Decompression-bomb safeguards bypassed when following HTTP redirects (streaming API)
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.
urllib3 can perform decoding or decompression based on the HTTP Content-Encoding header (e.g., gzip, deflate, br, or zstd). When using the streaming API, the library decompresses only the necessary bytes, enabling partial content consumption.
However, for HTTP redirect responses, the library would read the entire response body to drain the connection and decompress the content unnecessarily. This decompression occurred even before any read methods were called, and configured read limits did not restrict the amount of decompressed data. As a result, there was no safeguard against decompression bombs. A malicious server could exploit this to trigger excessive resource consumption on the client (high CPU usage and large memory allocations for decompressed data; CWE-409).
Affected usages
Applications and libraries using urllib3 version 2.6.2 and earlier to stream content from untrusted sources by setting preload_content=False when they do not disable redirects.
Remediation
Upgrade to at least urllib3 v2.6.3 in which the library does not decode content of redirect responses when preload_content=False.
If upgrading is not immediately possible, disable redirects by setting redirect=False for requests to untrusted source.
Affected software
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| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| Python (PyPI) | urllib3 | >= 1.22, < 2.6.3 | 2.6.3 |
References
- https://github.com/urllib3/urllib3/security/advisories/GHSA-38jv-5279-wg99
- https://nvd.nist.gov/vuln/detail/CVE-2026-21441
- https://github.com/urllib3/urllib3/commit/8864ac407bba8607950025e0979c4c69bc7abc7b
- https://github.com/urllib3/urllib3
- https://lists.debian.org/debian-lts-announce/2026/01/msg00017.html
- https://pypi.org/project/urllib3
- https://github.com/advisories/GHSA-38jv-5279-wg99
- CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H- Weakness (CWE)
CWE-409- Platform
- Python (PyPI)
- Package
urllib3- Disclosed
- 07.01.2026
- Updated
- 11.10.2026 15:17
Sources and licenses
OSV / GitHub Advisory Database - PYSEC-2026-1996 - PyPA Advisory Database; CC BY 4.0
OSV / GitHub Advisory Database - GHSA-38jv-5279-wg99 - 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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Decompression-bomb safeguards bypassed when following HTTP redirects (streaming API) this entry
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