CVE-2021-21330 | Open redirect vulnerability in aiohttp

aiohttp is an asynchronous HTTP client/server framework for asyncio and Python. In aiohttp before version 3.7.4 there is an 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. This security problem has been fixed in 3.7.4. Upgrade your dependency using pip as follows "pip install aiohttp >= 3.7.4". If upgrading is not an option for you, a workaround can be to avoid using `aiohttp.web_middlewares.normalize_path_middleware` in your applications.

Published: 2021-02-25 Last update: 2026-06-16 Assigner: [email protected] Source: [email protected]
NVD Status：Modified，CVE State: published
View at NVD, CVE.org, EUVD

CVE-2021-21330 is rated Moderate Risk (40.8/100): CVSS Low severity, with medium exploitation likelihood (EPSS 1.91%). EPSS rose +1.42% over the last day, indicating growing attacker interest. Review affected assets and schedule remediation.

Risk is dynamic; we continuously reassess and refresh what is shown on this page as upstream context changes.

Exploit prediction scoring system (EPSS) score for CVE-2021-21330

EPSS lead: Daily EPSS estimates relative likelihood of exploitation; percentile ranks this CVE among scored vulnerabilities (higher = more severe relative rank).

# Date Old EPSS score New EPSS score Delta (New - Old)
1 2026-06-15 0.49% 1.91% +1.42%
2 2025-11-21 1.10% 0.49% -0.62%
3 2025-11-18 — 1.10% —

Full EPSS history (18 records total)

Common vulnerability scoring system (CVSS) metrics for CVE-2021-21330

CVSS metrics for this CVE.

Base score Version Severity Vector Exploitability Impact Score source
3.1 3.1 LOW
CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:N/I:L/A:N Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:H)
Even with access, the exploit needs extra luck, timing, or a fussy environment to actually work.
Privileges required (PR:N)
No account or special rights needed—anonymous or random user is enough.
User interaction (UI:R)
A real person has to do something—click, install, enable—otherwise it doesn’t land.
Scope (S:U)
Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:N)
Doesn’t really leak secrets in a meaningful way.
Integrity (I:L)
Attackers could change some data, but it’s limited—not everything goes.
Availability (A:N)
Service keeps running; no real outage angle.
 1.6 1.4 [email protected]
6.1 3.1 MEDIUM
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:N)
No account or special rights needed—anonymous or random user is enough.
User interaction (UI:R)
A real person has to do something—click, install, enable—otherwise it doesn’t land.
Scope (S:C)
Breaking this can reach past the original component and bite other resources—bigger blast radius.
Confidentiality (C:L)
Some sensitive info could get out, but not a total data dump.
Integrity (I:L)
Attackers could change some data, but it’s limited—not everything goes.
Availability (A:N)
Service keeps running; no real outage angle.
 2.8 2.7 [email protected]
5.8 2.0 MEDIUM
AV:N/AC:M/Au:N/C:P/I:P/A:N Click to expand
Access vector (AV:N)
Can be exploited remotely over network reachability.
Access complexity (AC:M)
Exploitation needs some favorable conditions, but not exceptional ones.
Authentication (AU:N)
No authentication is required.
Confidentiality impact (C:P)
Partial confidentiality impact.
Integrity impact (I:P)
Partial integrity impact.
Availability impact (A:N)
No availability impact.
 8.6 4.9 [email protected]

Weakness enumeration for CVE-2021-21330

GitHub Security Advisory for CVE-2021-21330

GHSA-v6wp-4m6f-gcjg · Severity: low · Ecosystem: pip — `aiohttp` Open Redirect vulnerability (`normalize_path_middleware` middleware)

OS Trackers for CVE-2021-21330

vendor priority summary link
debian not yet assigned CVE-2021-21330 not yet assigned priority: Debian including 1 source packages (python-aiohttp), 5 status rows across 5 suites (bookworm, bullseye, forky, sid, trixie): resolved 5. https://security-tracker.debian.org/tracker/CVE-2021-21330
gentoo low CVE-2021-21330: 1 GLSA(s) (202208-19), 1 atom(s) (dev-python/aiohttp); latest impact low. https://bugs.gentoo.org/buglist.cgi?quicksearch=CVE-2021-21330
redhat medium — https://access.redhat.com/security/cve/CVE-2021-21330
suse high CVE-2021-21330 severity important: SUSE including 12 source package names (python-aiohttp-doc-3.4.4-3.6.1, python-aiohttp-doc-3.6.0-150100.3.9.1, …), 46 product×package rows across 28 product lines (Image SLES15-SP3-BYOS-Azure, Image SLES15-SP3-HPC-BYOS-Azure, … (28 product lines)): Fixed 46. https://www.suse.com/security/cve/CVE-2021-21330/
ubuntu medium CVE-2021-21330 medium priority: Ubuntu including 1 source packages (python-aiohttp), 15 status rows across 15 suites (bionic, focal, groovy, hirsute, impish, jammy, kinetic, lunar, mantic, noble, oracular, plucky, trusty, upstream, xenial): not-affected 9, released 3, ignored 2, DNE 1. https://ubuntu.com/security/CVE-2021-21330

Affected software / configurations for CVE-2021-21330

Vendor Product Version Raw CPE
aiohttp aiohttp < 3.7.4 cpe:2.3:a:aiohttp:aiohttp:*:*:*:*:*:*:*:*
debian debian_linux 10.0 cpe:2.3:o:debian:debian_linux:10.0:*:*:*:*:*:*:*
fedoraproject fedora 33 cpe:2.3:o:fedoraproject:fedora:33:*:*:*:*:*:*:*
fedoraproject fedora 34 cpe:2.3:o:fedoraproject:fedora:34:*:*:*:*:*:*:*

References for CVE-2021-21330

cvelogic Threat Intelligence