CVE-2021-33503 is a high-severity (CVSS 7.5) Uncontrolled Resource Consumption vulnerability in urllib3. A fix is available for urllib3 — see the affected versions and patch details below.
Catastrophic backtracking in URL authority parser when passed URL containing many @ characters
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2021-33503 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.
Where this sits among everything scored
Of 385,987 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.
Real-World Exposure
urllib3Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.
Description
Impact
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.
Patches
The issue has been fixed in urllib3 v1.26.5.
References
For more information
If you have any questions or comments about this advisory:
- Ask in our community Discord
- Email [email protected]
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | urllib3 | ≥ 1.25.4&&< 1.26.5 | 1.26.5pip install --upgrade 'urllib3==1.26.5' |
Affected Products
fedorafedoraprojectenterprise manager ops centeroracleinstantis enterprisetrackoraclezfs storage appliance kitoracleurllib3pythonDetection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for urllib3, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update urllib3 to 1.26.5 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-33503 is resolved across your whole dependency graph.
Workarounds
Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.
Fixing This On Your OS
If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.
* Red Hat OpenShift Container Platform (OCP) 4 delivers the python-urllib3 package which includes a vulnerable version of urllib3 module, however from OCP 4.6 the python-urllib3 package is no longer shipped. OCP 4.5 is out of support scope for Moderate and Low impact vulnerabilities, hence is marked Out Of Support…
Mitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base or stability.Source: Red Hat security advisory for CVE-2021-33503 (CC BY 4.0)
| Product | Fixed in | Advisory |
|---|---|---|
| Red Hat Automation Hub 4.2 for RHEL 7 | automation-hub-0:4.2.6-1.el7pc | RHSA-2021:3473 |
| Red Hat Enterprise Linux 8 | python39:3.9-8050020210811100211.d428a79b | RHSA-2021:4160 |
| Red Hat Enterprise Linux 8 | python38:3.8-8050020210811101222.e3d35cca | RHSA-2021:4162 |
| Red Hat Satellite 6.10 for RHEL 7 | python-urllib3-0:1.26.5-1.el7pc | RHSA-2021:4702 |
| Red Hat Software Collections for Red Hat Enterprise Linux 7 | rh-python38-babel-0:2.7.0-12.el7 | RHSA-2021:3254 |
Frequently Asked Questions
Is CVE-2021-33503 in your dependencies?
Find it across PyPI, including transitive dependencies.