GHSA-xhv5-w9c5-2r2w
HIGHUnbounded connection acceptance in http4s-blaze-server
Blast Radius
org.http4s:http4s-blaze-server_2.12☕org.http4s:http4s-blaze-server_2.13Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Maven packages — download data is not available via public APIs for these ecosystems.
Description
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.
Patches
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.
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
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| ☕Maven | org.http4s:http4s-blaze-server_2.12 | all versions | 0.21.17 |
| ☕Maven | org.http4s:http4s-blaze-server_2.13 | all versions | 0.21.17 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for org.http4s:http4s-blaze-server_2.12. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update org.http4s:http4s-blaze-server_2.12 to 0.21.17 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-xhv5-w9c5-2r2w is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether GHSA-xhv5-w9c5-2r2w is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to GHSA-xhv5-w9c5-2r2w. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is GHSA-xhv5-w9c5-2r2w in your dependencies?
O3 detects GHSA-xhv5-w9c5-2r2w across Maven dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.