Exploited Hunt for compromise (CISA)CVE from 2021, added in 2026

CVE-2021-27137

DD-WRT · Home and small-office routers, cameras and IoT

DD-WRT Stack-Based Buffer Overflow Vulnerability

At a glance

Exploited. Added to CISA’s catalog of exploited vulnerabilities on July 21, 2026.

CISA asks you to hunt for signs of compromise: patching is not enough if the device has already been breached.

Affected product: DD-WRT (category Home and small-office routers, cameras and IoT, indicative classification). Name in CISA’s catalog: DD-WRT DD-WRT.

What should I do?

Apply the patches or mitigations from DD-WRT; if none are available, stop using the product. Start with internet-facing devices.

CISA also asks you to hunt for signs of compromise: patching is not enough if the device has already been breached.

Deadline set by CISA for US federal agencies: 3 days (due July 24, 2026). It only binds those agencies, but it is a useful measure of urgency.

The vendor’s specific advisory is listed in the NVD references: CVE-2021-27137 on the NVD website.

Original text of the required action (CISA)

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.

Timeline

July 16, 2026 Vulnerability published Publication date (NVD)
July 21, 2026 Exploitation confirmed Added to CISA’s KEV catalog
July 24, 2026 CISA deadline Remediation deadline set for US federal agencies

The facts, with their sources

Exploitation confirmed: Yes, on July 21, 2026 Date added to the catalog of exploited vulnerabilities. Exploitation itself may have started earlier. Source: KEV catalog, collected September 29, 2026 at 14:57 UTC
Ransomware: No known use to date That does not guarantee it is not being used: the information is not public. Source: KEV catalog, collected September 29, 2026 at 14:57 UTC
Hunt for compromise: Requested by CISA Source: KEV catalog, collected September 29, 2026 at 14:57 UTC
CISA required action: Apply the patches or mitigations from DD-WRT; if none are available, stop using the product. Start with internet-facing devices. Rewritten by this site from CISA’s original text. Source: KEV catalog, collected September 29, 2026 at 14:57 UTC
CISA deadline: July 24, 2026, 3 days Set for US federal agencies: a measure of urgency, not an obligation elsewhere. Source: KEV catalog, collected September 29, 2026 at 14:57 UTC
Vulnerability published: July 16, 2026 Source: National Vulnerability Database (NVD), collected September 29, 2026 at 15:00 UTC
Product according to CISA: DD-WRT DD-WRT Filed by this site under “Home and small-office routers, cameras and IoT” (indicative classification). Source: KEV catalog, collected September 29, 2026 at 14:57 UTC
CVSS vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H Technical detail of the score: attack vector, complexity, impact. Source: National Vulnerability Database (NVD), collected September 29, 2026 at 15:00 UTC

Severity and activity

8.1 / 10 CVSS severity: high

Score from the vendor or CISA, relayed by the NVD. Collected Sep 29, 2026.

— Probability of exploitation in the next 30 days (EPSS)

FIRST’s EPSS model. Since exploitation is already confirmed, this figure is mostly useful to compare activity. Collected Sep 29, 2026.

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).

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.

Summary from CISA (KEV catalog).

CERT-FR advisories

No advisory among the latest CERT-FR publications this site follows. That does not mean there are none.

Information reused under the Open Licence 2.0 (Etalab); the date of last update is shown for each advisory. This site is neither affiliated with nor endorsed by ANSSI.

Sources for this page

CISA KEV catalog: exploitation, date added, required action, deadline, ransomware.
NVD (NIST): CVSS severity, publication date, description, vendor references.
EPSS (FIRST): probability of exploitation.
CERT-FR: advisories and alerts (in French).
Brand and category: indicative classification by this site (method).