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CVE Record

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

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.

HighCVSS 8.9Not KEV-listedUpdated
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Glexia's TakeAutomated analysishigh

Security readout for executives and security teams

Plain-English summary

A Python HTTP library can be forced to waste memory and CPU when an application streams data from an untrusted server and follows redirects. The sources describe availability impact, not data theft or code execution.

Executive priority

Treat as a high-priority availability risk for internet-facing or data-ingestion systems that fetch untrusted URLs. Prioritize upgrade where Python services process external content at scale.

Technical view

urllib3 versions 1.22 through before 2.6.3 may decompress an entire compressed redirect response while draining the connection when preload_content=False. Read limits do not cap the decompressed data, creating a decompression-bomb denial-of-service condition. Fixed behavior avoids decoding redirect responses in this streaming mode.

Likely exposure

Exposure is most likely in Python applications, services, or dependencies using urllib3 >=1.22 and <2.6.3 with preload_content=False, redirects enabled, and URLs controlled by or reachable from untrusted servers.

Exploitation context

The bundle does not show CISA KEV listing or cited evidence of active exploitation. A malicious server could trigger excessive client resource consumption through redirect handling, but the attack requires the vulnerable streaming configuration and redirect behavior.

Researcher notes

This is CWE-409 with CVSS 4.0 score 8.9. The key condition is streaming API use with redirect following. Validate configuration before assuming broad exposure across all urllib3 users.

Mitigation direction

  • Upgrade urllib3 to version 2.6.3 or later.
  • If immediate upgrade is not possible, disable redirects for untrusted sources.
  • Review OS vendor advisories for fixed distribution packages.
  • Check transitive Python dependencies that bundle or constrain urllib3.

Validation and detection

  • Inventory deployed urllib3 versions and package sources.
  • Search code for preload_content=False and redirect-enabled requests.
  • Confirm untrusted URLs cannot trigger vulnerable streaming paths.
  • Verify patched urllib3 or vendor-fixed packages are installed.
Prepared
Confidence
high
Sources
8

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

Conservative CVE-to-ATT&CK context

These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.

ATT&CK lookup starting points

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cwe · low confidence lookup

CWE-409: Exact CWE lookup

Use the exact CWE identifier as the starting point before reviewing related ATT&CK behavior. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.

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cve · low confidence lookup

CVE-2026-21441 mapping review

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Vulnerability profileCVE Program record
Severity
High
CVSS
8.9 (4.0)
Known Exploited
No
Published

Vector: 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

Official CVE source material

CNA and ADP enrichment extracted from CVE v5

These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.

2CVSS vectors
5Timeline events
3ADP providers
44Source links

SSVC decision data

CISA-ADPCISA Coordinator
Timestamp
Version
2.0.3
Exploitation: noneAutomatable: yesTechnical Impact: partial

CVSS vector scores

2 official scores

We collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.

ScoreVersionSeverityVectorExploitImpactSource
8.9CVSS 4.0HighCVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:H——GitHub_M
7.5CVSS 3.1HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H3.93.6redhat-SADP

Vulnerability scoring details

Base CVSS 4.0 score

8.9High
CVSS 4.0 vector shape for CVE-2026-21441Attack VectorAttack ComplexityAttack RequirementsPrivileges RequiredUser InteractionVS ConfidentialityVS IntegrityVS AvailabilitySS ConfidentialitySS IntegritySS Availability

Vector: 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
NetworkAdjacentLocalPhysical
Attack Complexity
LowHigh
Attack Requirements
NonePresent
Privileges Required
NoneLowHigh
User Interaction
NonePassiveActive
VS Confidentiality
HighLowNone
VS Integrity
HighLowNone
VS Availability
HighLowNone
SS Confidentiality
HighLowNone
SS Integrity
HighLowNone
SS Availability
HighLowNone

Vulnerability timeline

Timeline events are normalized from CVE metadata, CNA source timelines, ADP timelines, and KEV metadata when present.

  1. CVE reservedCVE Program

    The CVE ID was reserved by the assigning CNA.

  2. ADP timelineredhat-SADP

    Made public.

  3. CVE publishedCVE Program

    The CVE record was published.

  4. ADP timelineredhat-SADP

    Reported to Red Hat.

  5. CVE updatedCVE Program

    The CVE record metadata indicates this as the latest update time.

ADP provider summaries

CISA-ADPCISA ADP Vulnrichment
other:ssvc
CVECVE Program Container
redhat-SADPurllib3: urllib3 vulnerable to decompression-bomb safeguard bypass when following HTTP redirects (streaming API)
other:Red Hat severity ratingcvssV3_1
  • 2026-01-07T23:01:59.422Z: Reported to Red Hat.
  • 2026-01-07T22:09:01.936Z: Made public.

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
urllib3urllib3>= 1.22, < 2.6.3Listed
Weakness

CWE details

CWE-409

CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.

CWE-409 · source CWE mapping

Improper Handling of Highly Compressed Data (Data Amplification)

Improper Handling of Highly Compressed Data (Data Amplification) represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.

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