CVE-2021-27137

DD-WRT remote code execution, confirmed exploited in the wild.

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.

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4.0%chance of exploitation in the next 30 days (EPSS), 90th percentile

21 Juladded to CISA's Known Exploited Vulnerabilities list, 2026

24 Julfederal remediation deadline, already passed

Passedpast the deadline: patch now

CISA's name for it	DD-WRT Stack-Based Buffer Overflow Vulnerability
Vendor and product	DD-WRT · DD-WRT
Type	remote code execution · CWE-121
Exploited	Confirmed by CISA listed 21 July 2026
Likelihood	more likely to be exploited than most
Deadline	24 July 2026 · already passed
Used in ransomware	Not reported
Advisories	svn.dd-wrt.com ↗ · www.cisa.gov ↗ · www.cisa.gov ↗ · nvd.nist.gov ↗

What to do

Three steps, in this order

Step 1

Patch to the fixed version the vendor names, today if the product faces the internet.

Step 2

Until it is patched, take the affected service off the internet or put it behind a VPN or allow-list.

Step 3

Look for signs it was already used: new accounts, web shells, unexpected processes and outbound connections since the date it was listed.

CISA's required action	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.

CISA's required action

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.

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Questions

CVE-2021-27137 in short

Is CVE-2021-27137 being exploited?

Yes. CISA added CVE-2021-27137 to its Known Exploited Vulnerabilities catalogue on 21 July 2026, which it does only on evidence of exploitation in the wild.

What is the CISA deadline for CVE-2021-27137?

US federal agencies must remediate it by 24 July 2026. Everyone else should treat that date as the latest safe point to patch an internet-facing system.

How likely is CVE-2021-27137 to be exploited in the next 30 days?

FIRST's Exploit Prediction Scoring System gives it 4.0%, the 90th percentile of every CVE it scores.

How do I fix CVE-2021-27137?

Patch to the fixed version the vendor names, today if the product faces the internet. Until it is patched, take the affected service off the internet or put it behind a VPN or allow-list. Look for signs it was already used: new accounts, web shells, unexpected processes and outbound connections since the date it was listed.

Sources: CISA Known Exploited Vulnerabilities catalogue (released 10 November 2026) and FIRST EPSS. Page compiled automatically by SAK T.L.M, the threat-intelligence tool of SAK T.E.C, on 13 November 2026.