CVE-2021-27137

HIGH CISA KEV
Published Jul 16, 2026 Modified Jul 21, 2026 CWE-121

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).

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CVSS v3.1 Score

8.1
HIGH
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H

EPSS — Exploit Prediction

0.0545
Probability of exploitation
0.92%
Percentile rank

EPSS estimates the probability that this vulnerability will be exploited in the wild within the next 30 days. A higher score means more likely to be exploited.

CISA Known Exploited Vulnerability

This vulnerability is actively exploited in the wild.

Added: Jul 21, 2026 Remediation due: Jul 24, 2026

Weakness Type (CWE)

CWE-121 CWE-121

Affected Products

Vendor Product
dd-wrt dd-wrt

References

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). It has a CVSS v3.1 base score of 8.1 (HIGH). This vulnerability is listed in CISA's Known Exploited Vulnerabilities catalog, indicating active exploitation in the wild.
How severe is CVE-2021-27137? +
CVE-2021-27137 has a CVSS v3.1 score of 8.1 out of 10, rated HIGH. This is a high-severity vulnerability that should be prioritized for patching. The EPSS score is 0.0545, placing it in the 1th percentile for exploitation probability.
What products are affected by CVE-2021-27137? +
CVE-2021-27137 affects products from dd-wrt, specifically: dd-wrt. Check the affected products table above for specific version ranges.
How do I check if I'm vulnerable to CVE-2021-27137? +
You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

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