How severe is CVE-2021-21328?

CVE-2021-21328 has a CVSS score of 5.3/10, rated MEDIUM. Review your exposure and patch according to your risk tolerance.

Which packages are affected by CVE-2021-21328?

CVE-2021-21328 affects the following packages: github.com/vapor/vapor (SwiftURL). Ecosystems affected: SwiftURL.

How do I fix CVE-2021-21328?

Update github.com/vapor/vapor to 4.40.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-21328 is resolved across your whole dependency graph.

How do I detect CVE-2021-21328 in my SwiftURL dependencies?

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

How do I mitigate CVE-2021-21328 if there is no patch (or I can't update yet)?

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.

Is CVE-2021-21328 actively exploited in the wild?

No public exploit code has been indexed for CVE-2021-21328 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-2021-21328?

CVE-2021-21328 has an EPSS (Exploit Prediction Scoring System) score of 1.8%, 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-2021-21328?

CVE-2021-21328 is classified as Uncontrolled Resource Consumption (CWE-400). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2021-21328 published, and has it been updated?

CVE-2021-21328 was published on February 26, 2021 and was last updated on October 20, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

📦

📦 SwiftURL

Not in CISA KEV

MEDIUM severity

CVE-2021-21328 — vapor

MEDIUMFix: vapor/vapor@e3aa712

CVE-2021-21328 is a medium-severity (CVSS 5.3) Uncontrolled Resource Consumption vulnerability in github.com/vapor/vapor. A fix is available for github.com/vapor/vapor — see the affected versions and patch details below.

Vapor's Metrics integration could cause a system drain

Also known asGHSA-gcj9-jj38-hwmc

Published

Feb 26, 2021

Updated

Oct 20, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Nov 3, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

1.8%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

Where this sits among everything scored

Real-World Exposure

1 pkg affected

📦github.com/vapor/vapor

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects SwiftURL packages — download data is not available via public APIs for these ecosystems.

Description

Impact

This is a DoS attack against anyone who Bootstraps a metrics backend for their Vapor app with the following attack vector:

send unlimited requests against a vapor instance with different paths. this will create “unlimited” counters and timers, which will eventually drain the system.
downstream services might suffer from this attack as well by being spammed with error paths

Patches

This has been patched in 4.40.1. The DefaultResponder will rewrite any undefined route paths for to vapor_route_undefined to avoid unlimited counters.

Workarounds

Don't bootstrap a metrics system or upgrade to 4.40.1

For more information

If you have any questions or comments about this advisory:

Open an issue in Vapor
Ask in Discord

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
📦SwiftURL	`github.com/vapor/vapor`	all versions	4.40.1

Affected Products

1 product · 1 configurations

Application

vaporvapor_project

< 4.40.1

range

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/vapor/vapor, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update github.com/vapor/vapor to 4.40.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-21328 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.

Frequently Asked Questions

### Impact This is a DoS attack against anyone who Bootstraps a metrics backend for their Vapor app with the following 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 paths ### Patches This has been patched in 4.40.1. The `DefaultResponder` will rewrite any undefined route paths for to `vapor_route_undefined` to avoid unlimited counters. ### Workarounds Don't bootstrap a met