How severe is CVE-2021-27137?

CVE-2021-27137 has a CVSS 3.x base score of 8.1, rated high severity. It is exploitable over network with high attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability high.

Is CVE-2021-27137 being actively exploited?

Yes. CVE-2021-27137 is on CISA's Known Exploited Vulnerabilities (KEV) catalog, added on 2026-07-21, which means active exploitation has been confirmed. It should be prioritised for remediation.

What products are affected by CVE-2021-27137?

CVE-2021-27137 affects Dd-wrt. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2021-27137?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Because this CVE is known to be actively exploited, treat remediation as urgent — CISA KEV typically sets a short remediation deadline.

When was CVE-2021-27137 published?

CVE-2021-27137 was published on 2026-07-16 and last updated on 2026-07-22.

CVE-2021-27137

CVE-2021-27137 is a high-severity vulnerability in Dd-wrt with a CVSS 3.x base score of 8.1. It is listed in CISA's Known Exploited Vulnerabilities (KEV) catalog, confirming it has been exploited in the wild (added 2026-07-21). The underlying weakness is classified as CWE-121.

Key facts

Severity: High (CVSS 3.x base score 8.1)
EPSS exploit prediction: 16% (97th percentile)
Actively exploited: Yes — listed in CISA KEV (added 2026-07-21)
Weakness: CWE-121
Affected product: Dd-wrt
Published: 2026-07-16
Last modified: 2026-07-22

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).

Frequently asked questions

What is CVE-2021-27137?: 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).
How severe is CVE-2021-27137?: CVE-2021-27137 has a CVSS 3.x base score of 8.1, rated high severity. It is exploitable over network with high attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability high.
Is CVE-2021-27137 being actively exploited?: Yes. CVE-2021-27137 is on CISA's Known Exploited Vulnerabilities (KEV) catalog, added on 2026-07-21, which means active exploitation has been confirmed. It should be prioritised for remediation.
What products are affected by CVE-2021-27137?: CVE-2021-27137 affects Dd-wrt. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2021-27137?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Because this CVE is known to be actively exploited, treat remediation as urgent — CISA KEV typically sets a short remediation deadline.
When was CVE-2021-27137 published?: CVE-2021-27137 was published on 2026-07-16 and last updated on 2026-07-22.

References

Affected products (1)

cpe:2.3:o:dd-wrt:dd-wrt:*:*:*:*:*:*:*:*

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