DD-WRT SSDP Stack-Based Buffer Overflow (UPnP)
An unsafe strcpy in DD-WRT's SSDP handling lets an unauthenticated attacker overflow an internal buffer via the UPnP listener and trigger code execution. Added to CISA KEV on 2026-07-21.
- Vendor
- DD-WRT
- Product
- DD-WRT router firmware (builds prior to revision 45724)
- CVSS
- 8.1
- EPSS (exploit probability)
- N/A
- Status
- kev
- Published
CVE-2021-27137 is a stack-based buffer overflow in the SSDP handling of DD-WRT, an open-source firmware for consumer and SOHO routers. An unsafe strcpy in router/upnp/src/ssdp.c — specifically inside ssdp_msearch — lets an unauthenticated remote attacker send an M-SEARCH request that overflows a fixed-size internal buffer, enabling code execution in the context of the UPnP daemon.
Exploitation requires the DD-WRT user to have enabled UPnP, which is disabled by default. The SSDP listener also binds only to internal interfaces by default, so external reachability depends on user configuration. The fix landed in DD-WRT revision 45724 in 2021.
Why it mattered
The vulnerability class is well understood — a pre-authentication stack overflow in a network daemon, requiring no credentials and reachable via a single UDP packet where the service is exposed — and DD-WRT is widely deployed on consumer and SMB router hardware that is often installed once and never updated. CISA added the CVE to its Known Exploited Vulnerabilities catalog on 2026-07-21, indicating observed in-the-wild exploitation of the residual unpatched population.
DD-WRT’s changeset r45724 landed the fix, and the NVD record above tracks the definitive metadata.
