CVE-2021-20251Network attack vector

Brute-force access via non-atomic lockout updates

Published Mar 6, 2023 · Updated Mar 6, 2025

Race condition in Samba 4.1 and newer allows remote attackers to bypass bad-password lockouts through concurrent login attempts. The bad-password count path updates the per-account counter non-atomically, so simultaneous failed authentications can pass before excess attempts are refused. Exploitation requires concurrent guesses against the same account and server with lockout enforcement, enabling additional guesses beyond the configured threshold.

CVSS severity5.9
Medium
EPSS probability0.76%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Race condition in Samba 4.1 and newer allows remote attackers to bypass bad-password lockouts through concurrent login attempts. The bad-password count path updates the per-account counter non-atomically, so simultaneous failed authentications can pass before excess attempts are refused. Exploitation requires concurrent guesses against the same account and server with lockout enforcement, enabling additional guesses beyond the configured threshold.

The record

CVE
CVE-2021-20251
Published
Mar 6, 2023
Updated
Mar 6, 2025
Vendor
PPP Project
Product
Samba
Classifications
T1110.001
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Mar 6, 2023CVE publishedPublication date reported by the CVE source.
  2. Mar 6, 2025Record 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=Samba 4.1 and newer

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

PPP Project · Sambaversion=Samba 4.1 and newer

Published CVSS scores

5.9CanonicalCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/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.

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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

T1110.001Password Guessing

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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