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Known Exploited Vulnerability
8.1
HIGH CVSS 3.1
CVE-2021-27137
DD-WRT Stack-Based Buffer Overflow Vulnerability - [Actively Exploited]
Overview
Description

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).

Details
INFO

Published Date :

July 16, 2026, 6:16 p.m.

Last Modified :

July 22, 2026, 5:17 a.m.

Remotely Exploit :

Yes !

Source :

[email protected]
CISA Notification
CISA KEV (Known Exploited Vulnerabilities)

For the benefit of the cybersecurity community and network defenders—and to help every organization better manage vulnerabilities and keep pace with threat activity—CISA maintains the authoritative source of vulnerabilities that have been exploited in the wild.

Description :

DD-WRT contains a stack-based buffer overflow vulnerability that could allow an unauthenticated attacker to overflow an internal buffer used by UPnP and trigger a code execution vulnerability.

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.

Known Ransomware Campaign Use:

Unknown

Notes :

This vulnerability affects a common open-source component, third-party library, proprietary implementation, or a protocol used by different products. Please check with specific vendors for information on patching status. For more information, please see: https://svn.dd-wrt.com/changeset/45724 ; BOD 26-04: https://www.cisa.gov/news-events/directives/bod-26-04-prioritizing-security-updates-based-risk ; Forensics Triage Requirements: https://www.cisa.gov/news-events/directives/bod-26-04-implementation-guidance-prioritizing-security-updates-based-risk ; https://nvd.nist.gov/vuln/detail/CVE-2021-27137

Impact
Affected Products

The following products are affected by CVE-2021-27137 vulnerability. Even if cvefeed.io is aware of the exact versions of the products that are affected, the information is not represented in the table below.

ID Vendor Product Action
1 Dd-wrt dd-wrt
2 Dd-wrt dd-wrt
: Total Affected Vendor : 1 | Products : 2
Scoring
CVSS Scores
The Common Vulnerability Scoring System is a standardized framework for assessing the severity of vulnerabilities in software and systems. We collect and displays CVSS scores from various sources for each CVE.
Score Version Severity Vector Exploitability Score Impact Score Source
CVSS 134c704f-9b21-4f2e-91b3-4a467353bcc0
CVSS 3.1 HIGH 8254265b-2729-46b6-b9e3-3dfca2d5bfca
CVSS 3.1 HIGH [email protected]
CVSS 3.1 HIGH MITRE-CVE
Solution
Update DD-WRT to version 45724 or later to fix buffer overflow vulnerabilities in UPnP handling.
  • Update DD-WRT firmware to version 45724 or later.
  • Disable UPnP functionality if not required.
  • Restrict UPnP access to trusted internal networks.
References
References to Advisories, Solutions, and Tools

Here, you will find a curated list of external links that provide in-depth information, practical solutions, and valuable tools related to CVE-2021-27137.

URL Resource
https://securityaffairs.com/193290/uncategorized/iot-botnet-c0xmo-adds-competitor-killing-capability.html Exploit Third Party Advisory
https://ssd-disclosure.com/ssd-advisory-dd-wrt-upnp-buffer-overflow/ Exploit Third Party Advisory
https://svn.dd-wrt.com/changeset/45724 Patch
https://www.bleepingcomputer.com/news/security/c0xmo-botnet-spreads-via-dd-wrt-router-flaw-kills-rival-malware/ Exploit Third Party Advisory
https://www.fortinet.com/blog/threat-research/inside-cross-platform-propagation-of-new-gafgyt-variant-c0xmo Exploit
https://ssd-disclosure.com/ssd-advisory-dd-wrt-upnp-buffer-overflow/ Exploit Third Party Advisory
https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-27137 US Government Resource
CWE - Common Weakness Enumeration

While CVE identifies specific instances of vulnerabilities, CWE categorizes the common flaws or weaknesses that can lead to vulnerabilities. CVE-2021-27137 is associated with the following CWEs:

Common Attack Pattern Enumeration and Classification (CAPEC)

Common Attack Pattern Enumeration and Classification (CAPEC) stores attack patterns, which are descriptions of the common attributes and approaches employed by adversaries to exploit the CVE-2021-27137 weaknesses.

We scan GitHub repositories to detect new proof-of-concept exploits. Following list is a collection of public exploits and proof-of-concepts, which have been published on GitHub (sorted by the most recently updated).

Results are limited to the first 15 repositories due to potential performance issues.

The following list is the news that have been mention CVE-2021-27137 vulnerability anywhere in the article.

  • TheCyberThrone
CISA adds six vulnerabilities to KEV – July 2026

On 21 and 22 July 2026, the Cybersecurity and Infrastructure Security Agency (CISA) expanded its Known Exploited Vulnerabilities (KEV) Catalog by adding six vulnerabilities spanning enterprise collabo ... Read more

Published Date: Jul 23, 2026 (1 week, 6 days ago)
  • The Hacker News
Hackers Exploit Windmill Flaw to Read Arbitrary Server Files Without Authentication

A high-severity security flaw impacting open-source developer platform Windmill has come under active exploitation in the wild, per VulnCheck. The vulnerability in question is CVE-2026-29059 (CVSS sco ... Read more

Published Date: Jul 22, 2026 (2 weeks ago)

Results are limited to the first 20 news articles due to potential performance issues.

The following table lists the changes that have been made to the CVE-2021-27137 vulnerability over time.

