (`thumbor/filters/convolution.py`): - Require `columns` to be non-zero (e.g., use `BaseFilter.PositiveNonZeroNumber`).

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