CVE-2021-20501Network attack vector

Unauthenticated resource exhaustion via nonexistent recipients

Published Apr 21, 2021 · Updated Sep 17, 2024

Denial of service in IBM i 7.1 through 7.4 SMTP allows remote attackers to exhaust disk space and network bandwidth via email. The SMTP server accepts traffic addressed to nonexistent recipients in its local domain, consuming resources and enabling spam delivery. The behavior is reachable without authentication only under an affected non-default SMTP configuration; IBM has not disclosed the responsible code path.

CVSS severity5.9
Medium
EPSS probability1.34%
Next 30 days · Sep 17, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Denial of service in IBM i 7.1 through 7.4 SMTP allows remote attackers to exhaust disk space and network bandwidth via email. The SMTP server accepts traffic addressed to nonexistent recipients in its local domain, consuming resources and enabling spam delivery. The behavior is reachable without authentication only under an affected non-default SMTP configuration; IBM has not disclosed the responsible code path.

The record

CVE
CVE-2021-20501
Published
Apr 21, 2021
Updated
Sep 17, 2024
Vendor
IBM
Product
IBM i
Classifications
T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Apr 21, 2021CVE publishedPublication date reported by the CVE source.
  2. Sep 17, 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=7.1
  2. Affected versionversion=7.2
  3. Affected versionversion=7.3
  4. Affected versionversion=7.4

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

IBM · IBM iversion=7.1; version=7.2; version=7.3; version=7.4

Published CVSS scores

5.9IBM CorporationCVSS:3.0/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
8.2NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:H
6.4NIST NVDAV:N/AC:L/Au:N/C:N/I:P/A:P
5.2IBM CorporationCVSS:3.0/AV:N/C:N/S:U/AC:H/A:H/UI:N/I:N/PR:N/RC:C/E:U/RL:O

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 11, 2026Sep 17, 2026
Latest reporting daySep 17, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

T1499.004Application or System Exploitation

Public exploit references

No public exploit references are available in this record.

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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