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Exploited vulnerability · CISA KEV

Is CVE-2021-27137 exploited? Yes: in CISA KEV since 21 Jul 2026

Gociux · DD-WRT · data: CISA KEV, CVE record, FIRST EPSS

Yes. CVE-2021-27137, DD-WRT Stack-Based Buffer Overflow Vulnerability, is being exploited in the wild. CISA added it to its Known Exploited Vulnerabilities catalog on 21 Jul 2026, 5 days after it was published. It affects DD-WRT. If you run it, CISA's required action is: 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.

exploitedYesCISA KEV · 21 Jul 2026
ransomwareNot knownno confirmed use
EPSStop 10%more likely than most to be exploited
severity8.1CVSS 3.1 · high
US federal due24 Jul 2026BOD 22-01 deadline

What it is

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.

VendorDD-WRT (1 exploited CVE in KEV)
ProductDD-WRT
VulnerabilityDD-WRT Stack-Based Buffer Overflow Vulnerability
WeaknessCWE-121
Published16 Jul 2026
Added to KEV21 Jul 2026
Reported by (CNA)mitre

What to do

  1. Check whether you run it. Look for DD-WRT in your asset inventory, including appliances and cloud images. Stack check lists every exploited DD-WRT CVE at once.
  2. Apply the fix. 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.
  3. If you can't patch today, take the affected service off the internet or restrict who can reach it, and check its logs for signs of compromise since 16 Jul 2026.
  4. Get told next time. Email alerts when CISA adds a CVE for DD-WRT.

References

Questions

Is CVE-2021-27137 being exploited?

Yes. CISA added CVE-2021-27137 to the Known Exploited Vulnerabilities catalog on 21 Jul 2026, which means there is reliable evidence of exploitation in the wild.

Is CVE-2021-27137 used in ransomware attacks?

CISA does not list known ransomware use for it. That can change, and exploitation alone is reason to fix it first.

How do I fix CVE-2021-27137?

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. Check DD-WRT's advisory for the fixed versions, and if you can't patch quickly, restrict access to the affected DD-WRT and watch it for signs of compromise.

Sources: CISA KEV (exploitation, ransomware use, required action, due date), the CVE record (publication date, CVSS, CNA), NVD and FIRST EPSS (live score). More: all KEV additions in July 2026 · act-now radar.

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