CVE-2021-40516Network attack vector

Unauthenticated crash via unchecked WebSocket frame lengths

Published Sep 5, 2021 · Updated Aug 4, 2024

Out-of-bounds read in the WeeChat Relay plugin before 3.2.1 allows remote attackers to crash WeeChat with a malformed WebSocket frame. relay_websocket_decode_frame trusts frame-length fields and reads the extended length, four-byte mask, and payload without checking each access against the remaining receive buffer. A listening Relay endpoint is required, but the frame is processed before client authentication even when a Relay password is configured; the bounded consequence is a process crash.

CVSS severity7.5
High
EPSS probability1.59%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Out-of-bounds read in the WeeChat Relay plugin before 3.2.1 allows remote attackers to crash WeeChat with a malformed WebSocket frame. relay_websocket_decode_frame trusts frame-length fields and reads the extended length, four-byte mask, and payload without checking each access against the remaining receive buffer. A listening Relay endpoint is required, but the frame is processed before client authentication even when a Relay password is configured; the bounded consequence is a process crash.

The record

CVE
CVE-2021-40516
Published
Sep 5, 2021
Updated
Aug 4, 2024
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-125, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Sep 5, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 4, 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?

Affected buildAffected product and version details are unavailable.

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Affected products and versions are unavailable in this record.

Published CVSS scores

7.5NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
5.0NVDAV:N/AC:L/Au:N/C:N/I:N/A:P
7.5WeeChatCVSS: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.

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No observations available

Sep 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-125Out-of-bounds Read
T1499.004Application or System Exploitation

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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