CVE-2021-20594Public exploitNetwork attack vector

Username disclosure via login-name brute force

Published Aug 6, 2021 · Updated Aug 3, 2024

Information disclosure in MELSEC iQ-R CPU firmware allows remote attackers to enumerate registered usernames through login requests. The unauthenticated MELSOFT Username Login command accepts repeated candidate names and reveals which guesses correspond to registered accounts. Network access to Safety CPU firmware 26 and earlier or SIL2 Process CPU firmware 11 and earlier is sufficient; a disclosed username does not permit login without its password.

CVSS severity5.9
Medium
EPSS probability2.37%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Information disclosure in MELSEC iQ-R CPU firmware allows remote attackers to enumerate registered usernames through login requests. The unauthenticated MELSOFT Username Login command accepts repeated candidate names and reveals which guesses correspond to registered accounts. Network access to Safety CPU firmware 26 and earlier or SIL2 Process CPU firmware 11 and earlier is sufficient; a disclosed username does not permit login without its password.

The record

CVE
CVE-2021-20594
Published
Aug 6, 2021
Updated
Aug 3, 2024
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-200, T1087
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Aug 6, 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?

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

7.5NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N
5.9JPCERT/CC and IPACVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N
5.0NIST NVDAV:N/AC:L/Au:N/C:P/I:N/A:N

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-200Exposure of Sensitive Information to an Unauthorized Actor
T1087Account Discovery

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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