PYSEC-2026-3846

Summary

When decoding a compressed response body (gzip, deflate, br, or zstd), HTTPX2 fully decompressed each network read before yielding content to the application. A small compressed input could therefore cause a large intermediate memory allocation, even when the application streamed the response to keep memory usage bounded.

Details

HTTPX2's default transport reads the socket in pieces of up to 64 KiB. Before 2.12.0, each piece was inflated completely into one intermediate allocation before any decompressed bytes were yielded.

At DEFLATE's maximum compression ratio of roughly 1032:1, a 64 KiB compressed chunk can expand to about 64 MiB in one allocation. Brotli and Zstandard responses can cause similarly large amplification. Streaming the response did not prevent these transient allocations.

Impact

Applications that fetch resources from untrusted or attacker-influenced servers - such as webhook receivers, link unfurlers, crawlers, SSRF-reachable fetchers, and redirect followers - can experience memory pressure or out-of-memory termination when processing a malicious compressed response. No authentication or user interaction is required beyond issuing a request to the server.

Mitigation

Upgrade to HTTPX2 2.12.0 or later. Patched versions decompress responses incrementally with bounded intermediate buffers, including responses with multiple content encodings.