CVE-2026-21441: Urllib3 Decompression-bomb Safeguards Bypassed When Following Vulnerability

HIGH
Advisory Identifier: GHSA-38jv-5279-wg99
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CVSS 7.5
CWE-409
Published
Advisory Summary

CVE-2026-21441 is a HIGH (CVSS 7.5) decompression-bomb safeguards bypassed when following vulnerability (CWE-409: Improper Handling of Highly Compressed Data (Data Amplification)) affecting Urllib3 libraries in the PyPI ecosystem. The issue involves decompression-bomb safeguards bypassed when following http redirects (streaming api). Remediation is available by upgrading affected packages to version 2.6.3.

CVE-2026-21441 at a Glance

CVE Identifier CVE-2026-21441
GHSA Identifier GHSA-38jv-5279-wg99
Severity HIGH
CVSS v3.1 Base Score 7.5
CWE Classification CWE-409: Improper Handling of Highly Compressed Data (Data Amplification)
Affected Product Urllib3
Package Ecosystem PyPI
Published Date
Last Updated

Security Impact

CVSS: 7.5

The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H).

Attack Vector Network (AV:N)
Attack Complexity Low (AC:L)
Privileges Required None (PR:N)
User Interaction None (UI:N)
Scope Unchanged (S:U)
Confidentiality None (C:N)
Integrity None (I:N)
Availability High (A:H)

How to Fix CVE-2026-21441

To remediate this vulnerability, upgrade all affected Urllib3 dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

urllib3 fixed: 2.6.3 
pip install urllib3>=2.6.3
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

pip show urllib3

How to Check if You Are Affected

To determine whether your Python environment is affected by CVE-2026-21441, inspect your installed packages using pip:

pip show urllib3

Analyze the terminal output to locate any version of Urllib3 in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 2.6.3), your application is affected.

Note: The pip show urllib3 command checks currently installed packages in your active Python environment.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-409: Improper Handling of Highly Compressed Data (Data Amplification), where the product does not handle or incorrectly handles a compressed input with a very high compression ratio that produces a large output. In Urllib3, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

### Impact urllib3's [streaming API](https://urllib3.readthedocs.io/en/2.6.2/advanced-usage.html#streaming-and-i-o) 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](https://urllib3.readthedocs.io/en/2.6.2/user-guide.html#retrying-requests) by setting `redirect=False` for requests to untrusted source.

CVE-2026-21441 Affected Urllib3 Versions

1 component(s)
Fixed Versions:
2.6.3
Product / Package Ecosystem Affected Versions Fixed Version
urllib3
pypi/urllib3
PyPI
≥ 1.22
< 2.6.3
2.6.3

References

GitHub Advisory / Wiki ADVISORY
https://github.com/urllib3/urllib3/security/advisories/GHSA-38jv-5279-wg99
NVD — CVE-2026-21441 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-21441
Security Fix Commit FIX COMMIT
https://github.com/urllib3/urllib3/commit/8864ac407bba8607950025e0979c4c69bc7abc7b
Urllib3 Source Repository REPOSITORY
https://github.com/urllib3/urllib3
lists.debian.org/debian-lts-announce... WEB
https://lists.debian.org/debian-lts-announce/2026/01/msg00017.html

Frequently Asked Questions

What is CVE-2026-21441?
CVE-2026-21441 is a security vulnerability in Urllib3 involving Decompression-bomb safeguards bypassed when following HTTP redirects (streaming API) under CWE-409: Improper Handling of Highly Compressed Data (Data Amplification).
Which Urllib3 versions are affected?
Affected versions include releases prior to 2.6.3 across affected packages: urllib3.
What version fixes CVE-2026-21441?
Upgrading to 2.6.3 (or the latest patched release for your respective branch) resolves the vulnerability.
Is urllib3 affected?
Yes, urllib3 is affected in releases prior to 2.6.3. Upgrade your dependency to 2.6.3 or later.
What is the CVSS score of CVE-2026-21441?
CVE-2026-21441 has a CVSS v3.1 base score of 7.5 (HIGH) with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H.
What CWE is associated with CVE-2026-21441?
CVE-2026-21441 is classified under 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.).
How can I check PyPI dependencies for CVE-2026-21441?
Run the dependency tree command: 'pip show urllib3'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-38jv-5279-wg99",
  "modified" : "2026-01-23T16:05:21Z",
  "published" : "2026-01-07T19:18:14Z",
  "aliases" : [ "CVE-2026-21441" ],
  "summary" : "Decompression-bomb safeguards bypassed when following HTTP redirects (streaming API)",
  "details" : "### Impact\n\nurllib3's [streaming API](https://urllib3.readthedocs.io/en/2.6.2/advanced-usage.html#streaming-and-i-o) 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.\n\nurllib3 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.\n\nHowever, 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).\n\n### Affected usages\n\nApplications 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.\n\n\n### Remediation\n\nUpgrade to at least urllib3 v2.6.3 in which the library does not decode content of redirect responses when `preload_content=False`.\n\nIf upgrading is not immediately possible, disable [redirects](https://urllib3.readthedocs.io/en/2.6.2/user-guide.html#retrying-requests) by setting `redirect=False` for requests to untrusted source.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"
  }, {
    "type" : "CVSS_V4",
    "score" : "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"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "urllib3"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "1.22"
      }, {
        "fixed" : "2.6.3"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/urllib3/urllib3/security/advisories/GHSA-38jv-5279-wg99"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-21441"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/urllib3/urllib3/commit/8864ac407bba8607950025e0979c4c69bc7abc7b"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/urllib3/urllib3"
  }, {
    "type" : "WEB",
    "url" : "https://lists.debian.org/debian-lts-announce/2026/01/msg00017.html"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-409" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-01-07T19:18:14Z",
    "nvd_published_at" : "2026-01-07T22:15:44Z"
  }
}