urllib3 streaming API improperly handles highly compressed data
Impact
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; CWE-409) on the client side, even if the application only requested a small chunk of data.
Affected usages
Applications and libraries using urllib3 version 2.5.0 and earlier to stream large compressed responses or content from untrusted sources.
stream(), read(amt=256), read1(amt=256), read_chunked(amt=256), readinto(b) are examples of urllib3.HTTPResponse method calls using the affected logic unless decoding is disabled explicitly.
Remediation
Upgrade to at least urllib3 v2.6.0 in which the library avoids decompressing data that exceeds the requested amount.
If your environment contains a package facilitating the Brotli encoding, upgrade to at least Brotli 1.2.0 or brotlicffi 1.2.0.0 too. These versions are enforced by the urllib3[brotli] extra in the patched versions of urllib3.
Credits
The issue was reported by @Cycloctane. Supplemental information was provided by @stamparm during a security audit performed by 7ASecurity and facilitated by OSTIF.
Affected software
Click a technology to see all vulnerabilities affecting it.
| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| Python (PyPI) | urllib3 | >= 1.0, < 2.6.0 | 2.6.0 |
References
- https://github.com/urllib3/urllib3/security/advisories/GHSA-2xpw-w6gg-jr37
- https://nvd.nist.gov/vuln/detail/CVE-2025-66471
- https://github.com/urllib3/urllib3/commit/c19571de34c47de3a766541b041637ba5f716ed7
- https://github.com/urllib3/urllib3
- https://pypi.org/project/urllib3
- https://github.com/advisories/GHSA-2xpw-w6gg-jr37
- https://github.com/urllib3/urllib3/commit/d0fde3672e4a4093f7587858dccfc298eb66e46c
- https://github.com/pypa/advisory-database/tree/main/vulns/urllib3/PYSEC-2026-1994.yaml
- CVSS vector
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- Weakness (CWE)
CWE-409- Platform
- Python (PyPI)
- Package
urllib3- Disclosed
- 05.12.2025
- Updated
- 11.10.2026 15:18
Sources and licenses
OSV / GitHub Advisory Database - PYSEC-2026-1994 - PyPA Advisory Database; CC BY 4.0
OSV / GitHub Advisory Database - GHSA-2xpw-w6gg-jr37 - CC BY 4.0
Vulnerability history: urllib3
All published vulnerabilities of this package (Python (PyPI)), newest first. Shows whether this is a one-off slip or a package that needs watching.
- Total entries
- 22
- First → latest
- 01.2017 09.2026
- Without a fix
- 0
- Exploited in attacks
- 0
Severity breakdown
- Critical 1
- High 11
- Medium 10
Disclosures per year
- 2017: 1
- 2018: 1
- 2019: 2
- 2020: 2
- 2021: 2
- 2022: 0
- 2023: 3
- 2024: 1
- 2025: 4
- 2026: 6
-
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