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urllib3 CVE-2026-21441

HIGH
Improper Handling of Highly Compressed Data (Data Amplification) (CWE-409)
2026-01-07 security-advisories@github.com GHSA-38jv-5279-wg99
Python Information Disclosure Urllib3
8.9
CVSS 4.0 · Vendor: github
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Severity by source

Vendor (github) PRIMARY
8.9 HIGH
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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
vuln.today AI
5.9 MEDIUM

Network-reachable and unauthenticated (AV:N/PR:N), but AC:H because it requires the non-default streaming-plus-redirects-to-untrusted-source configuration; impact is availability-only (A:H, C/I:N).

3.1 AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
4.0 AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:H
SUSE
7.5 HIGH
AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Red Hat
7.5 HIGH
qualitative

Primary rating from Vendor (github).

CVSS VectorVendor: github

Attack Vector
Network
Attack Complexity
Low
Privileges Required
None
User Interaction
None
Scope
X

Lifecycle Timeline

9
Analysis Updated
Jun 30, 2026 - 06:15 vuln.today
v5 (cvss_changed)
Analysis Updated
Jun 30, 2026 - 06:15 vuln.today
v4 (cvss_changed)
Analysis Updated
Jun 30, 2026 - 06:15 vuln.today
v3 (cvss_changed)
Analysis Updated
Jun 30, 2026 - 06:14 vuln.today
v2 (cvss_changed)
Re-analysis Queued
Jun 30, 2026 - 03:23 vuln.today
cvss_changed
CVSS changed
Jun 30, 2026 - 03:23 NVD
7.5 (HIGH) 8.9 (HIGH)
Analysis Generated
Mar 12, 2026 - 21:54 vuln.today
Patch released
Jan 23, 2026 - 09:15 nvd
Patch available
CVE Published
Jan 07, 2026 - 22:15 nvd
HIGH 7.5

Blast Radius

ecosystem impact
† from your stack dependencies † transitive graph · vuln.today resolves 4-path depth
  • 403 pypi packages depend on urllib3 (121 direct, 286 indirect)

Ecosystem-wide dependent count for version 1.22.

DescriptionCVE.org

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.

AnalysisAI

Uncontrolled resource consumption in the Python urllib3 HTTP client library (versions 1.22 through 2.6.2) lets a malicious or compromised server crash or exhaust memory on the client by serving a compressed redirect response. When an application streams from an untrusted source with preload_content=False and has not disabled redirects, urllib3 eagerly drains and decompresses the entire redirect body before any read call, ignoring configured read limits, leaving no defense against decompression bombs. EPSS is low (0.03%), there is no public exploit identified at time of analysis, and it is not in CISA KEV, but the bug is broadly reachable given urllib3's ubiquity.

Technical ContextAI

urllib3 is the foundational HTTP client for the Python ecosystem (it underpins requests and countless tools), and its streaming API reads response bodies in chunks while transparently decoding the Content-Encoding header - gzip, deflate, br (Brotli), or zstd. The flaw is a classic CWE-409 (Improper Handling of Highly Compressed Data, i.e., a decompression/data-amplification bomb): for HTTP 3xx redirect responses the library reads the full body to drain the socket so the connection can be reused, and that drain path decompressed the content immediately and unconditionally. Because decompression happened before any user read() and the read limit was not applied to the decompressed stream, a server advertising e.g. gzip could return a few kilobytes that expand to gigabytes, with the amplification fully absorbed by the client. The CPE cpe:2.3:a:python:urllib3 confirms the affected component is the urllib3 package itself, not any single downstream consumer.

RemediationAI

Vendor-released patch: upgrade urllib3 to at least 2.6.3, in which the library no longer decodes the content of redirect responses when preload_content=False; this is the primary and complete fix (see GHSA-38jv-5279-wg99 and commit 8864ac407bba8607950025e0979c4c69bc7abc7b). On Red Hat or Debian systems, apply the corresponding distribution package updates (e.g., the listed RHSA-2026 errata or the Debian LTS announcement) rather than pip-upgrading a system package. If immediate upgrade is not possible, the vendor-recommended workaround is to set redirect=False on requests to untrusted sources, which eliminates the vulnerable redirect-drain path; the trade-off is that the application must then handle redirects itself or will fail to follow them, so this is safe only where redirects to untrusted hosts are not required. Where feasible, also avoid streaming (leave preload_content at its default) for untrusted endpoints, accepting the higher memory cost of buffering full responses.

