GHSA-mf9v-mfxr-j63j CVE-2026-44432 PYSEC-2026-142

urllib3: Decompression-bomb safeguards bypassed in parts of the streaming API

High PyPI

Severity: 8.9 / 10; High · CVSS 4.0
Exploit chance (EPSS): 0.68%; Chance of exploitation in the next 30 days. Higher than 52.40% of scored vulnerabilities.
Known exploited: No; Not in CISA's catalog
Weakness: CWE-409 Improper Handling of Highly Compressed Data (Data Amplification)
Fix: 2.7.0; Upgrade urllib3 to this version or later

Description

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 br is read incrementally with at least two HTTPResponse.read(amt=N) or HTTPResponse.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(), call HTTPResponse.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 versions

PyPI

Package	Vulnerable	Fixed in
urllib3	`>= 2.6.0, < 2.7.0`	`2.7.0`
urllib3	`>= 2.6.0, < 2.7.0`	`2.7.0`

How to fix

Move to the fixed version or later with your package manager, update the lockfile, and run your tests. If the package only arrives through another dependency, upgrade that dependency first. If none of its releases pulls a fixed version yet, force the version with your package manager's override.

urllib3 PyPI → `2.7.0`

pip

pip install --upgrade "urllib3>=2.7.0"

poetry

poetry add "urllib3>=2.7.0"

uv

uv add "urllib3>=2.7.0"

Severity breakdown

CVSS 4.0 8.9

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

Attack vector: Network
Attack complexity: Low
Attack requirements: Present
Privileges required: None
User interaction: None
Confidentiality (vulnerable system): None
Integrity (vulnerable system): None
Availability (vulnerable system): High
Confidentiality (subsequent systems): None
Integrity (subsequent systems): None
Availability (subsequent systems): High

CVSS 3.1 7.5

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Attack vector: Network
Attack complexity: Low
Privileges required: None
User interaction: None
Scope: Unchanged
Confidentiality: None
Integrity: None
Availability: High

Weakness

CWE-409: Improper Handling of Highly Compressed Data (Data Amplification)

The product does not handle or incorrectly handles a compressed input with a very high compression ratio that produces a large output.

References

Advisories

Packages

github.com/urllib3/urllib3

Timeline

May 11, 2026 Published
Sep 10, 2026 Last updated by the source
Nov 1, 2026 Latest EPSS score

Advisory	Severity	Why it's related
GHSA-vxq7-64xx-v4gw urllib3: HTTPResponse.stream()/read_chunked() buffers an unbounded chunk-size line into memory	High	Also affects urllib3
GHSA-gh4c-6fx4-qh6g urllib3: Chunked Deflate streaming can enter an infinite loop	Medium	Also affects urllib3
GHSA-8988-9cw3-xx77 urllib3: HTTPS proxy TLS configuration may be ignored or overridden	High	Also affects urllib3
GHSA-qccp-gfcp-xxvc urllib3: Sensitive headers forwarded across origins in proxied low-level redirects	High	Also affects urllib3
GHSA-38jv-5279-wg99 Decompression-bomb safeguards bypassed when following HTTP redirects (streaming API)	High	Also affects urllib3
GHSA-2xpw-w6gg-jr37 urllib3 streaming API improperly handles highly compressed data	High	Also affects urllib3
GHSA-gm62-xv2j-4w53 urllib3 allows an unbounded number of links in the decompression chain	High	Also affects urllib3
GHSA-48p4-8xcf-vxj5 urllib3 does not control redirects in browsers and Node.js	Medium	Also affects urllib3

Credits: Cycloctane, illia-v, kimkou2024, pquentin

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