Vulnerability history details can be useful for understanding the evolution of a vulnerability, and for identifying the most recent changes that may impact the vulnerability's severity, exploitability, or other characteristics.

  • CVE Modified by 134c704f-9b21-4f2e-91b3-4a467353bcc0

    Jul. 22, 2026

    Action Type Old Value New Value
    Changed SSVC {'id': 'CVE-2021-27137', 'role': 'CISA Coordinator', 'options': [{'exploitation': 'active'}, {'automatable': 'no'}, {'technicalImpact': 'total'}], 'version': '2.0.3', 'timestamp': '2026-07-21T15:45:25.742968Z'} {'id': 'CVE-2021-27137', 'role': 'CISA Coordinator', 'options': [{'exploitation': 'active'}, {'automatable': 'no'}, {'technicalImpact': 'total'}], 'version': '2.0.3', 'timestamp': '2026-07-21T00:00:00+00:00'}
  • Initial Analysis by [email protected]

    Jul. 21, 2026

    Action Type Old Value New Value
    Added CPE Configuration OR *cpe:2.3:o:dd-wrt:dd-wrt:*:*:*:*:*:*:*:* versions up to (excluding) 45724
    Added Reference Type MITRE: https://securityaffairs.com/193290/uncategorized/iot-botnet-c0xmo-adds-competitor-killing-capability.html Types: Exploit, Third Party Advisory
    Added Reference Type MITRE: https://ssd-disclosure.com/ssd-advisory-dd-wrt-upnp-buffer-overflow/ Types: Exploit, Third Party Advisory
    Added Reference Type MITRE: https://svn.dd-wrt.com/changeset/45724 Types: Patch
    Added Reference Type MITRE: https://www.bleepingcomputer.com/news/security/c0xmo-botnet-spreads-via-dd-wrt-router-flaw-kills-rival-malware/ Types: Exploit, Third Party Advisory
    Added Reference Type MITRE: https://www.fortinet.com/blog/threat-research/inside-cross-platform-propagation-of-new-gafgyt-variant-c0xmo Types: Exploit
    Added Reference Type CISA-ADP: https://ssd-disclosure.com/ssd-advisory-dd-wrt-upnp-buffer-overflow/ Types: Exploit, Third Party Advisory
    Added Reference Type CISA-ADP: https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-27137 Types: US Government Resource
  • CVE Modified by 134c704f-9b21-4f2e-91b3-4a467353bcc0

    Jul. 21, 2026

    Action Type Old Value New Value
    Added Reference https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-27137
    Changed SSVC {'id': 'CVE-2021-27137', 'role': 'CISA Coordinator', 'options': [{'exploitation': 'poc'}, {'automatable': 'no'}, {'technicalImpact': 'total'}], 'version': '2.0.3', 'timestamp': '2026-07-16T18:24:17.054985Z'} {'id': 'CVE-2021-27137', 'role': 'CISA Coordinator', 'options': [{'exploitation': 'active'}, {'automatable': 'no'}, {'technicalImpact': 'total'}], 'version': '2.0.3', 'timestamp': '2026-07-21T15:45:25.742968Z'}
  • CVE CISA KEV Update by 9119a7d8-5eab-497f-8521-727c672e3725

    Jul. 21, 2026

    Action Type Old Value New Value
    Added Date Added 2026-07-21
    Added Due Date 2026-07-21
    Added Required Action 2026-07-21
    Added Vulnerability Name 2026-07-21
  • CVE Modified by 134c704f-9b21-4f2e-91b3-4a467353bcc0

    Jul. 16, 2026

    Action Type Old Value New Value
    Added Reference https://ssd-disclosure.com/ssd-advisory-dd-wrt-upnp-buffer-overflow/
    Added SSVC {'id': 'CVE-2021-27137', 'role': 'CISA Coordinator', 'options': [{'exploitation': 'poc'}, {'automatable': 'no'}, {'technicalImpact': 'total'}], 'version': '2.0.3', 'timestamp': '2026-07-16T18:24:17.054985Z'}
  • New CVE Received by [email protected]

    Jul. 16, 2026

    Action Type Old Value New Value
    Added Affected [{'vendor': 'DD-WRT', 'product': 'DD-WRT', 'versions': [{'status': 'affected', 'version': '0', 'lessThan': '45724', 'versionType': 'custom'}], 'defaultStatus': 'unaffected'}]
    Added Description 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).
    Added CVSS V3.1 AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
    Added CWE CWE-121
    Added Reference https://securityaffairs.com/193290/uncategorized/iot-botnet-c0xmo-adds-competitor-killing-capability.html
    Added Reference https://ssd-disclosure.com/ssd-advisory-dd-wrt-upnp-buffer-overflow/
    Added Reference https://svn.dd-wrt.com/changeset/45724
    Added Reference https://www.bleepingcomputer.com/news/security/c0xmo-botnet-spreads-via-dd-wrt-router-flaw-kills-rival-malware/
    Added Reference https://www.fortinet.com/blog/threat-research/inside-cross-platform-propagation-of-new-gafgyt-variant-c0xmo
EPSS is a daily estimate of the probability of exploitation activity being observed over the next 30 days. Following chart shows the EPSS score history of the vulnerability.
CVSS
Vulnerability Scoring Details
Base CVSS Score: 8.1
Attack Vector
Attack Complexity
Privileges Required
User Interaction
Scope
Confidentiality Impact
Integrity Impact
Availability Impact