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.
Betroffene Software
Klicken Sie auf eine Technologie, um alle zugehörigen Schwachstellen anzuzeigen.
| Plattform | Paket | Anfällige Versionen | Behoben in |
|---|---|---|---|
| Python (PyPI) | urllib3 | >= 1.0, < 2.6.0 | 2.6.0 |
Verweise
- 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-Vektor
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- Schwäche (CWE)
CWE-409- Plattform
- Python (PyPI)
- Paket
urllib3- Veröffentlicht
- 05.12.2025
- Aktualisiert
- 11.10.2026 15:18
Quellen und Lizenzen
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
Schwachstellenhistorie: urllib3
Alle veröffentlichten Schwachstellen dieses Pakets (Python (PyPI)), neueste zuerst. Zeigt, ob es ein einmaliger Ausrutscher ist oder ein Paket, das man im Auge behalten muss.
- Einträge insgesamt
- 22
- Erste → letzte
- 01.2017 09.2026
- Ohne Fix
- 0
- In Angriffen ausgenutzt
- 0
Verteilung nach Schweregrad
- Kritisch 1
- Hoch 11
- Mittel 10
Veröffentlichungen pro Jahr
- 2017: 1
- 2018: 1
- 2019: 2
- 2020: 2
- 2021: 2
- 2022: 0
- 2023: 3
- 2024: 1
- 2025: 4
- 2026: 6
-
urllib3 streaming API improperly handles highly compressed data dieser Eintrag
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