Known exploited vulnerability
CVE-2021-27137
The short answer
CVE-2021-27137 affects DD-WRT DD-WRT and is rated high (CVSS 8.1). CISA added it to the Known Exploited Vulnerabilities catalog on 2026-07-21: attackers are using it, so patch it first. The vendor has linked a patch.
What it is
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).
Weakness: CWE-121.
Severity
8.1 high
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
Exploited (CISA KEV)
Yes, since Jul 21, 2026
Federal due date Jul 24, 2026
EPSS, next 30 days
4.0%
Higher than 91% of scored CVEs
How it can be exploited
- Attack vector
- Network
- Attack complexity
- High
- Privileges required
- None
- User interaction
- None
- Scope
- Unchanged
- Confidentiality impact
- High
- Integrity impact
- High
- Availability impact
- High
What CISA says to do
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…
Affected products
DD-WRT. NVD's record lists the exact affected versions.
References
- securityaffairs.com/193290/uncategorized/iot-botnet-c0xmo-adds-competitor-killing-capability.html ExploitThird Party Advisory
- ssd-disclosure.com/ssd-advisory-dd-wrt-upnp-buffer-overflow/ ExploitThird Party Advisory
- svn.dd-wrt.com/changeset/45724 Patch
- www.bleepingcomputer.com/news/security/c0xmo-botnet-spreads-via-dd-wrt-router-flaw-kills-rival-malware/ ExploitThird Party Advisory
- www.fortinet.com/blog/threat-research/inside-cross-platform-propagation-of-new-gafgyt-variant-c0xmo Exploit
- ssd-disclosure.com/ssd-advisory-dd-wrt-upnp-buffer-overflow/ ExploitThird Party Advisory
- www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-27137 US Government Resource
Questions
Is CVE-2021-27137 being exploited?
Yes. CISA added CVE-2021-27137 to its Known Exploited Vulnerabilities catalog on 2026-07-21, with a federal remediation due date of 2026-07-24.
How do I fix CVE-2021-27137?
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…
From NVD, CISA KEV, FIRST EPSS and OSV.dev, checked . Status on NVD: Analyzed. The description is NVD's text, quoted as published.
Related
Latest CVEs this week
Critical and High CVEs from the last 7 days, with KEV and EPSS.
Exploited vulnerabilities by vendor
Every vendor in CISA's KEV catalog, with its count.
DD-WRT exploited vulnerabilities
Everything CISA lists as exploited for DD-WRT.
Vulnerability watch
A briefing and a fix pull request when an advisory hits your dependencies.
Know the day a CVE lands in your code
Connect a GitHub repository and ZeusQA checks its dependencies daily, briefs each advisory and opens the fix. Three test runs free, no card.