CVE-2021-27137: DD-WRT DD-WRT

Should you act?

Exploited: patch now if you run it

CISA has confirmed exploitation in the wild. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

Yes, if you run DD-WRT DD-WRT. CISA added this CVE to its Known Exploited Vulnerabilities catalog on 2026-07-21, which means there is reliable evidence of exploitation in the wild. US federal agencies must remediate by 2026-07-24; treat that as the outer limit, not the target.
EPSS 4.0%, percentile 90: FIRST’s model estimates a 4.0% chance of exploitation activity in the next 30 days, higher than 90% of all scored CVEs (elevated). EPSS is a probability, not a verdict.
Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
Editorial verdict: Check exposure. DD-WRT CVE-2021-27137: UPnP Overflow Used by C0XMO Botnet — Only routers running an old DD-WRT build with UPnP turned on are at risk; find any you own, update or retire them, and switch UPnP off where nobody needs it.

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 ssdpmsearch (reachable by an M-SEARCH request).

CISA’s 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.

Weakness type: CWE-121.

Product: DD-WRT DD-WRT · Assigned by: mitre · Published: 2026-07-16

Affected and fixed versions

Vendor	Product	Affected versions (per CVE record)
DD-WRT	DD-WRT	< 45724

Severity scores

Scored by	CVSS	Score	Vector
mitre	3.1	8.1 High	`CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H`

Attack vector: network (remotely reachable)
Attack complexity: high (needs special conditions)
Privileges required: none (no login)
User interaction: none
Scope: unchanged

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

CISA KEV: added 2026-07-21; federal due date 2026-07-24. Known ransomware use: Unknown.
CISA 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.
CISA SSVC assessment: exploitation active, automatable no, technical impact total.
FIRST EPSS (model of 2026-10-21): 4.00% probability of exploitation activity in the next 30 days; percentile 90.

Timeline

2021-03-24 — erroneo.us brief: DD-WRT CVE-2021-27137: UPnP Overflow Used by C0XMO Botnet
2026-07-16 — CVE record published by mitre
2026-07-21 — Added to CISA’s Known Exploited Vulnerabilities catalog
2026-07-22 — CVE record last updated
2026-07-24 — CISA remediation deadline for US federal agencies

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Other references from the CVE record and advisory

ssd-disclosure.com ssd-disclosure.com
securityaffairs.com securityaffairs.com
bleepingcomputer.com bleepingcomputer.com
fortinet.com fortinet.com

All exploited DD-WRT vulnerabilities · Today’s Security Digest