Vulnerability in Python Urllib3
CVE-2025-66471
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.
Published · last modified .
CVSS v3 metric
CVSS v3 base score 7.5 (High). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H.
EPSS exploit prediction
EPSS: 0.007 (51.5th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 52nd percentile — 51.5% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.007 · newest: 0.007 · change: 0.000
Affected products
- Python Urllib3
- Urllib3 — versions >= 1.0, < 2.6.0
Weakness classification (CWE)
References
Frequently asked questions
- What is CVE-2025-66471?
- CVE-2025-66471 is a high-severity vulnerability in Python Urllib3, classified under Improper Handling of Highly Compressed Data (Data Amplification). CVSS score: 7.5/10. Published 2025-12-05.
- How severe is CVE-2025-66471?
- High severity. CVSS v3 base score is 7.5 out of 10.