How severe is CVE-2021-21293?

CVE-2021-21293 has a CVSS score of 7.5/10, rated HIGH. Immediate patching is strongly recommended.

Which packages are affected by CVE-2021-21293?

CVE-2021-21293 affects the following packages: org.http4s:blaze-core_2.11 (Maven), org.http4s:blaze-core_2.12 (Maven), org.http4s:blaze-core_2.13 (Maven). Ecosystems affected: Maven.

How do I fix CVE-2021-21293?

Update org.http4s:blaze-core_2.11 to 0.14.15 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-21293 is resolved across your whole dependency graph.

How do I detect CVE-2021-21293 in my Maven dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for org.http4s:blaze-core_2.11, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2021-21293 if there is no patch (or I can't update yet)?

Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.

Is CVE-2021-21293 actively exploited in the wild?

No public exploit code has been indexed for CVE-2021-21293 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2021-21293?

CVE-2021-21293 has an EPSS (Exploit Prediction Scoring System) score of 2.1%, placing it in the 81th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2021-21293?

CVE-2021-21293 is classified as Uncontrolled Resource Consumption (CWE-400), CWE-770 (CWE-770). These weakness types describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2021-21293 published, and has it been updated?

CVE-2021-21293 was published on February 2, 2021 and was last updated on July 8, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

☕

☕ Maven

Not in CISA KEV

HIGH severity

CVE-2021-21293 — blaze-core_2.11

HIGHFix: http4s/blaze@4f78617

CVE-2021-21293 is a high-severity (CVSS 7.5) Uncontrolled Resource Consumption vulnerability in org.http4s:blaze-core_2.11. A fix is available for org.http4s:blaze-core_2.11 — see the affected versions and patch details below.

Unbounded connection acceptance leads to file handle exhaustion

Also known asGHSA-xmw9-q7x9-j5qc

Published

Feb 2, 2021

Updated

Jul 8, 2026

Affected

3 pkgs

Patched

3 / 3

Exploits

None indexed

Exploitation data as of Oct 10, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

2.1%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs81th percentile — riskier than 81% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

CVE-2021-21293 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 385,987 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

3 pkgs affected

☕org.http4s:blaze-core_2.11☕org.http4s:blaze-core_2.12☕org.http4s:blaze-core_2.13

Real-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

All servers running blaze-core <= 0.14.14, including blaze-http and http4s-blaze-server users, are affected.

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.

Patches

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.

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.
A similar sidecar strategy is viable for other non-HTTP services running on blaze-core.
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.

For more information

If you have any questions or comments about this advisory:

Open an issue in http4s/blaze
Contact us according to the http4s security policy

Affected Packages

3 total 3 fixed

Ecosystem	Package	Vulnerable range	Fix
☕Maven	`org.http4s:blaze-core_2.11`	all versions	0.14.15`org.http4s:blaze-core_2.11:0.14.15`
☕Maven	`org.http4s:blaze-core_2.12`	all versions	0.14.15`org.http4s:blaze-core_2.12:0.14.15`
☕Maven	`org.http4s:blaze-core_2.13`	all versions	0.14.15`org.http4s:blaze-core_2.13:0.14.15`

Affected Products

1 product · 1 configurations

Application

blazetypelevel

< 0.14.15

range

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for org.http4s:blaze-core_2.11, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update org.http4s:blaze-core_2.11 to 0.14.15 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-21293 is resolved across your whole dependency graph.
Workarounds
Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.

Frequently Asked Questions

### Impact All servers running blaze-core <= 0.14.14, including blaze-http and http4s-blaze-server users, are affected. 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