CVE-2026-97688: urllib3: Chunked Deflate streaming can enter an infinite loop

urllib3’s streaming API is designed for the efficient handling of large HTTP responses by reading content in chunks instead of loading the entire response body into memory at once.

urllib3 can decompress response bodies according to the HTTP Content-Encoding header. When streaming a compressed, chunked response, urllib3 first consumes data already buffered by the decoder before reading the next HTTP chunk.

However, urllib3 versions from 2.6.2 through 2.7.0 could enter an infinite loop when a chunked response contained bytes after the end of the Deflate stream. If the decompressed body exceeded the requested streaming chunk size, Python’s zlib implementation could retain the trailing bytes as unconsumed input after reaching the end of the compressed stream. urllib3 would repeatedly attempt to decode those same bytes without making progress or reading more data from the network.

A malicious server could exploit this behavior to cause excessive CPU usage and prevent the affected request from completing on the client. Network read timeouts would not interrupt the loop because no further socket operation was required.