How severe is CVE-2021-21330?

CVE-2021-21330 has a CVSS score of 6.1/10, rated MEDIUM. Review your exposure and patch according to your risk tolerance.

Which packages are affected by CVE-2021-21330?

CVE-2021-21330 affects the following packages: aiohttp (PyPI). Ecosystems affected: PyPI.

How do I fix CVE-2021-21330?

Update aiohttp to 3.7.4 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-21330 is resolved across your whole dependency graph.

How do I detect CVE-2021-21330 in my PyPI dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for aiohttp, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2021-21330 if there is no patch (or I can't update yet)?

Stop reflecting attacker-controlled destinations: resolve every redirect target against an allowlist of paths or hosts you own, prefer a server-side key over a full URL in the request, and reject absolute URLs entirely where the flow only ever needs a relative one.

Is CVE-2021-21330 actively exploited in the wild?

No public exploit code has been indexed for CVE-2021-21330 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2021-21330?

CVE-2021-21330 has an EPSS (Exploit Prediction Scoring System) score of 1.9%, placing it in the 79th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2021-21330?

CVE-2021-21330 is classified as Open Redirect (CWE-601). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2021-21330 published, and has it been updated?

CVE-2021-21330 was published on February 26, 2021 and was last updated on August 7, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐍

🐍 PyPI

Not in CISA KEV

MEDIUM severity

CVE-2021-21330 — aiohttp

MEDIUMFix: aio-libs/aiohttp@2545222

CVE-2021-21330 is a medium-severity (CVSS 6.1) Open Redirect vulnerability in aiohttp. A fix is available for aiohttp — see the affected versions and patch details below.

`aiohttp` Open Redirect vulnerability (`normalize_path_middleware` middleware)

Also known asGHSA-v6wp-4m6f-gcjgPYSEC-2021-76

Published

Feb 26, 2021

Updated

Aug 7, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Oct 9, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

1.9%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs79th percentile — riskier than 79% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

CVE-2021-21330 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

1 pkg affected

🐍aiohttp

Real-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

Open redirect vulnerability — a maliciously crafted link to an aiohttp-based web-server could redirect the browser to a different website.

It is caused by a bug in the aiohttp.web_middlewares.normalize_path_middleware middleware.

Patches

This security problem has been fixed in v3.7.4. Upgrade your dependency as follows: pip install aiohttp >= 3.7.4

Workarounds

If upgrading is not an option for you, a workaround can be to avoid using aiohttp.web_middlewares.normalize_path_middleware in your applications.

References

For more information

If you have any questions or comments about this advisory:

Open an issue in the aiohttp repo
Email us at [email protected] and/or [email protected]

Credit: Jelmer Vernooĳ and Beast Glatisant.

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
🐍PyPI	`aiohttp`	all versions	3.7.4`pip install --upgrade 'aiohttp==3.7.4'`

Affected Products

3 products · 4 configurations

Application

aiohttpaiohttp

< 3.7.4

range

debian linuxdebian

1 version

10.0

fedorafedoraproject

2 versions

3334

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for aiohttp, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update aiohttp to 3.7.4 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-21330 is resolved across your whole dependency graph.
Workarounds
Stop reflecting attacker-controlled destinations: resolve every redirect target against an allowlist of paths or hosts you own, prefer a server-side key over a full URL in the request, and reject absolute URLs entirely where the flow only ever needs a relative one.

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 HatModerate

Red Hat Satellite version 6.7 onward (mostly pulp part) does ship an affected version of aiohttp, however, is not vulnerable since the product code does not use the normalize_path_middleware function, which the attacker may use for an attack. We may update the python-aiohttp dependency in a future release.

Product	Fixed in	Advisory
Red Hat Satellite 6.10 for RHEL 7	`python-aiohttp-0:3.7.4-1.el7pc`	RHSA-2021:4702

Frequently Asked Questions

### Impact Open redirect vulnerability — a maliciously crafted link to an aiohttp-based web-server could redirect the browser to a different website. It is caused by a bug in the `aiohttp.web_middlewares.normalize_path_middleware` middleware. ### Patches This security problem has been fixed in v3.7.4. Upgrade your dependency as follows: [`pip install aiohttp >= 3.7.4`] ### Workarounds If upgrading is not an option for you, a workaround can be to avoid using `aiohttp.web_middlewares.normalize_path_middleware` in your applications. ### References * [aiohttp @ PyPI] * [GHSA-v6wp-4m6f-gcjg