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Vendor StatusVendor

SUSE

Severity: High
Product Status
Container suse/manager/4.3/proxy-tftpd:4.3.16.2.9.63.10 Container suse/manager/5.0/x86_64/proxy-tftpd:latest Container suse/manager/5.0/x86_64/server:latest Container suse/multi-linux-manager/5.1/x86_64/proxy-tftpd:5.1.2.8.13.1 Image SLES15-SP5-SAP-Azure-LI-BYOS Image SLES15-SP5-SAP-Azure-LI-BYOS-Production Image SLES15-SP5-SAP-Azure-VLI-BYOS Image SLES15-SP5-SAP-Azure-VLI-BYOS-Production Image SLES15-SP7-CHOST-BYOS-Aliyun Image SLES15-SP7-CHOST-BYOS-Azure Image SLES15-SP7-CHOST-BYOS-EC2 Image SLES15-SP7-CHOST-BYOS-GDC Image SLES15-SP7-CHOST-BYOS-SAP-CCloud Image SLES15-SP7-GCE-3P Image SLES15-SP7-SAP-Azure-LI-BYOS-Production Image SLES15-SP7-SAP-Azure-VLI-BYOS-Production Image SLES15-SP7-SAP-GCE-3P Image proxy-tftpd-image Affected
Container suse/multi-linux-manager/5.1/x86_64/proxy-salt-broker:5.1.2.9.13.2 Image SLES15-SP4-BYOS-Azure Image SLES15-SP4-Hardened-BYOS-Azure Image SLES15-SP4-SAP Image SLES15-SP4-SAP-BYOS-Azure Image SLES15-SP4-SAP-EC2 Image SLES15-SP4-SAP-Hardened-BYOS-Azure Image SLES15-SP4-SAPCAL Image SLES15-SP4-SAPCAL-EC2 Image SLES15-SP5-Azure-3P Image SLES15-SP5-BYOS-Azure Image SLES15-SP5-EC2 Image SLES15-SP5-Hardened-BYOS-Azure Image SLES15-SP5-SAP-Azure-3P Image SLES15-SP5-SAP-BYOS-Azure Image SLES15-SP5-SAP-Hardened-Azure Image SLES15-SP5-SAP-Hardened-BYOS-Azure Image SLES15-SP5-SAPCAL-EC2 Image SLES15-SP6 Image SLES15-SP6-Azure-3P Image SLES15-SP6-Azure-Basic Image SLES15-SP6-Azure-Standard Image SLES15-SP6-BYOS-Azure Image SLES15-SP6-EC2 Image SLES15-SP6-EC2-ECS-HVM Image SLES15-SP6-HPC Image SLES15-SP6-HPC-Azure Image SLES15-SP6-HPC-BYOS-Azure Image SLES15-SP6-Hardened-BYOS-Azure Image SLES15-SP6-SAP-Azure Image SLES15-SP6-SAP-Azure-3P Image SLES15-SP6-SAP-BYOS-Azure Image SLES15-SP6-SAP-EC2 Image SLES15-SP6-SAP-Hardened-Azure Image SLES15-SP6-SAP-Hardened-BYOS-Azure Image SLES15-SP6-SAPCAL-Azure Image SLES15-SP6-SAPCAL-EC2 Image SLES15-SP7-Azure-3P Image SLES15-SP7-Azure-Basic Image SLES15-SP7-Azure-Standard Image SLES15-SP7-BYOS-Azure Image SLES15-SP7-BYOS-GCE Image SLES15-SP7-EC2 Image SLES15-SP7-EC2-ECS-HVM Image SLES15-SP7-HPC-Azure Image SLES15-SP7-HPC-BYOS-Azure Image SLES15-SP7-HPC-BYOS-GCE Image SLES15-SP7-Hardened-BYOS-Azure Image SLES15-SP7-Hardened-BYOS-GCE Image SLES15-SP7-SAP-Azure Image SLES15-SP7-SAP-Azure-3P Image SLES15-SP7-SAP-BYOS-Azure Image SLES15-SP7-SAP-BYOS-GCE Image SLES15-SP7-SAP-Hardened-Azure Image SLES15-SP7-SAP-Hardened-BYOS-Azure Image SLES15-SP7-SAP-Hardened-BYOS-GCE Image SLES15-SP7-SAPCAL-Azure Image SLES15-SP7-SAPCAL-EC2 Image proxy-salt-broker-image Affected
Container suse/multi-linux-manager/5.1/x86_64/server:5.1.2.8.13.2 Image SLES15-SP4-BYOS Image SLES15-SP4-BYOS-EC2 Image SLES15-SP4-HPC-BYOS Image SLES15-SP4-HPC-BYOS-EC2 Image SLES15-SP4-HPC-EC2 Image SLES15-SP4-Hardened-BYOS Image SLES15-SP4-Hardened-BYOS-EC2 Image SLES15-SP4-SAP-BYOS Image SLES15-SP4-SAP-BYOS-EC2 Image SLES15-SP4-SAP-Hardened-BYOS Image SLES15-SP4-SAP-Hardened-BYOS-EC2 Image SLES15-SP5-BYOS-EC2 Image SLES15-SP5-HPC-BYOS-EC2 Image SLES15-SP5-Hardened-BYOS-EC2 Image SLES15-SP5-SAP-BYOS-EC2 Image SLES15-SP5-SAP-Hardened-BYOS-EC2 Image SLES15-SP6-BYOS Image SLES15-SP6-BYOS-EC2 Image SLES15-SP6-HPC-BYOS Image SLES15-SP6-HPC-BYOS-EC2 Image SLES15-SP6-HPC-EC2 Image SLES15-SP6-Hardened-BYOS Image SLES15-SP6-Hardened-BYOS-EC2 Image SLES15-SP6-SAP-BYOS Image SLES15-SP6-SAP-BYOS-EC2 Image SLES15-SP6-SAP-Hardened-BYOS Image SLES15-SP6-SAP-Hardened-BYOS-EC2 Image SLES15-SP6-SAP-Hardened-EC2 Image SLES15-SP7-BYOS-EC2 Image SLES15-SP7-HPC-BYOS-EC2 Image SLES15-SP7-Hardened-BYOS-EC2 Image SLES15-SP7-SAP-BYOS-EC2 Image SLES15-SP7-SAP-EC2 Image SLES15-SP7-SAP-Hardened-BYOS-EC2 Image server-image Affected
Container suse/sl-micro/6.0/baremetal-os-container:2.1.3-6.24 Affected
Container suse/sl-micro/6.0/base-os-container:2.1.3-7.4 Container suse/sl-micro/6.0/kvm-os-container:2.1.3-6.31 Container suse/sl-micro/6.0/rt-os-container:2.1.3-7.36 Affected

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CVE-2026-21441 vulnerability details – vuln.today
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