High severity · published Jul 16, 2026
CVE-2021-27137
DD-WRT Stack-Based Buffer Overflow Vulnerability
In wildKEV
8.1
CVSS
4%
EPSS
Yes
On KEV
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).
CVSS vector
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
Attack vector
Network
Complexity
High
Privileges
None
User interaction
None
Confidentiality
High
Integrity
High
Availability
High
Scope
Unchanged
Exploitation
Is it being used?
Exploit prediction (EPSS)4.0% · higher than 90% of CVEs
Exploited in the wildYes
CISA KEVListed Jul 21, 2026
Federal remediation dueJul 24, 2026
Ransomware campaign useNot known
Affected products
Which versions
DD
DD-WRT · DD-WRT
before 45724 · source: vendor, NVD, KEV
In the news
AdvisoryHigh
CISA Adds Four Known Exploited Vulnerabilities to Catalog
CISA has added four new vulnerabilities to its Known Exploited Vulnerabilities (KEV) Catalog , based on evidence of active exploitation.
CVE-2021-27137 DD-WRT Stack-Based Buffer Overflow Vulnerability
CVE-2026-0770 Langflow Inclusion of Functionality from Untrusted Control Sphere Vulnerability
CVE-2026-63030 WordPress Core Interpretation Conflict Vulnerability
CVE-2026-60137 WordPress Core SQL Injection Vulnerability
These types of vulnerabilities are frequent attack vectors for malicious cyber actors and pose significant risks to the federal enterprise.
Binding Operational Directive (BOD) 26-04: Prioritizing Security Updates Based on Risk establishes vulnerability management requirements for Federal Civilian Executive Branch (FCEB) agencies. BOD 26-04 reinforces the importance of the KEV Catalog and requires federal agencies to prioritize rapid remediation of high-risk vulnerabilities, specifically those identified by Common Vulnerabilities and Exposures (CVEs) listed in CISA’s KEV Catalog on publicly exposed assets that grant total control of the asset post-exploitation, while deferring action for lower-risk vulnerabilities. BOD 26-04 further establishes basic expectations for when agencies must check whether threat actors compromised the system before the patch was applied.
While BOD 26-04 applies only to FCEB agencies, CISA encourages all organizations to adopt risk-based vulnerability management and prioritize remediation of KE...
Jul 21, 2026·CISA Advisories
VulnerabilityHigh
[CISA KEV] CVE-2021-27137: DD-WRT DD-WRT - DD-WRT Stack-Based Buffer Overflow Vulnerability
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.
DDDD-WRTCVE-2021-27137
Jul 21, 2026·CISA KEV
ThreatIntelHigh
IoT Botnet C0XMO Adds Competitor-Killing Capability
C0XMO is a new Gafgyt botnet variant exploiting old router flaws, spreading across IoT devices, killing rivals, and enabling large-scale DDoS attacks. In March 2026, FortiGuard Labs discovered a new variant of the Gafgyt botnet, dubbed C0XMO, which is noticeably more capable than its predecessors. The malware spreads through CVE-2021-27137, a stack buffer overflow in […]
Jun 8, 2026·Security Affairs
References
NVD
https://nvd.nist.gov/vuln/detail/CVE-2021-27137
CVE.org
https://www.cve.org/CVERecord?id=CVE-2021-27137
CISA KEV catalogue
https://www.cisa.gov/known-exploited-vulnerabilities-catalog
svn.dd-wrt.com
https://svn.dd-wrt.com/changeset/45724
ssd-disclosure.com
https://ssd-disclosure.com/ssd-advisory-dd-wrt-upnp-buffer-overflow/
securityaffairs.com
https://securityaffairs.com/193290/uncategorized/iot-botnet-c0xmo-adds-competitor-killing-capability.html
www.bleepingcomputer.com
https://www.bleepingcomputer.com/news/security/c0xmo-botnet-spreads-via-dd-wrt-router-flaw-kills-rival-malware/
www.fortinet.com
https://www.fortinet.com/blog/threat-research/inside-cross-platform-propagation-of-new-gafgyt-variant-c0xmo
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