Summary
Unbounded connection acceptance in http4s-blaze-server
Workarounds
- An Nginx side-car acting as a reverse proxy for the local http4s-blaze-server instance would be able to apply a connection limiting semantic before the sockets reach blaze-core. Nginx’s connection bounding is both asynchronous and properly respects backpressure.
- http4s-ember-server is an alternative to http4s-blaze-server, but does not yet have HTTP/2 or web socket support. Its performance in terms of RPS is appreciably behind Blaze’s, and as the newest backend, has substantially less industrial uptake.
- http4s-jetty is an alternative to http4s-blaze-server, but does not yet have web socket support. Its performance in terms of requests per second is somewhat behind Blaze’s, and despite Jetty's industrial adoption, the http4s integration has substantially less industrial uptake.
- http4s-tomcat is an alternative to http4s-blaze-server, but does not yet have HTTP/2 web socket support. Its performance in terms of requests per second is somewhat behind Blaze’s, and despite Jetty's industrial adoption, the http4s integration has substantially less industrial uptake.
References
See the Blaze GHSA for more on the underlying issue.
For more information
If you have any questions or comments about this advisory:
- Open an issue in http4s/http4s
- Contact us according to the http4s security policy
Impact
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.
Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service. Typical impact: denial of service.
CVE-2021-21294 has a CVSS score of 7.5 (High). 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 (0.21.17); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
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. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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In 0.21.18, 0.22.0-M3, and 1.0.0-M16, a newmaxConnections 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.
The connections are bounded in 0.21.17, 0.22.0-M2, and 1.0.0-M14, but the maxConnections parameter was passed incorrectly, making it impossible to change the Blaze default of 512.
Frequently Asked Questions
- What is CVE-2021-21294? CVE-2021-21294 is a high-severity uncontrolled resource consumption vulnerability in org.http4s:http4s-blaze-server_2.12 (maven), affecting versions < 0.21.17. It is fixed in 0.21.17. Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service.
- How severe is CVE-2021-21294? CVE-2021-21294 has a CVSS score of 7.5 (High). 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.
- Which packages are affected by CVE-2021-21294?
org.http4s:http4s-blaze-server_2.12(maven) (versions < 0.21.17)org.http4s:http4s-blaze-server_2.13(maven) (versions < 0.21.17)
- Is there a fix for CVE-2021-21294? Yes. CVE-2021-21294 is fixed in 0.21.17. Upgrade to this version or later.
- Is CVE-2021-21294 exploitable, and should I be worried? Whether CVE-2021-21294 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
- What actually determines whether CVE-2021-21294 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.
- How do I fix CVE-2021-21294?
- Upgrade
org.http4s:http4s-blaze-server_2.12to 0.21.17 or later - Upgrade
org.http4s:http4s-blaze-server_2.13to 0.21.17 or later
- Upgrade