CVE-2021-32640Public exploitNetwork attack vector

Service slowdown via backtracking subprotocol-header parsing

Published May 25, 2021 · Updated Aug 3, 2024

Regular expression denial of service in WebSockets ws allows remote attackers to slow servers via a crafted Sec-WebSocket-Protocol header. The WebSocketServer upgrade path trims and splits that header with a backtracking expression whose processing cost grows sharply on long whitespace runs. An unauthenticated WebSocket handshake can trigger service degradation, but the documented consequence is slowdown rather than a crash or data access.

CVSS severity5.3
Medium
EPSS probability2.82%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Regular expression denial of service in WebSockets ws allows remote attackers to slow servers via a crafted Sec-WebSocket-Protocol header. The WebSocketServer upgrade path trims and splits that header with a backtracking expression whose processing cost grows sharply on long whitespace runs. An unauthenticated WebSocket handshake can trigger service degradation, but the documented consequence is slowdown rather than a crash or data access.

The record

CVE
CVE-2021-32640
Published
May 25, 2021
Updated
Aug 3, 2024
Vendor
WebSockets
Product
ws
Classifications
CWE-400, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. May 25, 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=>= 5.0.0 <= 7.4.5

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

WebSockets · wsversion=>= 5.0.0 <= 7.4.5

Published CVSS scores

5.3GitHub, Inc.CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
5.0NIST NVDAV: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 exploitPublished

Weakness, pattern, technique

CWE-400Uncontrolled Resource Consumption
T1499.004Application or System Exploitation

Public exploit references

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

Technologies

Your stack

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Your stack

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