CVE-2021-42911Public exploitNetwork attack vector

Unauthenticated code execution via CGI format string

Published Mar 29, 2022 · Updated Aug 4, 2024

Format string flaws in DrayTek Vigor 2960, Vigor 3900, and Vigor 300B 1.5.1.3 and earlier allow remote attackers to execute code. The mainfunction.cgi handler treats attacker-controlled malformed query-string data as a format string rather than fixed data. A crafted HTTP request reaches the router web interface without authentication, enabling code execution in the CGI process with no user interaction.

CVSS severity9.8
Critical
EPSS probability3.52%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Format string flaws in DrayTek Vigor 2960, Vigor 3900, and Vigor 300B 1.5.1.3 and earlier allow remote attackers to execute code. The mainfunction.cgi handler treats attacker-controlled malformed query-string data as a format string rather than fixed data. A crafted HTTP request reaches the router web interface without authentication, enabling code execution in the CGI process with no user interaction.

The record

CVE
CVE-2021-42911
Published
Mar 29, 2022
Updated
Aug 4, 2024
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-134, T1190
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Mar 29, 2022CVE publishedPublication date reported by the CVE source.
  2. Aug 4, 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 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

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-134Use of Externally-Controlled Format String
T1190Exploit Public-Facing Application

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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