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CVE-2026-21441 high CVSS 7.5

Vulnerability in urllib3 (CVE-2026-21441)

Summary

Vulnerability in urllib3 (CVE-2026-21441). Risk of unauthorized operations or information disclosure. Exploitable via gzip. Mitigation: upgrade to 2.6.3 or later.

Original description

The description as published by the source.

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](https://urllib3…

Source: NIST National Vulnerability Database NVD GitHub Advisory

Auto-generated summary

Assembled by SecPulse from the source data (CVSS, CWE, affected versions) using templates — no AI involved. Check the original description and references below for exact details.

A vulnerability tracked as CVE-2026-21441 has been found in urllib3.
Attackers can target a specific entry point like gzip over the network to misuse the product.
Risk of unauthorized operations or information disclosure. CVSS score: 7.5/10.
What to do: upgrade urllib3 to 2.6.3 or later.
If unsure, ask your IT team or search "urllib3 CVE-2026-21441" on the vendor's site.

CVE-2026-21441 (urllib3) — CWE-409 / CVSS v3 7.5
Attack vector: remote (network-reachable) / unauthenticated / no user interaction
Attack surface: gzip / deflate / zstd
Patched: 2.6.3 — apply immediately
Workaround: disable [redirects](https://urllib3.
Plan: 1) Audit SBOM/dependencies, 2) Stage→prod upgrade, 3) Add WAF/proxy monitoring on affected endpoints, 4) Hunt IOCs in logs.
Refs: see the GHSA / vendor advisory / patched release linked on this page.

❓ What is the problem

A vulnerability (CWE-409) exists in urllib3. Attackers reach the vulnerable code path via gzip without authentication.

📍 Affected scope

Check the vendor advisory for the affected versions of urllib3. Attack surface: gzip / deflate / zstd.

🔥 Severity

Severity: High (CVSS 7.5/10). Risk of unauthorized operations or information disclosure.

🔧 How to fix

Update to 2.6.3.

🛡️ Workaround

Workaround: disable [redirects](https://urllib3.

🔍 Detection

Search webserver/proxy logs for unusual request patterns matching this CVE's known IOCs. Run grep -r 'urllib3' . against your dependency files (package-lock.json, requirements.txt, go.sum) to find affected services.

Response actions

Concrete steps and command examples for SOC/SRE teams to execute in order

  1. 1
    Identify exposure identify 
    grep -r 'urllib3' . | grep -v node_modules

    リポジトリと本番環境の依存ファイル (package-lock.json / requirements.txt / go.sum / Gemfile.lock 等) で urllib3 を grep し、稼働しているサービス・バージョンを把握する。

  2. 2
    Apply temporary workaround mitigate 
    disable [redirects](https://urllib3.

    パッチが適用されるまでの応急処置として、disable [redirects](https://urllib3. を実施。回避策の副作用 (機能低下) を確認した上で。

  3. 3
    Apply patch patch 
    Upgrade urllib3 to 2.6.3

    ステージング環境で 2.6.3 に上げて回帰テスト → 本番反映。回帰テストはアプリの主要ハッピーパスを含めること。

  4. 4
    Post-deployment verification verify 
    Confirm patched version is live in production

    パッチ適用後、ステージングで PoC または同等の悪用パターンを再現して脆弱性が閉じたことを確認。本番ではアクセスログ・WAF でアラート再発が無いか継続監視。

Affected packages

PyPI urllib3
[{"type":"ECOSYSTEM","events":[{"introduced":"1.22"},{"fixed":"2.6.3"}]}]

Fix status by distribution

Fixed packages published by each distribution for this CVE

References

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