urllib3: Decompression-bomb safeguards bypassed in parts of the 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 decompression based on the HTTP Content-Encoding header (e.g., gzip, deflate, br, or zstd). When using the streaming API since version 2.6.0, the library decompresses only the necessary bytes, enabling partial content consumption.
However, urllib3 before version 2.7.0 could still decompress the whole response instead of the requested portion in two cases:
- During the second
HTTPResponse.read(amt=N)call when the response was decompressed using the official Brotli library. - When
HTTPResponse.drain_conn()was called after the response had been read and decompressed partially (compression algorithm did not matter here).
These issues could cause urllib3 to fully decode a small amount of highly compressed data in a single operation. This could result in excessive resource consumption (high CPU usage and massive memory allocation for the decompressed data; CWE-409) on the client side.
Affected usages
Applications and libraries using urllib3 versions earlier than 2.7.0 may be affected when streaming compressed responses from untrusted sources in either of these cases, unless decompression is explicitly disabled:
- A response encoded with
bris read incrementally with at least twoHTTPResponse.read(amt=N)orHTTPResponse.stream(amt=N)calls while using the official Brotli library. HTTPResponse.drain_conn()is called after response decompression has already started.
Remediation
Upgrade to at least urllib3 version 2.7.0 in which the library:
- Is more efficient for reads with Brotli.
- Always skips decompression for
HTTPResponse.drain_conn().
If upgrading is not immediately possible, the following workarounds may reduce exposure in specific cases:
- For the Brotli-specific issue only, switch from brotli to brotlicffi until you can upgrade urllib3; the official Brotli package is affected because of https://github.com/google/brotli/issues/1396.
- If your code explicitly calls
HTTPResponse.drain_conn(), callHTTPResponse.close()instead when connection reuse is not important.
Credits
The Brotli-specific issue was reported by @kimkou2024.
HTTPResponse.drain_conn() inefficiency was reported by @Cycloctane.
Affected software
Click a technology to see all vulnerabilities affecting it.
| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| Python (PyPI) | urllib3 | >= 2.6.0, < 2.7.0 | 2.7.0 |
References
- 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
- 11.05.2026
- Updated
- 11.10.2026 15:17
Sources and licenses
OSV / GitHub Advisory Database - PYSEC-2026-142 - PyPA Advisory Database; CC BY 4.0
OSV / GitHub Advisory Database - GHSA-mf9v-mfxr-j63j - 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
-
urllib3: Decompression-bomb safeguards bypassed in parts of the streaming API this entry
Know which vulnerabilities affect your application
With the DockRay integration and software inventory enabled, DockRay checks reported component versions against known vulnerabilities and notifies you about confirmed matches. Coverage depends on the supported technology and available advisory data.