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Q1What is this vulnerability? (Essence + Consequences)
🚨 **Essence**: OS Command Injection in Yealink Device Management. 💥 **Consequences**: Attackers can execute arbitrary commands with **root privileges** on the target device, leading to full system compromise.
Q2Root Cause? (CWE/Flaw)
🛡️ **Root Cause**: Improper input validation in the `/sm/api/v1/firewall/zone/services` endpoint. The software fails to sanitize user input, allowing shell metacharacters to be interpreted as commands.
Q3Who is affected? (Versions/Components)
📦 **Affected**: Yealink Device Management (DM) **Version 3.6.0.20 and earlier**. Specifically targets Yealink Skype For Business IP phones managed by this DM server.
Q4What can hackers do? (Privileges/Data)
🕵️ **Attacker Capabilities**: Can run commands as **root**. This means total control over the device, including reading sensitive data, installing backdoors, or pivoting to internal networks.
Q5Is exploitation threshold high? (Auth/Config)
⚡ **Exploitation Threshold**: **Extremely Low**. The vulnerability requires **NO authentication**. Any unauthenticated user on the network can trigger the injection via the specific URI.
Q6Is there a public Exp? (PoC/Wild Exploitation)
🔍 **Public Exploit**: Yes. A Proof of Concept (PoC) is available via **ProjectDiscovery Nuclei templates**. This makes automated scanning and widespread exploitation highly likely.
Q7How to self-check? (Features/Scanning)
🔎 **Self-Check**: Scan for the specific endpoint `/sm/api/v1/firewall/zone/services`. Use Nuclei or similar tools with the CVE-2021-27561 template to detect vulnerable instances automatically.
Q8Is it fixed officially? (Patch/Mitigation)
🩹 **Official Fix**: Yes. The vendor released patches. You must upgrade Yealink Device Management to a version **newer than 3.6.0.20** to resolve the issue.
Q9What if no patch? (Workaround)
🚧 **No Patch Workaround**: If patching is delayed, **block external access** to the DM server. Restrict network access to the `/sm/api/v1/firewall/zone/services` URI to trusted IPs only via firewall rules.
Q10Is it urgent? (Priority Suggestion)
🔥 **Urgency**: **CRITICAL**. Due to the lack of authentication and root-level impact, this is a high-priority vulnerability. Immediate patching or network isolation is required.