CVE-2021-21328: Vapor Project Vapor

Medium severity, CVSS 5.3. EPSS: 1.8% chance of exploitation in the next 30 days.

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)

Published 2021-02-26. Last modified 2026-06-17.