CVE-2021-21293

blaze is a Scala library for building asynchronous pipelines, with a focus on network IO. All servers running blaze-core before version 0.14.15 are affected by a vulnerability in which unbounded connection acceptance leads to file handle exhaustion. Blaze, accepts connections unconditionally on a dedicated thread 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. The vast majority of affected users are using it as part of http4s-blaze-server <= 0.21.16. http4s provides a mechanism for limiting open connections, but 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. The issue is fixed in version 0.14.15 for "NIO1SocketServerGroup". A "maxConnections" parameter is added, with a default value of 512. Concurrent connections beyond this limit are rejected. To run unbounded, which is not recommended, set a negative number. The "NIO2SocketServerGroup" has no such setting and is now deprecated. There are several possible workarounds described in the refrenced GitHub Advisory GHSA-xmw9-q7x9-j5qc.

• CWE-400
• CWE-770

• https://github.com/http4s/blaze/commit/4f786177f9fb71ab272f3a5f6c80bca3e5662aa1Patch, Third Party Advisory
• https://github.com/http4s/blaze/security/advisories/GHSA-xmw9-q7x9-j5qcThird Party Advisory
• https://github.com/http4s/http4s/security/advisories/GHSA-xhv5-w9c5-2r2wThird Party Advisory
• https://github.com/http4s/blaze/commit/4f786177f9fb71ab272f3a5f6c80bca3e5662aa1Patch, Third Party Advisory
• https://github.com/http4s/blaze/security/advisories/GHSA-xmw9-q7x9-j5qcThird Party Advisory
• https://github.com/http4s/http4s/security/advisories/GHSA-xhv5-w9c5-2r2wThird Party Advisory

Published

Feb 2, 2021

Last Modified

Jun 17, 2026

Status

Modified

• How Strix found a critical auth bypass in etcdStrix autonomously discovered a critical authentication bypass in etcd, later designated CVE-2026-33413.
• Autonomous PentestingAI agents that find and validate exploitable vulnerabilities like this one across your applications.
• PR ReviewsPentest every pull request so vulnerable code is caught before it ships to production.
• AI Penetration TestingHow AI-driven penetration testing continuously covers your attack surface.

• CVE-2021-21288CarrierWave is an open-source RubyGem which provides a simpl…4.3
• CVE-2021-21289Mechanize is an open-source ruby library that makes automate…8.3
• CVE-2021-2129Vulnerability in the Oracle VM VirtualBox product of Oracle …7.9
• CVE-2021-21290Netty is an open-source, asynchronous event-driven network a…5.5
• CVE-2021-21291OAuth2 Proxy is an open-source reverse proxy and static file…6.1
• CVE-2021-21292Traccar is an open source GPS tracking system. In Traccar be…6.3
• CVE-2021-21294Http4s (http4s-blaze-server) is a minimal, idiomatic Scala i…7.5
• CVE-2021-21295Netty is an open-source, asynchronous event-driven network a…5.9
• CVE-2021-21296Fleet is an open source osquery manager. In Fleet before ver…2.7
• CVE-2021-21297Node-Red is a low-code programming for event-driven applicat…6.5
• CVE-2021-21298Node-Red is a low-code programming for event-driven applicat…6.5
• CVE-2021-21299hyper is an open-source HTTP library for Rust (crates.io). I…8.1