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CVE Record

CVE-2021-33503: An issue was discovered in urllib3 before 1.26.5.

An issue was discovered in urllib3 before 1.26.5. When provided with a URL containing many @ characters in the authority component, the authority regular expression exhibits catastrophic backtracking, causing a denial of service if a URL were passed as a parameter or redirected to via an HTTP redirect.

UnknownCVSS not scoredNot KEV-listedUpdated
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Glexia's TakeAutomated analysismoderate

Security readout for executives and security teams

Plain-English summary

CVE-2021-33503 is an availability issue in urllib3 before 1.26.5. Specially shaped URLs can make urllib3 spend excessive CPU time while parsing, potentially slowing or stopping services that process untrusted URLs or redirects.

Executive priority

Treat this as a targeted availability risk, not a confirmed breach indicator. Prioritize internet-facing or automation-heavy services that process external URLs, then update dependencies through normal patch cycles.

Technical view

urllib3’s authority parsing regular expression could catastrophically backtrack when handling URLs containing many @ characters in the authority component. The CVE states denial of service is possible if such a URL is supplied as a parameter or reached through an HTTP redirect.

Likely exposure

Exposure is most likely in Python applications or packaged products using urllib3 before 1.26.5, especially where services accept URLs from users, process URL parameters, or follow redirects from external HTTP responses.

Exploitation context

The provided sources do not show CISA KEV listing or confirmed active exploitation. The issue is theoretically reachable through crafted URL input or redirect behavior, but no exploit-in-the-wild evidence is included in the bundle.

Researcher notes

The key condition is urllib3 URL authority parsing before 1.26.5. Validate reachability by mapping dependency use and untrusted URL flows. Avoid assuming impact where urllib3 is present but no external URL or redirect handling path exists.

Mitigation direction

  • Upgrade urllib3 to 1.26.5 or later where directly or transitively used.
  • Rebuild and redeploy affected Python workloads after dependency updates.
  • Check Fedora, Gentoo, and Oracle advisories for packaged fixes where applicable.
  • Reduce exposure of services that process untrusted redirect targets or URL parameters.

Validation and detection

  • Inventory Python environments for urllib3 versions earlier than 1.26.5.
  • Review dependency lockfiles and OS package manifests for urllib3.
  • Identify code paths that parse user-supplied URLs or follow redirects.
  • Confirm remediation through package-manager or dependency-scanner results.
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

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CVE-2021-33503 mapping review

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Vulnerability profileCVE Program record
Severity
Unknown
CVSS
Not scored
Known Exploited
No
Published
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.

0CVSS vectors
0Timeline events
0ADP providers
7Source links

CVSS and timeline data

No CVSS vectors or timeline events were available in the normalized CVE source material.

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
n/an/an/aListed
Weakness

CWE details

No CWE listed

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