GHSA-xhv5-w9c5-2r2w · Severity: high · Ecosystem: maven — Unbounded connection acceptance in http4s-blaze-server
Http4s (http4s-blaze-server) is a minimal, idiomatic Scala interface for HTTP services. Http4s before versions 0.21.17, 0.22.0-M2, and 1.0.0-M14 have a vulnerability which can lead to a denial-of-service. Blaze-core, a library underlying http4s-blaze-server, accepts connections unboundedly on its selector pool. This has the net effect of amplifying degradation in services that are unable to handle their current request load, since incoming connections are still accepted and added to an unbounded queue. Each connection allocates a socket handle, which drains a scarce OS resource. This can also confound higher level circuit breakers which work based on detecting failed connections. http4s provides a general "MaxActiveRequests" middleware mechanism for limiting open connections, but it is enforced inside the Blaze accept loop, after the connection is accepted and the socket opened. Thus, the limit only prevents the number of connections which can be simultaneously processed, not the number of connections which can be held open. In 0.21.17, 0.22.0-M2, and 1.0.0-M14, a new "maxConnections" property, with a default value of 1024, has been added to the `BlazeServerBuilder`. Setting the value to a negative number restores unbounded behavior, but is strongly disrecommended. The NIO2 backend does not respect `maxConnections`. Its use is now deprecated in http4s-0.21, and the option is removed altogether starting in http4s-0.22. There are several possible workarounds described in the refrenced GitHub Advisory GHSA-xhv5-w9c5-2r2w.
Conclusion & alert: CVE-2021-21294 is rated Moderate Risk (59.6/100): CVSS High severity, with medium exploitation likelihood (EPSS 2.15%). Core evidence: EPSS rose +1.74% over the last day, indicating growing attacker interest. Mandatory action: Review affected assets and schedule remediation.
Risk is dynamic; we continuously reassess and refresh what is shown on this page as upstream context changes.
EPSS lead: Daily EPSS estimates relative likelihood of exploitation; percentile ranks this CVE among scored vulnerabilities (higher = more severe relative rank).
| # | Date | Old EPSS score | New EPSS score | Delta (New - Old) |
|---|---|---|---|---|
| 1 | 2026-06-15 | 0.41% | 2.15% | +1.74% |
| 2 | 2025-10-25 | 0.53% | 0.41% | -0.12% |
| 3 | 2025-04-29 | — | 0.53% | — |
Full EPSS history (12 records total)
CVSS metrics for this CVE.
| Base score | Version | Severity | Vector | Exploitability | Impact | Score source |
|---|---|---|---|---|---|---|
| 7.5 | 3.1 | HIGH |
|
3.9 | 3.6 | [email protected] |
| 7.5 | 3.1 | HIGH |
|
3.9 | 3.6 | [email protected] |
| 5.0 | 2.0 | MEDIUM |
|
10.0 | 2.9 | [email protected] |
GHSA-xhv5-w9c5-2r2w · Severity: high · Ecosystem: maven — Unbounded connection acceptance in http4s-blaze-server
| URL | Tags |
|---|---|
| https://github.com/http4s/blaze/security/advisories/GHSA-xmw9-q7x9-j5qc | Third Party Advisory |
| https://github.com/http4s/http4s/commit/987d6589ef79545b9bb2324ac4bdebf82d9a0171 | Patch Third Party Advisory |
| https://github.com/http4s/http4s/security/advisories/GHSA-xhv5-w9c5-2r2w | Third Party Advisory |