CVE-2021-21419Network attack vector

Denial of service via unbounded WebSocket frames

Published May 7, 2021 · Updated Aug 3, 2024

Denial of service in Eventlet 0.10.0 through 0.30.3 allows remote attackers to exhaust process memory through crafted WebSocket frames. In eventlet/websocket.py, RFC6455WebSocket accepts an unbounded declared frame length and decompresses permessage-deflate data without limiting output. Any unauthenticated peer that can reach an Eventlet WebSocket endpoint can exhaust the process memory and interrupt service without affecting confidentiality or integrity.

CVSS severity5.3
Medium
EPSS probability1.81%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
Coverage for this vulnerability

See if you're affected

Explore vulnerability checks for your environment with Hinoki.

Book a demo

Summary

What happened

Denial of service in Eventlet 0.10.0 through 0.30.3 allows remote attackers to exhaust process memory through crafted WebSocket frames. In eventlet/websocket.py, RFC6455WebSocket accepts an unbounded declared frame length and decompresses permessage-deflate data without limiting output. Any unauthenticated peer that can reach an Eventlet WebSocket endpoint can exhaust the process memory and interrupt service without affecting confidentiality or integrity.

The record

CVE
CVE-2021-21419
Published
May 7, 2021
Updated
Aug 3, 2024
Vendor
Eventlet
Product
Eventlet
Classifications
CWE-409, CWE-789, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. May 7, 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

Compare your product and version with the public record. A matching version still requires validation against your environment.

Is a vulnerable build present?

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

  1. Affected versionversion=>= 0.10, < 0.31.0

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Eventlet · Eventletversion=>= 0.10, < 0.31.0

Published CVSS scores

5.3GitHubCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
5.3NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
5.0NVDAV:N/AC:L/Au:N/C:N/I:N/A:P

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-409Improper Handling of Highly Compressed Data (Data Amplification)
CWE-789Memory Allocation with Excessive Size Value
T1499.004Application or System Exploitation

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

See the directory against your own environment.

Your stack

Check the software in your environment

Book a demo to see how Hinoki identifies affected software and validates exploitability in your environment.

Book a demo