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No exploitation reportedFix available

CVE-2026-21441

No exploitation reported by the sources we read.

  • Published2026-01-07
  • Fixed in2.6.3
  • WeaknessCWE-409

Am I affected?

With a domain, we read the version its public headers and pages expose. A distribution build can carry the fix under an older number.

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.

Timeline

dates in UTC
  1. 2026-01-0722:15CVE publishedNVD

Observed activity

isMalicious corpus · updated 2026-10-23 18:48 UTC
Indicators tagged with this CVE, per day0 indicators over 7 days
Block these indicators
  • Firewall list (EDL, text): https://api.ismalicious.com/edl/cve/CVE-2026-21441?format=txt
  • CSV export: https://api.ismalicious.com/edl/cve/CVE-2026-21441?format=csv
  • TAXII collection of exploited CVEs
  • The EDL and CSV take an API key on the Pro plan.

Versions per branch

NVD
Python urllib3
BranchAffectedFixed in
2.6≥ 1.22, < 2.6.32.6.3

as publishedRanges as NVD and OSV publish them; a distribution’s build can be fixed under an older number.

Distribution builds and vendor advisories

Packages from OSV.dev and products from the vendors' CSAF advisories.

version not verifiedVersions not verified: read as OSV.dev and the vendors publish them, not checked against each vendor's release list.

Vendor advisories

  • RHSA-2026:86618 Red Hat Product Security4 products fixed

    Red Hat Security Advisory: New container image: rhceph-9.2

  • RHSA-2026:61628 Red Hat Product Security2 products fixed

    Red Hat Security Advisory: Red Hat AI Inference Server Model Optimization Tools 3.2.2 (CUDA)

  • RHSA-2026:28441 Red Hat Product Security3 products fixed

    Red Hat Security Advisory: Red Hat Openshift Mirror Registry v2.0.11

  • RHSA-2026:25127 Red Hat Product Security28 products fixed

    Red Hat Security Advisory: Submariner v0.21 security fixes and container updates

  • RHSA-2026:19712 Red Hat Product Security12 products fixed

    Red Hat Security Advisory: RHOAI 3.3.3 - Red Hat OpenShift AI

  • RHSA-2026:17463 Red Hat Product Security4 products fixed

    Red Hat Security Advisory: zero trust workload identity manager for Red Hat OpenShift 1.0.1

24 more advisories

Full record on NVD

Fix

  1. Update Python urllib3 to 2.6.3
  2. Vendor fix Red Hat Product Security RHSA-2026:86618 · 4 productsThe container images provided by this update can be downloaded from the Red Hat container registry at registry.redhat.io using the "podman pull" command.
  3. Vendor fix Red Hat Product Security RHSA-2026:61628 · 2 productsFor more information visit https://access.redhat.com/errata/RHSA-2026:61628
  4. Vendor fix Red Hat Product Security RHSA-2026:28441 · 3 productsBefore applying this update, make sure all previously released errata relevant to your system have been applied. For details on how to apply this update, refer to: https://access.redhat.com/articles/11258
  5. Vendor fix Red Hat Product Security RHSA-2026:25127 · 28 productsFor release note details, see the upstream Submariner release notes: https://submariner.io/community/releases/ Downstream-specific issues resolved: * ACM-27238 * ACM-28294 * ACM-28295 * ACM-28311 * ACM-28313 * ACM-28314 * ACM-28315 * ACM-28316 * ACM-28329 * ACM-28331 * ACM-28333 * ACM-28335 * ACM…
  6. Vendor fix Red Hat Product Security RHSA-2026:19712 · 12 productsFor Red Hat OpenShift AI 3.3.3 see the following documentation, which will be updated shortly for this release, for important instructions on how to upgrade your cluster and fully apply this errata update: https://docs.redhat.com/en/documentation/red_hat_openshift_ai/
  7. Vendor fix Red Hat Product Security RHSA-2026:17463 · 4 productsBefore installing the operator, make sure all previously released errata relevant to your system have been applied. The steps to apply the upgraded images will differ depending on the installation plan approval policy that will be used while installing thezero trust workload identity manager for R…

Advisories and patches

Detection

Nuclei template
none so far
ProjectDiscovery
Exploit-DB
no entry
Exploit-DB
Proof-of-concept repositories
none listed
PoC-in-GitHub

Sources

  • CISA-ADPSSVC decisionnone2026-01-08
  • OpenCVESSVC decisionnone—
  • CNACVSS 4.08.9read 2026-10-23
  • CNACVSS 3.17.5read 2026-10-23
  • NVDCVSS 3.17.5read 2026-10-23
  • CNAweakness (CWE)CWE-409read 2026-10-23
  • FIRST EPSSEPSS score2.9 %2026-10-22

Technical details

CVSS v3 Vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
CVSS 4.0 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/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
Published
2026-01-07
Last Modified
2026-09-15
NVD status
Modified

Frequently asked questions

What is CVE-2026-21441?

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.

Is CVE-2026-21441 actively exploited?

Active exploitation of CVE-2026-21441 has not been confirmed. Its EPSS score was 2.9% on 2026-10-22, the estimated probability of exploitation in the next 30 days.

What is the CVSS score for CVE-2026-21441?

CVE-2026-21441 has a CVSS v3 base score of 7.5 (HIGH severity), with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H.

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