~/exploited / cve-2021-27137

CVE-2021-27137: DD-WRT

According to CISA this vulnerability is already used in attacks (added on 21 July 2026).

Key facts

Description (CISA)

DD-WRT contains a stack-based buffer overflow vulnerability that could allow an unauthenticated attacker to overflow an internal buffer used by UPnP and trigger a code execution vulnerability.

Required action (CISA)

Apply mitigations in accordance with vendor instructions, ensuring compliance with CISA’s BOD 26-04 Prioritizing Security Updates Based on Risk (see URL in Notes) guidance and CISA’s “Forensics Triage Requirements” (see URL in Notes). Follow applicable BOD 26-04 guidance for cloud services or discontinue use of the product if mitigations are unavailable. Stakeholders are responsible for evaluating each asset's internet exposure and ensuring adherence to BOD 26-04 patching guidelines.

What admins should do now

1. Affected? Check whether DD-WRT is in use and which version runs (package manager, admin UI, --version).
2. Apply the vendor patch or workaround as soon as it exists. If that is not possible right away: restrict outside access (firewall, VPN, reverse proxy).
3. Review logs back to 21 July 2026: unusual logins, new files, unexpected processes, outbound connections.
4. If you suspect a compromise: isolate the system, rotate credentials, check the last backup from before the incident.

The deadline applies to US federal agencies. It is still a useful yardstick for everyone: when a flaw is attacked in the wild, waiting is not an option.

Sources

• CISA KEV Catalog
• NVD CVE-2021-27137
• svn.dd-wrt.com/changeset/45724
• www.cisa.gov/news-events/directives/bod-26-04-prioritizing-security-updates-base
• www.cisa.gov/news-events/directives/bod-26-04-implementation-guidance-prioritizi
• nvd.nist.gov/vuln/detail/CVE-2021-27137

Data: CISA Known Exploited Vulnerabilities Catalog, as of 30 October 2026. No guarantee — always check the vendor's advisory.

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