DD-WRT Stack-Based Buffer Overflow
CVE-2021-27137 is a high-severity stack-based buffer overflow vulnerability in DD-WRT, with a CVSS 3.1 base score of 8.1 out of 10. It can be exploited remotely over the network by an unauthenticated attacker, without any action from a user. The record lists version 45724 as the first without the flaw, so updating DD-WRT to that release or later removes the exposure. CISA added it to the Known Exploited Vulnerabilities catalogue on July 21, 2026, which means attacks have been observed in the wild.
Attackers are already exploiting this vulnerability, according to CISA.
What to do
- Update DD-WRT to 45724 or a later version.
- 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 evaluating each asset's internet exposure and ensuring adherence to BOD 26-04 patching guidelines.
- Until you can update, restrict network access to the affected service so it is not reachable from untrusted networks.
Affected products and versions
Version data comes from the organisation that assigned the CVE. Confirm against the vendor advisory before deciding you are unaffected.
How the severity score breaks down
- Attack vector
- NetworkCan be exploited remotely over a network, including the internet.
- Attack complexity
- HighSuccess depends on conditions outside the attacker's control.
- Privileges required
- NoneNo account or login is needed.
- User interaction
- NoneNo action by a victim is required.
- Scope
- UnchangedImpact stays within the vulnerable component.
- Confidentiality
- HighAll data handled by the component can be read.
- Integrity
- HighData or code can be modified at will.
- Availability
- HighThe component can be taken down completely.
Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H. Scored by CNA (mitre.org).
Official 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).
Source: the CVE record as published by its numbering authority, via NVD and CVE.org.
The underlying weakness: Stack-based Buffer Overflow
The product writes past the end of a buffer that was allocated on the stack.
How developers prevent this class of flaw
- Use automatic buffer overflow detection mechanisms that are offered by certain compilers or compiler extensions. Examples include: the Microsoft Visual Studio /GS flag, Fedora/Red Hat FORTIFY_SOURCE GCC flag, StackGuard, and ProPolice, which provide various mechanisms including canary-based detection and range/index checking. D3-SFCV (Stack Frame Canary Validation) from D3FEND discusses canary-based detection in detail.
- Use an abstraction library to abstract away risky APIs. Not a complete solution.
- Implement and perform bounds checking on input.
Timeline
- CVE record published.
- CISA published its SSVC assessment (exploitation: active, automatable: no, technical impact: total).
- Added to CISA's Known Exploited Vulnerabilities catalogue.
- Record last changed materially (score, affected products, status or references).
- Deadline for US federal agencies to remediate, set by CISA.
References
Patches and release notes
Third-party and government advisories
- cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-27137US Government Resource
Technical write-ups (may include proof of concept)
- securityaffairs.com/193290/uncategorized/iot-botnet-c0xmo-adds-competitor-killing-capability.htmlExploit, Third Party Advisory
- ssd-disclosure.com/ssd-advisory-dd-wrt-upnp-buffer-overflow/Exploit, Third Party Advisory
- bleepingcomputer.com/news/security/c0xmo-botnet-spreads-via-dd-wrt-router-flaw-kills-rival-malware/Exploit, Third Party Advisory
- fortinet.com/blog/threat-research/inside-cross-platform-propagation-of-new-gafgyt-variant-c0xmoExploit
Other vulnerabilities in DD-WRT
- 9.8 Critical severity
Recent Stack-based Buffer Overflow vulnerabilities
- 7.5 High severity
- 7.5 High severity
- 8.3 High severity
- 7.8 High severity
- 7.5 High severity
- 7.4 High severity
Spotted an error? Tell us.