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)
- 4.0%
- Status
- kev
- CISA patch-by (BOD 22-01)
- Published
Is CVE-2021-27137 being exploited?
Yes. CISA added CVE-2021-27137 to its Known Exploited Vulnerabilities catalog on . CISA only lists a CVE when it has reliable evidence of exploitation in the wild.
FIRST's EPSS model puts the probability of exploitation activity in the next 30 days at 4.0% (scored October 14, 2026).
- CISA KEV[ LISTED ]
- ENISA EUVD (exploited)[ — ]
- CIRCL KEV[ — ]
Catalog snapshot 2026-11-13. Full cross-catalog view: /kev-comparison
CVE-2021-27137 timeline
Every event links to its source. Catalog dates are each catalog's own; day offsets count from NVD publication. Data: JSON
What to do
US federal civilian agencies must remediate by (3 days after listing) under BOD 22-01. Everyone else: that deadline is a sensible upper bound, not a target. Vendor guidance and the NVD record are linked under Source above.
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.
