CVE-2021-4070Network attack vector

Client crash via truncated VMess SwitchAccount command

Published Feb 23, 2022 · Updated Aug 3, 2024

Off-by-one error in V2Ray Core before 4.44.0 allows remote attackers controlling a VMess server to crash connecting clients. CommandSwitchAccountFactory.Unmarshal accepts a one-byte-short SwitchAccount command because its length check permits reading data[timeStart] when no byte remains. Exploitation requires a client to connect to an attacker-controlled VMess server; the result is a client-process crash without data disclosure or code execution.

CVSS severity9.1
Critical
EPSS probability0.86%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Off-by-one error in V2Ray Core before 4.44.0 allows remote attackers controlling a VMess server to crash connecting clients. CommandSwitchAccountFactory.Unmarshal accepts a one-byte-short SwitchAccount command because its length check permits reading data[timeStart] when no byte remains. Exploitation requires a client to connect to an attacker-controlled VMess server; the result is a client-process crash without data disclosure or code execution.

The record

CVE
CVE-2021-4070
Published
Feb 23, 2022
Updated
Aug 3, 2024
Vendor
V2Fly
Product
V2Ray Core
Classifications
CWE-193, T1499
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Feb 23, 2022CVE 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=unspecified <4.44.0

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

V2Fly · V2Ray Coreversion=unspecified <4.44.0

Published CVSS scores

6.4NIST NVDAV:N/AC:L/Au:N/C:P/I:N/A:P
5.9Protect AI (formerly huntr.dev)CVSS:3.0/AV:L/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:L
9.1NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/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-193Off-by-one Error
T1499Endpoint Denial of Service

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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