CVE-2025-66471: high-severity vulnerability in urllib3
urllib3 Streaming API improperly handles highly compressed data
Published
E-mail alert when it lands in KEV, gets a public PoC or its score goes up.
21Vexday Risk Score
No sign of exploitation. No public exploitation artifact known so far.
ssvc Trackcvss 8.9epss 0.7%
urllib3 is a user-friendly HTTP client library for Python. Starting in version 1.0 and prior to 2.6.0, the Streaming API improperly handles highly compressed data. urllib3's streaming API is designed for the efficient handling of large HTTP responses by reading the content in chunks, rather than loading the entire response body into memory at once. When streaming a compressed response, urllib3 can perform decoding or decompression based on the HTTP Content-Encoding header (e.g., gzip, deflate, br, or zstd). The library must read compressed data from the network and decompress it until the requested chunk size is met. Any resulting decompressed data that exceeds the requested amount is held in an internal buffer for the next read operation. The decompression logic could cause urllib3 to fully decode a small amount of highly compressed data in a single operation. This can result in excessive resource consumption (high CPU usage and massive memory allocation for the decompressed data.
CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:H
Affected products
urllib3 · urllib3Related CVEs — urllib3
In the same product, most dangerous first.
CVE-2026-21441HIGHurllib3 vulnerable to decompression-bomb safeguard bypass when following HTTP redirects (streaming API)EPSS 2.9%CVE-2023-43804MEDIUM`Cookie` HTTP header isn't stripped on cross-origin redirectsEPSS 1.4%CVE-2024-37891MEDIUMProxy-Authorization request header isn't stripped during cross-origin redirects in urllib3EPSS 1.1%CVE-2025-66418HIGHurllib3 allows an unbounded number of links in the decompression chainEPSS 0.7%CVE-2026-44432HIGHurllib3: Decompression-bomb safeguards bypassed in parts of the streaming APIEPSS 0.7%CVE-2025-50182MEDIUMurllib3 does not control redirects in browsers and Node.jsEPSS 0.6%