What’s Vulnerable › Vulnerabilities

CVE-2021-21328: Denial of Service

• Severity: medium (CVSS 5.3 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 1.8% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2021-02-26 · updated 2026-06-17
• Weakness: CWE-400

Description

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 paths. 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.

Affected products

• Vapor Project Vapor: before 4.40.1 — fixed in 4.40.1

How to fix

Upgrade to a fixed version: Vapor Project Vapor 4.40.1. Check the vendor advisory for mitigations.

References

• https://github.com/vapor/vapor/commit/e3aa712508db2854ac0ab905696c65fd88fa7e23
• https://github.com/vapor/vapor/releases/tag/4.40.1
• https://github.com/vapor/vapor/security/advisories/GHSA-gcj9-jj38-hwmc
• https://vapor.codes/
• https://github.com/vapor/vapor/commit/e3aa712508db2854ac0ab905696c65fd88fa7e23
• https://github.com/vapor/vapor/releases/tag/4.40.1
• https://github.com/vapor/vapor/security/advisories/GHSA-gcj9-jj38-hwmc
• https://vapor.codes/

Get alerted when CVE-2021-21328 or the next vulnerability affects your stack