CVE-2021-21294Network attack vector

Service interruption via unbounded connection acceptance

Published Feb 2, 2021 · Updated Aug 3, 2024

Uncontrolled resource consumption in http4s-blaze-server before 0.21.17 allows remote attackers to interrupt services with open connections. The Blaze selector pool accepts every incoming connection into an unbounded queue, allocating a socket handle before MaxActiveRequests limits request processing. An unauthenticated network path to a Blaze-backed server is sufficient to drain file handles under load, causing availability loss without reported data disclosure or modification.

CVSS severity7.5
High
EPSS probability2.15%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Uncontrolled resource consumption in http4s-blaze-server before 0.21.17 allows remote attackers to interrupt services with open connections. The Blaze selector pool accepts every incoming connection into an unbounded queue, allocating a socket handle before MaxActiveRequests limits request processing. An unauthenticated network path to a Blaze-backed server is sufficient to drain file handles under load, causing availability loss without reported data disclosure or modification.

The record

CVE
CVE-2021-21294
Published
Feb 2, 2021
Updated
Aug 3, 2024
Vendor
http4s
Product
http4s
Classifications
CWE-770, CWE-400, T1499.003
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Feb 2, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=< 0.21.17

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

http4s · http4sversion=< 0.21.17

Published CVSS scores

5.0NIST NVDAV:N/AC:L/Au:N/C:N/I:N/A:P
7.5GitHub, Inc.CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-770Allocation of Resources Without Limits or Throttling
CWE-400Uncontrolled Resource Consumption
T1499.003Application Exhaustion Flood

Public exploit references

No public exploit references are available in this record.

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Technologies

Your stack

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Your stack

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