Out-of-bounds Write (HTTP2Shell) Affecting brotli package, versions [,1.2.1)
Threat Intelligence
This vulnerability is trending on Twitter; this may indicate a growing threat.
Snyk has reported that there have been attempts or successful attacks targeting this vulnerability.
The probability is the direct output of the EPSS model, and conveys an overall sense of the threat of exploitation in the wild. The percentile measures the EPSS probability relative to all known EPSS scores. Note: This data is updated daily, relying on the latest available EPSS model version. Check out the EPSS documentation for more details.
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Test your applications- Snyk IDSNYK-PYTHON-BROTLI-21717207
- published26 Oct 2026
- disclosed21 Oct 2026
- creditmcollina
Introduced: 21 Oct 2026
NewCVE-2026-111497 Â (opens in a new tab)How to fix?
Upgrade brotli to version 1.2.1 or higher.
Overview
brotli is a Brotli is a generic-purpose lossless compression algorithm that compresses data using a combination of a modern variant of the LZ77 algorithm, Huffman coding and 2nd order context modeling, with a compression ratio comparable to the best currently available general-purpose compression methods. It is similar in speed with deflate but offers more dense compression.
Affected versions of this package are vulnerable to Out-of-bounds Write (HTTP2Shell) via block-switch handling in the Brotli decoder, specifically DecodeBlockTypeAndLength in deps/brotli/c/dec/decode.c. An attacker can execute arbitrary code by supplying a crafted Brotli-compressed stream with an invalid block switch to the code that decodes it; the decoder fails to propagate the block-switch error during command processing, allowing the out-of-bounds write.
Notes
- At a literal, command, or distance block-switch, the vulnerable path is reached when that stream’s block-type count is at most one (max_block_type <= 1); the decoder’s boolean failure is then discarded by the unsafe switch path, which continues processing.