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Vulnerabilities

Vulnerability · CVE-2021-42911

DrayTek vigor2960_firmware Use of Externally-Controlled Format String

Published Nov 5, 2026 · catalog updated Nov 5, 2026 · DrayTek

Last checked Nov 8, 2026. A connected source was read within the last 7 days.

Patch now

CVE-2021-42911 is in the CISA known-exploited catalog. Confirm DrayTek against the vendor advisory and apply that fix. The CVSS score does not change the priority.

Same product and reporting

Severity
Unknown—
Known exploitation
CISA KEV and VulnCheck KEV
EPSS
3.52%
Remediation
Remediation guidance
Affected products
stated vigor2960_firmware · vendor DrayTek vigor2960_firmware

A Format String vulnerability exists in DrayTek Vigor 2960 <= 1.5.1.3, DrayTek Vigor 3900 <= 1.5.1.3, and DrayTek Vigor 300B <= 1.5.1.3 in the mainfunction.cgi file via a crafted HTTP message containing malformed QUERY STRING, which could let a remote malicious user execute arbitrary code. VulnCheck canary evidence.

Are you affected?

Enter the version you run. The result uses only the ranges stored on this record.

The check returns affected, not affected, or incomplete. It does not guess a version that is missing from the record.

  • stated vigor2960_firmware<= 1.5.1
  • vendor DrayTek vigor2960_firmwareversion unspecified

How to fix CVE-2021-42911

  • Vendor action

    Apply remediations or mitigations per vendor instructions or discontinue use of the product if remediation or mitigations are unavailable.

  • Version check

    Affected <= 1.5.1

  • Advisory

    Open the cve_org record and follow its fix.

  • Official advisories

What to do

Three questions, each answered from this record only. Confirm the vendor advisory before you patch.

Should you patch?

CVE-2021-42911 is in the CISA known-exploited catalog. Confirm DrayTek against the vendor advisory and apply that fix. The CVSS score does not change the priority.

Which versions are affected?

Compare what you have installed with <= 1.5.1. Apply remediations or mitigations per vendor instructions or discontinue use of the product if remediation or mitigations are unavailable.

What should an analyst verify?

No weakness class is stored. EPSS 3.52%. The CISA KEV listing is the exploitation evidence. No Exploit-DB or Metasploit reference is linked. Use the source links for the original advisory.

Code

The project this record applies to. Exploit listings stay in References.

No public source repository is identified for this product.

Timeline

Detection, updates, and resolution for this record.

  1. Nov 5, 2026 · VulnCheck KEV

    Detected

    A Format String vulnerability exists in DrayTek Vigor 2960 <= 1.5.1.3, DrayTek Vigor 3900 <= 1.5.1.3, and DrayTek Vigor 300B <= 1.5.1.3 in the mainfunction.cgi file via a crafted HTTP message containing malformed QUERY STRING, which could let a remote malicious user execute arbitrary code. VulnCheck canary evidence.

  2. Nov 5, 2026 · CISA KEV

    Known exploited

    Apply remediations or mitigations per vendor instructions or discontinue use of the product if remediation or mitigations are unavailable.

Sources

  • CVE.orgVulnerability recordCVE-2021-42911

    Official CVE identifier

    cve.org
  • VulnCheck KEVConfirmed exploitationCVE-2021-42911

    A Format String vulnerability exists in DrayTek Vigor 2960 <= 1.5.1.3, DrayTek Vigor 3900 <= 1.5.1.3, and DrayTek Vigor 300B <= 1.5.1.3 in the mainfunction.cgi file via a crafted HTTP message containing malformed QUERY STRING, which could let a remote malicious user execute arbit

    vulncheck.com

References

Freshness

Last verified in this catalog Nov 8, 2026. Significant claims (KEV, EPSS, CVSS, affected ranges) come from the source rows above — open the original publisher for the authoritative advisory.

  • VulnCheck KEVNov 8, 2026
  • CVE.orgNov 6, 2026