CVE-2021-21328: Vapor is a web framework for Swift

Q: How do I fix CVE-2021-21328?

vapor: upgrade to 4.40.1. Exact commands for each OS are on this page.

Q: Is CVE-2021-21328 being exploited?

CVE-2021-21328 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2021-21328 affect?

CVE-2021-21328 affects vapor. Severity is medium (CVSS 5.3). Vapor is a web framework for Swift. In Vapor before version 4.40.1, there is a DoS attack against anyone who Bootstraps a metrics backend for their Vapor app. The following is the attack vector: 1. send unlimited requests against a vapor in

Q: How urgent is CVE-2021-21328?

Priority 43/100. Remotely exploitable, no login, no user action

This month

MEDIUM 5.3REMOTEUPGRADE

CVE-2021-21328 is a medium-severity flaw (CVSS 5.3) affecting vapor. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Vapor is a web framework for Swift. In Vapor before version 4.40.1, there is a DoS attack against anyone who Bootstraps a metrics backend for their Vapor app. The following is the attack vector: 1. send unlimited requests against a vapor instance with different paths. this will create unlimited counters and timers, which will eventually drain the system. 2. downstream services might suffer from this attack as well by being spammed with error path

Why this priority

Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	5.3 (Medium)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	1.8% (top 21%)
CISA exploited list	No
Weakness	CWE-400
Published	2021-02-26
Last modified	2026-06-17

Publisher (CNA): GitHub, Inc. · Vendor: vapor project

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2021-02-26 02:15 UTCPublished CVE record published
2026-06-17 03:35 UTCUpdated Record last modified

vapor project vapor UPGRADE

Affected: < 4.40.1

Fixed in: 4.40.1

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy vapor || rpm -q vapor || vapor --version

2. UPGRADE to 4.40.1 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade vapor

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade vapor

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2021-21328?

vapor: upgrade to 4.40.1. Exact commands for each OS are on this page.

Is CVE-2021-21328 being exploited?

CVE-2021-21328 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2021-21328 affect?

CVE-2021-21328 affects vapor. Severity is medium (CVSS 5.3). Vapor is a web framework for Swift. In Vapor before version 4.40.1, there is a DoS attack against anyone who Bootstraps a metrics backend for their Vapor app. The following is the attack vector: 1. send unlimited requests against a vapor in

How urgent is CVE-2021-21328?

Priority 43/100. Remotely exploitable, no login, no user action

Software pages

Swift vulnerabilities

Related CVEs

CVE-2024-21631 MEDIUM 6.5 UPGRADE Vapor is an HTTP web framework for Swift
CVE-2023-44386 MEDIUM 5.3 UPGRADE Vapor is an HTTP web framework for Swift
CVE-2022-31019 HIGH 7.5 UPGRADE Vapor is a server-side Swift HTTP web framework
CVE-2022-31005 HIGH 7.5 UPGRADE Vapor is an HTTP web framework for Swift
CVE-2021-32742 HIGH 7.5 UPGRADE Vapor is a web framework for Swift

References

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