CVE-2021-33503: urllib3 ReDoS
CVE-2021-33503 (urllib3 ReDoS) is a regular expression denial of service (redos) vulnerability in urllib3 (PyPI). As of 2026-06-18 it carries a 3.3% probability of exploitation in the next 30 days (EPSS). It is fixed in 1.26.5 — if you depend on an earlier version, upgrade.
| Severity | Medium |
|---|---|
| CVSS vector | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H |
| EPSS (exploit probability, 30d) | 3.3% · 87th percentile |
| Vulnerability type | Regular Expression Denial of Service (ReDoS) |
| Affected package | urllib3 (PyPI) |
| Fixed in | 1.26.5 |
What is CVE-2021-33503?
urllib3 before 1.26.5 is vulnerable to catastrophic backtracking when parsing a crafted URL authority, allowing a denial of service against code that constructs URLs from untrusted input.
Is CVE-2021-33503 exploitable?
CVE-2021-33503 has an EPSS score of 3.3% (87th percentile), meaning a 3.3% probability of exploitation in the next 30 days. Its CVSS vector is CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H, which scores as Medium severity. It is a regular expression denial of service (redos) issue.
What is the EPSS score for CVE-2021-33503?
As of 2026-06-18, the EPSS exploit-prediction score for CVE-2021-33503 is 3.3% (87th percentile). EPSS estimates the probability that a vulnerability will be exploited in the wild within the next 30 days and is recomputed daily by FIRST.org.
How do I fix CVE-2021-33503?
Upgrade urllib3 to 1.26.5 or later. urllib3 before 1.26.5 is vulnerable to catastrophic backtracking when parsing a crafted URL authority, allowing a denial of service against code that constructs URLs from untrusted input.
Am I affected by CVE-2021-33503?
You are affected if your project (directly or transitively) depends on a vulnerable version of urllib3 in the PyPI ecosystem. Check your lockfile for the resolved version, or scan automatically with VulnFeed.
Check your own project automatically
Don't eyeball every dependency by hand. VulnFeed reads your lockfile, checks it against the same advisory data, and ranks findings by EPSS — free, no signup (10 scans/day):
curl -s https://vulnfeed-api.novadyne.ai/vulnscan/query \
-H 'content-type: application/json' \
-d '{"ecosystem":"npm","package":"PKG","version":"VERSION"}'
Or run it inside Claude Code with no API key — uvx vulnfeed-mcp — and just ask "scan this project for vulnerabilities."
Data sources: vulnerability metadata from OSV.dev and the National Vulnerability Database; exploit-probability (EPSS) from FIRST.org (recomputed daily; EPSS data shown here as of 2026-06-18). Page generated 2026-06-18.