CVE-2021-27137
8.1, high Known exploitedDD-WRT Stack-Based Buffer Overflow Vulnerability
- CVSS
- 8.1, high high CVSS v3.1
- EPSS, next 30 days
- 4% higher than 90.3% of scored CVEs
- Exploitation
- Confirmed in the wild
- Published
- 16 Jul 20263 months ago
Known exploited
CISA added this to the Known Exploited Vulnerabilities Catalog on 21 Jul 2026. US federal agencies had to remediate by 24 Jul 2026, a deadline that has passed.
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.
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).
Details
- Vendor
- DD-WRT
- Product
- DD-WRT
- Weakness
- CWE-121 (opens in a new tab)
- Status
- Analyzed
- Updated
- 22 Jul 2026
- Assigner
- cve@mitre.org
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
References 6
Exploit code 4
- securityaffairs.com /193290/uncategorized/iot-botnet-c0xmo-adds-competitor-killing-capability.html (opens in a new tab)
- ssd-disclosure.com /ssd-advisory-dd-wrt-upnp-buffer-overflow/ (opens in a new tab)
- bleepingcomputer.com /news/security/c0xmo-botnet-spreads-via-dd-wrt-router-flaw-kills-rival-malware/ (opens in a new tab)
- fortinet.com /blog/threat-research/inside-cross-platform-propagation-of-new-gafgyt-variant-c0xmo (opens in a new tab)
Patches and fixes 1
The kind is read from NVD's tags where present and from the shape of the URL otherwise, so treat it as a hint.
Beyond this page
Is CVE-2021-27137 in your network?
This page tells you how dangerous CVE-2021-27137 is out there. Whether it sits on one of your devices, only a scan of your own network can tell. That is what our systems do, every day, for the companies we protect.
CVE-2021-27137 is in the CISA catalogue of exploited vulnerabilities: attackers are already exploiting it.
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EXYS7600 OCELOT
Find it before attackers do
Ocelot scans your network on a schedule and reports every vulnerability it finds by its CVE identifier, with its CVSS score and how many of your hosts are affected. A finding like CVE-2021-27137 comes with the devices to patch.
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EXYS7950 KONDRA
Detect it if it's exploited
Until the patch is applied, Kondra monitors your network, events and endpoints in real time. It keeps an inventory of the software installed on every endpoint, and its anomaly detection rules flag what an attacker does once inside.
Learn more about EXYS7950 KONDRA
Assembled from the NVD 2.0 API, the CISA Known Exploited Vulnerabilities Catalog and FIRST EPSS. Scores are as of our last collection on 17 Oct 2026, not the live upstream state.