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CVE-2021-21294

24
FAUCET Score

CVE-2021-21294 is a denial-of-service vulnerability affecting Http4s (http4s-blaze-server) versions prior to 0.21.17, 0.22.0-M2, and 1.0.0-M14. The vulnerability stems from Blaze-core's unbounded acceptance of connections on its selector pool, allowing an attacker to exhaust system resources by holding open numerous connections. This can lead to service degradation and denial-of-service, with a CVSS score of 7.5 (HIGH). There is no evidence of active exploitation, publicly available exploit code, or significant community discussion surrounding this CVE.

Impacted Technologies

VendorProductVersion(s)CPE
< 0.21.17CPE matchmatch criteria
cpe:2.3:a:typelevel:http4s:*:*:*:*:*:*:*:*

CVSS Data

CVSS version used by this source: 3.1

7.5HIGH

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Attack Vector
NETWORK
Attack Complexity
LOW
Privileges Required
NONE
User Interaction
NONE
Scope
UNCHANGED
Confidentiality Impact
NONE
Integrity Impact
NONE
Availability Impact
HIGH
Exploitability Score
3.9
Impact Score
3.6
CvssVersion
3.1

Exploit Intelligence

EPSS Score
2.15%
Probability of exploitation in next 30 days
EPSS Percentile
80.2%
Percentile rank of EPSS score among Peer Group
As of 2026-07-27
Model: v2026.06.15
This CVE's current EPSS score of 0.0215 is in the 65th percentile among its peer group of 51,506 CVEs.

Social Chatter

The average CVE in this peer group has 0.0 Twitter, 0.0 Reddit, 0.1 Bluesky, 0.1 Mastodon, and 0.4 GitHub mentions.

Media Mentions

No media coverage found for this CVE.

The average CVE in this peer group has 0.1 InfoSec Media, 0.0 Vendor Blog, and 0.0 Security Researcher mentions.

Remediation

Patch Available

Vendor Patches (3)

github_advisorypatch availablevia nvd_reference
View patch
mavenpatch availablevia ghsa
Product: org.http4s:http4s-blaze-server_2.12Fixed in: 0.21.17
mavenpatch availablevia ghsa
Product: org.http4s:http4s-blaze-server_2.13Fixed in: 0.21.17

Vendor Advisories (1)

mavenGHSA-xhv5-w9c5-2r2whigh

Unbounded connection acceptance in http4s-blaze-server

Feb 2, 2021

References

github.com / http4s/blaze/security/advisories/GHSA-xmw9-q7x9-j5qc
Third Party Advisory
github.com / http4s/http4s/commit/987d6589ef79545b9bb2324ac4bdebf82d9a0171
PatchThird Party Advisory
github.com / http4s/http4s/security/advisories/GHSA-xhv5-w9c5-2r2w
Third Party Advisory