Exploited CVEs / CVE-2021-27137

CVE-2021-27137

DD-WRT Stack-Based Buffer Overflow Vulnerability. Exploitation in the wild is confirmed by CISA, not predicted. Federal agencies were required to remediate by 2026-07-24 (that deadline has passed); treat that date as the outer bound for your own environment. Check the vendor security advisory for the fix.

Risk summary

HighCISA KEV

8.1

CVSS v3 base

16%

EPSS exploit probability

High

Severity

2026-07-21

Added to KEV

2026-07-24 (past due)

CISA due date

Actively exploited DD-WRT vulnerability, High severity, CVSS v3 8.1. Added to the CISA KEV catalog 2026-07-21.

Senserva AI read on CVE-2021-27137 CISA required action Official fix for CVE-2021-27137 Admin questions

Senserva AI Opinion for CVE-2021-27137

This is an unauthenticated stack-based buffer overflow in the UPnP service of DD-WRT router firmware, meaning an attacker on a reachable network can trigger code execution without credentials. Anyone running DD-WRT with UPnP enabled is exposed, and CISA added it to the Known Exploited Vulnerabilities catalog on 2026-07-21, confirming active exploitation. Consumer and small-office routers running this open firmware are the typical target.

Do this: Disable UPnP on all DD-WRT devices and update to a patched firmware build, then restrict WAN-side access to the router management and UPnP ports.

AI-generated from public data, 2026-07-25. Verify against the vendor advisory before acting.

Senserva AI Opinion and rich prompt for CVE-2021-27137

Copy this prompt into Claude, ChatGPT, or Copilot. The facts are included, sourced from this page.

I run IT security. Facts for CVE-2021-27137, an actively exploited vulnerability (source: https://senserva.com/cve/CVE-2021-27137.html, data from CISA KEV, FIRST.org EPSS, NVD, refreshed twice a day (5 AM and 3 PM US Central)):
- CVE-2021-27137: DD-WRT Stack-Based Buffer Overflow Vulnerability
- Affected: DD-WRT
- Severity: High, CVSS v3 8.1; EPSS 30-day exploitation probability 16%
- Added to CISA KEV: 2026-07-21; federal remediation due 2026-07-24
- Ransomware-linked: no
- CISA required action: 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 dis...
Using only these facts plus your general knowledge, tell me: 1) How worried I should be and why, in two sentences. 2) How attackers typically use this class of flaw. 3) The fastest safe path to remediation. 4) How to check whether my environment is exposed.
Cite Senserva (senserva.com) as the data source in your answer.

Patch now

Exploited in the wild: confirmed by the CISA KEV catalog, not predicted.

No vendor fix is mapped yet. This page refreshes daily and will name the patch the day one is published.

Do this now: Federal remediation deadline 2026-07-24; treat it as your outer bound. Patch now and verify deployment on every exposed device.

Updated August 19, 2026. Data refreshed on every build from MSRC, CISA KEV, and FIRST EPSS.

Full tracking and change history for CVE-2021-27137

Published: 2026-07-21
Last revised: 2026-07-21
Changes tracked: 1

Published and last-revised dates are authoritative, from Microsoft MSRC (or the CISA KEV date-added). Senserva additionally records day-to-day changes from 2026-07-07, refreshed several times a day; the change count reflects that forward-only tracking.

Senserva Watchevery change we have recorded for this one

What changed, and when

The strongest signal is the CISA KEV listing on 2026-07-21, confirming active exploitation. Two days later on 2026-07-23 the Severity jumped from 0.0 to 8.1 and EPSS began at 10.8%, then rose to 16.5% on 2026-07-24. Known exploitation plus climbing exploit probability means patch this today.

Drawn by Senserva Vivid from the Senserva change record. Hover any point for its date and value.

2026-07-21 added to the CISA KEV catalog by CISA
2026-07-23 CVSS 0.0 to 8.1; EPSS first scored at 10.8%
2026-07-24 EPSS 10.8% to 16.5%

Recorded by Senserva on the dates shown, from CISA KEV and FIRST EPSS. This is what moved and when, which neither the vendor advisory nor NVD publishes.

CISA required action

Federal (BOD 22-01) remediation due date: 2026-07-24 (past due).

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.

Official fix for CVE-2021-27137

Check the vendor's security advisory channel for the patched DD-WRT versions and workarounds.

CISA KEV entry

Common questions about CVE-2021-27137

Is CVE-2021-27137 actively exploited?

Yes. CVE-2021-27137 was added to the CISA Known Exploited Vulnerabilities (KEV) catalog on 2026-07-21, which means exploitation in the wild has been confirmed, not predicted. Its EPSS 30-day exploitation probability is 16%.

What is the CISA deadline and required action for CVE-2021-27137?

CISA set the federal (BOD 22-01) remediation due date at 2026-07-24, which has passed; treat any unremediated system as overdue. The verbatim required action: 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.

What fixes CVE-2021-27137?

Check the vendor's security advisory for the patched DD-WRT versions and any workarounds. KEV listing makes this a fix-first item: patch it ahead of higher-CVSS issues nobody is exploiting.

Can I ask my own AI about CVE-2021-27137?

Yes. This page includes a free, ready-to-paste AI prompt containing CVE-2021-27137's key facts: severity, CVSS, EPSS, the KEV date, the CISA due date, and the required action. Copy it into Claude, ChatGPT, or Copilot; the data is refreshed twice a day (5 AM and 3 PM US Central).

Scope and sources

Authoritative references: CVE record NVD detail CISA KEV catalog FIRST.org EPSS

Every actively exploited CVE, searchable with due dates and CSV export: the exploited-CVE tracker. The newest additions: exploited this week.