PYSEC-2026-4323

CVE-2026-111497 ("Node2Shell") is an out-of-bounds write (CWE-787) in Google's Brotli decoder (c/dec/decode.c). DecodeBlockTypeAndLength() returns an ambiguous value for an invalid block switch, and the fast decode path discards it (CWE-252, unchecked return value), so a crafted Brotli stream makes the decoder continue on corrupted state and write past the ring buffer. Upstream rates it CVSS 4.0 9.2 and classifies the impact as potential remote code execution.

The vulnerability is in Google's C library, not in Node.js specifically. The PyPI Brotli package is Google's CPython binding and compiles the same c/dec/decode.c that has the original vulnerability. All Brotli releases up to and including 1.2.0 ship the vulnerable decoder; 1.2.1 contains the upstream fix (commit 0ed4268a) and differs from 1.2.0 only in that fix plus a version bump.

The decoder is reached whenever untrusted Brotli data is decompressed. With brotlicffi installed, common HTTP clients decode Brotli responses automatically (urllib3/requests, httpx with the brotli extra, aiohttp) and services that accept Content-Encoding: br request bodies decode inbound data, so the flaw is reachable from attacker-controlled input.