CVEs / CVE-2021-27137
CVE-2021-27137: DD-WRT Stack-Based Buffer Overflow Vulnerability
- Known exploited in the CISA KEV catalog since 21 Jul 2026
- high 8.1 CVSS 3.1 (from the CNA)
- EPSS 3.99% chance of exploitation in the next 30 days, higher than 90% of CVEs (30 Oct 2026)
- Last changed at its sources
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Summary
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.
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).
Affected and fixed versions
| Product | Affected | Fixed in |
|---|---|---|
| DD-WRT DD-WRT | before 45724 | 45724 |
Exploitation
| In CISA KEV since | 21 Jul 2026 |
|---|---|
| Due date for US federal agencies | 24 Jul 2026 |
| Used in ransomware campaigns | Unknown |
| 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. |
Severity
| CVSS 3.1 | 8.1 highCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:HScored by the CNA ([email protected]) |
|---|---|
| Weakness | CWE-121 |
References
- CVE-2021-27137 on NVD
- CVE-2021-27137 in the CISA KEV catalog
- https://svn.dd-wrt.com/changeset/45724 (cited by CISA)
- https://www.cisa.gov/news-events/directives/bod-26-04-prioritizing-security-updates-based-risk (cited by CISA)
- https://www.cisa.gov/news-events/directives/bod-26-04-implementation-guidance-prioritizing-security-updates-based-risk (cited by CISA)
- https://securityaffairs.com/193290/uncategorized/iot-botnet-c0xmo-adds-competitor-killing-capability.html Exploit, Third Party Advisory
- https://ssd-disclosure.com/ssd-advisory-dd-wrt-upnp-buffer-overflow/ Exploit, Third Party Advisory
- https://svn.dd-wrt.com/changeset/45724 Patch
- https://www.bleepingcomputer.com/news/security/c0xmo-botnet-spreads-via-dd-wrt-router-flaw-kills-rival-malware/ Exploit, Third Party Advisory
- https://www.fortinet.com/blog/threat-research/inside-cross-platform-propagation-of-new-gafgyt-variant-c0xmo Exploit
- https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-27137 US Government Resource
Sources
Exploitation status from the CISA Known Exploited Vulnerabilities catalog. EPSS scores from FIRST. Descriptions, severity and references from the National Vulnerability Database. Open-source package versions from OSV (CC-BY 4.0). Text is shown as each source states it; Stackurity does not edit it. Check the vendor's advisory before acting.
This product uses data from the NVD API but is not endorsed or certified by the NVD.