PatchTheCVECVE-2021-27137: DD-WRT Stack-Based Buffer Overflow Vulnerability
CVE-2021-27137 is a high-severity flaw (CVSS 8.1) affecting dd-wrt, reported as exploited in the wild. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.
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).
Why this priority
- High severity
- EPSS top 10%: very likely to be exploited
- Exploited in the wild (CISA KEV)
- Public exploit reference
- Remotely exploitable, no login, no user action
A fixed release or distro patch exists.
Risk facts
| CVSS score | 8.1 (High) |
| Attack vector | Network (remote) |
| Attack complexity | High |
| Privileges needed | None |
| User interaction | None |
| EPSS exploit chance | 4.0% (top 10%) |
| CISA exploited list | Yes, fix due 2026-07-24 |
| Weakness | CWE-121 |
| Published | 2026-07-16 |
| Last modified | 2026-07-22 |
Last updated 2026-07-22 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score
Am I affected?
Timeline
- Published CVE record published
- Exploited Added to the CISA Known Exploited Vulnerabilities list
- Updated Record last modified
- Deadline CISA due date for US federal agencies to patch
Exact fixes by distribution
Public exploits and detection
Public code exists for this CVE. Treat unpatched systems as at risk, and only run PoCs in a lab.
- Write-up: securityaffairs.com/193290/uncategorized/iot-botnet-c0xmo-adds-competitor-killing-capabili
- Write-up: www.bleepingcomputer.com/news/security/c0xmo-botnet-spreads-via-dd-wrt-router-flaw-kills-r
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.
Quick answers
How do I fix CVE-2021-27137?
dd-wrt: upgrade to 45724. Exact commands for each OS are on this page.
Is CVE-2021-27137 being exploited?
Yes. CVE-2021-27137 is on the CISA Known Exploited Vulnerabilities list, so treat it as patch-now.
What does CVE-2021-27137 affect?
CVE-2021-27137 affects dd-wrt. Severity is high (CVSS 8.1). 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. Exploita
How urgent is CVE-2021-27137?
Priority 86/100. High severity; EPSS top 10%: very likely to be exploited; Exploited in the wild (CISA KEV); Public exploit reference; Remotely exploitable, no login, no user action