CVE-2021-21328

Vapor's Metrics integration could cause a system drain

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

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

Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service. Typical impact: denial of service.

CVE-2021-21328 has a CVSS score of 5.3 (Medium). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (4.40.1); upgrading removes the vulnerable code path.

Kodem intelligence

Already deployed Kodem?

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.

1. What is CVE-2021-21328? CVE-2021-21328 is a medium-severity uncontrolled resource consumption vulnerability in github.com/vapor/vapor (swift), affecting versions <= 4.40.0. It is fixed in 4.40.1. Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service.
2. How severe is CVE-2021-21328? CVE-2021-21328 has a CVSS score of 5.3 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of github.com/vapor/vapor are affected by CVE-2021-21328? github.com/vapor/vapor (swift) versions <= 4.40.0 is affected.
4. Is there a fix for CVE-2021-21328? Yes. CVE-2021-21328 is fixed in 4.40.1. Upgrade to this version or later.
5. Is CVE-2021-21328 exploitable, and should I be worried? Whether CVE-2021-21328 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2021-21328 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2021-21328? Upgrade github.com/vapor/vapor to 4.40.1 or later.

Other vulnerabilities in github.com/vapor/vapor