Summary
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.
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.
Untrusted input controls a URL used for redirection, which can forward users to attacker-controlled sites. Typical impact: phishing and credential harvesting via a trusted domain.
CVE-2021-21330 has a CVSS score of 3.1 (Low). The vector is network-reachable, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (3.7.4); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
Remediation advice
This security problem has been fixed in v3.7.4. Upgrade your dependency as follows:
[pip install aiohttp >= 3.7.4]
Frequently Asked Questions
- What is CVE-2021-21330? CVE-2021-21330 is a low-severity open redirect vulnerability in aiohttp (pip), affecting versions < 3.7.4. It is fixed in 3.7.4. Untrusted input controls a URL used for redirection, which can forward users to attacker-controlled sites.
- How severe is CVE-2021-21330? CVE-2021-21330 has a CVSS score of 3.1 (Low). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of aiohttp are affected by CVE-2021-21330? aiohttp (pip) versions < 3.7.4 is affected.
- Is there a fix for CVE-2021-21330? Yes. CVE-2021-21330 is fixed in 3.7.4. Upgrade to this version or later.
- Is CVE-2021-21330 exploitable, and should I be worried? Whether CVE-2021-21330 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2021-21330 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2021-21330? Upgrade
aiohttpto 3.7.4 or later.