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).
The record classifies CVE-2021-27137 under weakness class CWE-121. Its CVSS v3 base vector states that the flaw is reachable remotely over the network, needs no prior authentication, needs no user interaction, and has high impact on confidentiality, integrity, availability. 1 affected-product entry is recorded, across 1 vendor, listed below. The identifier was first published 104 days ago.
Severity and exploitation probability
- CVSS v3.1 base score
- 8.1 — HIGH
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H - EPSS (FIRST)
- 3.9% probability of exploitation in the next 30 days, higher than 90.2% of all scored CVEs
- CISA KEV
- Listed since 2026-07-21, federal remediation deadline 2026-07-24
- Threadlinqs priority
- 9/10 — CISA KEV-listed, which Threadlinqs floors at 9
- Published
- 2026-07-16, last modified 2026-07-22
Is CVE-2021-27137 being exploited?
CISA added CVE-2021-27137 to the Known Exploited Vulnerabilities catalog on 2026-07-21, which means the agency holds evidence of exploitation in the wild; US federal civilian agencies had to remediate it by 2026-07-24 under BOD 22-01. Weaponised exploit code for CVE-2021-27137 is publicly available. 1 public proof-of-concept repository is tracked for this identifier. It currently carries a trending score of 54 in the Threadlinqs vulnerability feed.
- trickest/cve (trickest)
Affected products and versions
- DD-WRT: DD-WRT
How to fix CVE-2021-27137
Because CVE-2021-27137 is KEV-listed, US federal civilian agencies were required to apply the vendor fix, or stop using the product, by 2026-07-24. No vendor patch reference has been recorded for CVE-2021-27137 in the tracked sources. Follow the references below for a fix, and treat the products listed above as exposed until the vendor states otherwise.
Threat activity tracking CVE-2021-27137
1 tracked threat in the Threadlinqs corpus references CVE-2021-27137, either in the campaign’s CVE list or as an indicator on the campaign record.
Sources
Enriched from CVE.org, FIRST EPSS, CISA KEV, GitHub Security Advisories, public proof-of-concept repositories. Last verified by Threadlinqs on .