CVE-2021-27137

HIGH

8.1

CVSS Score

Published: July 16, 2026 at 12:00 AM UTC

Updated: July 22, 2026 at 05:17 AM UTC

1 product affected across 1 vendorDd Wrt

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Severity

HIGH8.1

Exploitation

CISA KEV

Exploit prediction

EPSS 4%

SSVC

Active

Actively Exploited

CISA Known Exploited Vulnerabilities (KEV) Catalog

EPSS Score4.0%

EPSS (Exploit Prediction Scoring System) estimates a 4.0% chance this CVE is exploited in the wild within the next 30 days — higher than 91% of all CVEs. It complements CVSS: a high EPSS on a mid-severity CVE can still be urgent.

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

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.

Ransomware Use

Unknown

Added to KEV

Jul 21, 2026

Remediation Due

Jul 24, 2026

99 days overdue

Notes

This vulnerability affects a common open-source component, third-party library, proprietary implementation, or a protocol used by different products. Please check with specific vendors for information on patching status. For more information, please see: https://svn.dd-wrt.com/changeset/45724

BOD 26-04: https://www.cisa.gov/news-events/directives/bod-26-04-prioritizing-security-updates-based-risk

Forensics Triage Requirements: https://www.cisa.gov/news-events/directives/bod-26-04-implementation-guidance-prioritizing-security-updates-based-risk

https://nvd.nist.gov/vuln/detail/CVE-2021-27137

Description

An issue was discovered in router/upnp/src/ssdp.c in DD-WRT before 45724. An unsafe strcpy in the UPnP handling functionality allows an unauthenticated remote attacker to send a request that would overflow an internal fixed buffer. Exploitation requires the DD-WRT user to enable UPnP (which is off by default, and only listens on internal interfaces by default). This occurs in ssdp_msearch (reachable by an M-SEARCH request).