CVE-2021-21820Public exploitNetwork attack vector

Remote code execution via default Libcli password

Published Jul 16, 2021 · Updated Aug 3, 2024

Hard-coded credentials in D-Link DIR-3040 1.13B03 allow remote attackers to execute code through the Libcli test environment. An unauthenticated request to /start_telnet activates a hidden Telnet service, which accepts a published default password and exposes command-capable Libcli access. The attack requires network access to the router and no prior account; successful login exposes system and reboot commands.

CVSS severity9.8
Critical
EPSS probability2.96%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Hard-coded credentials in D-Link DIR-3040 1.13B03 allow remote attackers to execute code through the Libcli test environment. An unauthenticated request to /start_telnet activates a hidden Telnet service, which accepts a published default password and exposes command-capable Libcli access. The attack requires network access to the router and no prior account; successful login exposes system and reboot commands.

The record

CVE
CVE-2021-21820
Published
Jul 16, 2021
Updated
Aug 3, 2024
Vendor
D-Link
Product
DIR-3040
Classifications
CWE-798, T1078.001
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jul 16, 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?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=D-LINK DIR-3040 1.13B03

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

D-Link · DIR-3040version=D-LINK DIR-3040 1.13B03

Published CVSS scores

7.5NIST NVDAV:N/AC:L/Au:N/C:P/I:P/A:P
9.8NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
10.0Cisco TalosCVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/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.

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-798Use of Hard-coded Credentials
T1078.001Valid Accounts Default Accounts

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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