CVE DatabaseCVE-2021-27561

Updated 2026-06-17한국어 권고문 →

CVE-2021-275619.8/CRITICALCISA KEVCWE-78CWE-78

Yealink Device Management (DM) Unauthenticated Remote Root Command Injection Vulnerability (CVE-2021-27561) Security Advisory

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Affected Vendors

yealink

Affected Components

device management

Published Date

2021-10-15

CVSS v3.1

9.8

CRITICAL

EPSS (30d)

82.9%

FIRST.org

CISA KEV

Exploited

NVD Source ↗

Executive & SecOps Summary

SECOPS ASSESSMENT

Affected Scope

Yealink Device Management (DM) server version 3.6.0.20 and prior.

Impact

Unauthenticated remote attackers can execute arbitrary system commands with root privileges by sending crafted requests to a vulnerable API endpoint.

Mitigation

Immediately apply the latest official security patches and restrict external internet access to the management console.

Affected Products & Version Scope

Yealink Device Management (DM) version 3.6.0.20 and prior.

Remediation & Live Workaround Commands

PATCH & WORKAROUND

1. Official Patch & Version Upgrade Commands

Apply the official security update provided by Yealink. Before upgrading, ensure you back up your current configuration and stop the active services.

bash

# Stop the Yealink DM service and create a backup archive
sudo systemctl stop yealink-dm.service
tar -czvf yealink_dm_backup_$(date +%F).tar.gz /usr/local/yealink/dm

# Install the updated package provided by Yealink support
# wget <Yealink_Official_Patch_URL>
sudo dpkg -i yealink-dm-upgrade-package.deb

# Restart the service and verify its status
sudo systemctl start yealink-dm.service
sudo systemctl status yealink-dm.service

2. Temporary Workaround & Hardening Configuration

If patching cannot be applied immediately, restrict access to the vulnerable API endpoint using a reverse proxy or firewall rules.

nginx

# Nginx reverse proxy hardening configuration (/etc/nginx/conf.d/yealink.conf)
server {
    listen 443 ssl;
    server_name yealink-dm.local;

    # Block external access to the vulnerable API endpoint
    location /sm/api/v1/firewall/zone/services {
        allow 127.0.0.1;
        allow 10.0.0.0/8; # Restrict to trusted internal admin networks
        deny all;
        proxy_pass http://127.0.0.1:8080;
    }

    location / {
        proxy_pass http://127.0.0.1:8080;
    }
}

3. Log Audit & IoC Verification Commands

Inspect web server access logs for unauthorized requests targeting the vulnerable URI containing command injection patterns.

bash

# Scan web server access logs for exploitation attempts targeting the specific URI
echo '=== Scanning Logs for Exploitation Attempts ==='
grep -E '/sm/api/v1/firewall/zone/services' /var/log/nginx/access.log /var/log/httpd/access.log 2>/dev/null | grep -E '(%3B|%26|%7C|;|\&|\|)'

# Check for suspicious active processes running under root privileges
ps aux --sort=-%cpu | grep -E '(sh|bash|curl|wget|nc)' | grep -v grep

Interactive SecOps Remediation Checklist

0/4 (0%)

Recommended Security Control & Mitigation Partner

NordLayer / Zero-Trust Network Access

As this vulnerability is actively exploited in the wild (CISA KEV), immediately isolate exposed Yealink DM servers from the public internet and apply the official patch.

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Technical Root Cause & Exploitation Mechanics

1. Vulnerability Overview & Root Cause

This vulnerability (CVE-2021-27561) is a critical OS Command Injection (CWE-78) flaw in the Yealink Device Management (DM) platform. The root cause lies in the improper validation and sanitization of user-supplied input within the /sm/api/v1/firewall/zone/services URI endpoint. Because the application directly passes unsanitized input parameters into system shell execution functions, attackers can inject arbitrary shell commands. Crucially, this endpoint does not enforce authentication, allowing remote attackers to exploit it without any credentials.

2. Exploitation Mechanics & Attack Scenario

An attacker exploits this vulnerability by sending a specially crafted HTTP POST or PUT request to the /sm/api/v1/firewall/zone/services endpoint. By embedding shell metacharacters (such as ;, &&, or |) followed by malicious payloads inside the request parameters, the attacker forces the backend server to execute the injected commands. Since the web application process runs with elevated privileges, the commands are executed with full root authority, enabling complete system compromise without requiring any prior authentication.

3. Infrastructure & Business Impact

With a CVSS score of 9.8 (Critical) and its inclusion in the CISA Known Exploited Vulnerabilities (KEV) catalog, this vulnerability poses an immediate threat and is actively exploited in the wild. Successful exploitation grants the attacker complete control over the DM server, allowing them to establish persistence, pivot laterally into the internal corporate network, and potentially compromise managed endpoint devices (such as IP phones) to intercept communications or steal sensitive credentials.

Official Vendor Advisories & References

NVD Original English Description

Yealink Device Management (DM) 3.6.0.20 allows command injection as root via the /sm/api/v1/firewall/zone/services URI, without authentication.

https://ssd-disclosure.com/?p=4688

Source: cve@mitre.org · Third Party Advisory

https://ssd-disclosure.com/?p=4688

Source: af854a3a-2127-422b-91ae-364da2661108 · Third Party Advisory

https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-27561

Source: 134c704f-9b21-4f2e-91b3-4a467353bcc0 · US Government Resource

Frequently Asked SecOps Questions (CVE-2021-27561)

FAQ

Q1. How critical is CVE-2021-27561 and is it actively exploited in the wild?

CVE-2021-27561 is rated CVSS 9.8 (CRITICAL) with a 30-day EPSS exploitation probability of 82.9%. It is listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, requiring immediate emergency remediation.

Q2. Which vendors and products are affected by CVE-2021-27561?

Primary impacted vendors include yealink across device management. Scope summary: Yealink Device Management (DM) version 3.6.0.20 and prior.

Q3. What temporary mitigations or workarounds exist for CVE-2021-27561 before patching?

[Mitigation] Immediately apply the latest official security patches and restrict external internet access to the management console. Follow the CLI remediation commands and interactive SecOps checklist on this page to restrict exposure prior to your maintenance window.

SECOPS WORKBENCH INTEGRATION