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CVE-2026-21441— urllib3 vulnerable to decompression-bomb safeguard bypass when following HTTP redirects (streaming API)

Quick assessment

Affected
urllib3 urllib3
Exploitation
Public or AI PoC available; prioritize validation
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

urllib3是urllib3开源的一款Python HTTP库。该产品具有线程安全连接池、文件发布支持等。 urllib3 2.6.3之前版本存在安全漏洞，该漏洞源于处理HTTP重定向响应时未限制解压缩数据量，可能导致资源消耗过多。

CVSS 8.9 · High EPSS 2.92% · P87

Possible ATT&CK Techniques 1 AI

T1496 · Resource Hijacking

Affected Version Matrix 1

VendorProduct Version RangeStatus
urllib3 urllib3 >= 1.22, < 2.6.3 affected
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I. Basic Information for CVE-2026-21441

Vulnerability Information
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Vulnerability Title
urllib3 vulnerable to decompression-bomb safeguard bypass when following HTTP redirects (streaming API)
Source: CVE Program / CVE List V5
Vulnerability Description
urllib3 is an HTTP client library for Python. 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. Starting in version 1.22 and prior to version 2.6.3, 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. Applications and libraries are affected when they stream content from untrusted sources by setting `preload_content=False` when they do not disable redirects. Users should 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.
Source: CVE Program / CVE List V5
CVSS Information
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
Source: CVE Program / CVE List V5
Vulnerability Type
对高度压缩数据的处理不恰当(数据放大攻击)
Source: CVE Program / CVE List V5
Vulnerability Title
urllib3 安全漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
urllib3是urllib3开源的一款Python HTTP库。该产品具有线程安全连接池、文件发布支持等。 urllib3 2.6.3之前版本存在安全漏洞，该漏洞源于处理HTTP重定向响应时未限制解压缩数据量，可能导致资源消耗过多。
Source: CNNVD (China National Vulnerability Database)
CVSS Information
N/A
Source: CNNVD (China National Vulnerability Database)
Vulnerability Type
N/A
Source: CNNVD (China National Vulnerability Database)

Affected Products

Vendor Product Affected Versions CPE Subscribe
urllib3 urllib3 >= 1.22, < 2.6.3 -

II. Public POCs for CVE-2026-21441

# POC Description Source Link Shenlong Link
AI-Generated POC Verified env Premium
Qwen3.6-35B-A3B · 10123 chars
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Vulnerability reproduction recording (real sandbox build + trigger, exclusive)
In-depth vulnerability mechanism
Trigger conditions & impact
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III. Intelligence Information for CVE-2026-21441

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Patches & Fixes for CVE-2026-21441 (1)

Vendor Advisories for CVE-2026-21441 (1)

IV. Related Vulnerabilities

Same product: urllib3
Same vendor: urllib3
Same weakness: CWE-409

V. Comments for CVE-2026-21441

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