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CVE-2021-27137

HIGH 8.1 KNOWN EXPLOITED ALL-YEARS

Published 2026-07-16 · Last modified 2026-07-22 · mitre · NVD: Analyzed

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).

ACTIVELY EXPLOITED

Confirmed exploited in the wild — in CISA KEV since 2026-07-21. Patch or mitigate now.

Exploitation likelihood

4.0%chance of exploitation in 30 days · 90th percentile

● In CISA KEV (2026-07-21)○ Catalogued PoC◆ SSVC: exploitation active, automatable no

Impact if exploited

8.1CVSS 3.1 · HIGH · CNA

  • ConfidentialityHigh
  • IntegrityHigh
  • AvailabilityHigh

What an attacker needs

  • ✓Access: Reachable over the network — no local access needed
  • ✓Privileges: No account or privileges required
  • ✓User interaction: No user interaction needed
  • ⚠Complexity: Needs a race window or specific setup

✓ lowers the bar for an attacker · ⚠ raises it

Proof of concept

No public PoC is currently catalogued.

Search for new PoCs

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Tracked sources are checked weekly for the full archive, including PoCs published after the original disclosure. A later release appears after a tracked source lists it and a refresh succeeds.

Data snapshot:

Affected

Vendors Dd Wrt

Products Dd Wrt

Weakness (CWE)

  • CWE-121: Stack-based Buffer Overflow

CVSS vector

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

References and research data (JSON)

Additional exploitation reports

Source-specific reports, separate from CISA KEV. Repeated observations from one source are not independent confirmations.

  • VulnCheck KEV · 2026-06-03T00:00:00Z

    Source guidance: Apply mitigations in accordance with vendor instructions, ensuring compliance with CISA’s BOD 26-04 Prioritizing Security Updates Based on Risk (see URL in Notes) guidance and CISA’s “Forensics Triage Requirements” (see URL in Notes). Follow applicable BOD 26-04 guidance for cloud services or discontinue use of the product if mitigations are unavailable. Stakeholders are responsible for evaluating each asset's internet exposure and ensuring adherence to BOD 26-04 patching guidelines.

    Source ransomware assessment: Unknown

    VulnCheck KEV · CVE-2021-27137 · Updated 2026-06-03T00:00:00Z
  • VulnCheck KEV · 2026-06-03T00:00:00ZVulnCheck KEV · CVE-2021-27137
  • VulnCheck KEV · 2026-07-10T00:00:00ZVulnCheck KEV · CVE-2021-27137
  • VulnCheck KEV · 2026-07-21T00:00:00ZVulnCheck KEV · CVE-2021-27137
    VulnCheck KEV · CVE-2021-27137
  • VulnCheck KEV · 2026-08-07T00:00:00ZVulnCheck KEV · CVE-2021-27137

Known Affected Software Configurations

VendorProductVersion range
Dd WrtDd Wrt< 45724

All CVSS metrics

  • HIGH 8.1 v3.1 · CNA Primary
    CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
  • HIGH 8.1 v3.1 · NVD Secondary
    CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H

References

Exploit-tagged references (not proof of a PoC)

Patches & mitigations

Advisories

Technical & other

Sources: NVD · CVE.org · All sources and attribution · EPSS