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CVE-2026-49854 — Tornado: Out-of-bounds memory access in C extension

Affected: tornadoweb tornado

Tornado is a Python web framework and asynchronous networking library. Prior to 6.5.6, the optional native extension tornado.speedups implemented websocket_mask without validating that the mask argument is exactly four bytes, allowing the C function to read up to three bytes beyond the provided buffer when reached through Tornado XSRF token decoding with the native extension active. This issue is fixed in version 6.5.6.

GHSA-cx3h-4qpv-8hc9 — Tornado has out-of-bounds memory access via C extension

Affected: PyPI: tornado

### Summary

Tornado's optional native extension `tornado.speedups` implements `websocket_mask` without validating that the `mask` argument is exactly four bytes long. The C function reads four bytes from `mask` unconditionally, even when Python passes a shorter byte string. This can read beyond the provided buffer, exposing up to 3 bytes of uninitialized memory.

The behavior is reachable from Tornado's XSRF token decoder when `xsrf_cookies=True` and the native extension is active.

### Mitigations

This bug is fixed in Tornado 6.5.6. Prior to upgrading to this version, setting the environment variable TORNADO_EXTENSION=0 will disable the vulnerable code (at the expense of reducing websocket performance).

PYSEC-2026-3388 — Tornado has out-of-bounds memory access via C extension

Affected: PyPI: tornado

### Summary

The behavior is reachable from Tornado's XSRF token decoder when `xsrf_cookies=True` and the native extension is active.

### Mitigations