CriticalCVE-2021-27561

    Yealink Device Management Server-Side Request Forgery (SSRF) Vulnerability (CVE-2021-27561)

    Published In CISA KEV since Sources: CVE.org

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    What is affected, what to do

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

    The vulnerability is being actively exploited; treat it as a priority. The available sources name no fixed version. Check the vendor guidance in the references below. Immediate actions, mitigation and detection rules are in the runbook below.

    Affected
    • Yealink Device Management lte *..3.6.0.20

    Not sure whether and how you are affected? Ask a question about this vulnerability

    Metrics

    9.8

    Priority

    XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.

    Source: NVD, CVSS v3

    9.8

    CVSS v3

    Severity Critical on the scale from 0 to 10.

    Source: CVE.org

    82.9 %

    EPSS score

    High — model estimates ≥ 50% chance of real-world exploitation within 30 days.

    Source: FIRST EPSS, percentile 99.67

    Show all metrics
    Severity
    Critical
    Priority (0–325)

    The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first?

    Read in glossary
    228.47 Methodology
    Exploit maturity

    An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack.

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    actively exploited
    XIEM Effective CVSS

    XIEM® is the tiered managed-security operating model from NEOSEC and a registered trademark of NEOSEC GmbH. It extends classic SIEM — the central collection, normalisation and correlation of security-relevant events — with endpoint forensics, network visibility and deception technology, and consolidates them into three building tiers: Sentry, Orchestrate and Command.

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    9.8 Methodology
    CVSS v3

    CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions.

    Read in glossary
    9.8
    EPSS score

    The EPSS score is the absolute model output for an individual vulnerability: the estimated probability of real-world exploitation within the next 30 days, expressed as a value between 0 and 1 (i.e. 0 % to 100 %). A score of 0.42 means „the model estimates a 42 % chance of real-world exploitation within 30 days“.

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    82.9 %
    High — model estimates ≥ 50% chance of real-world exploitation within 30 days.
    EPSS percentile

    The EPSS percentile is the relative position of a vulnerability within all EPSS-scored CVEs, expressed as a value between 0 and 1. A percentile of 0.90 means „this CVE has a higher EPSS score than 90 % of all other scored CVEs“. Unlike the EPSS score, it is not a probability — but a positioning within a ranking.

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    99.67
    Critical — this CVE ranks in the top fifth of all CVEs scored today (rank ≥ 80%).
    KEV status

    The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.

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    Actively exploited
    Published
    In CISA KEV since

    The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.

    Read in glossary
    CISA assessment (SSVC)
    Exploitation: active · automatable · Technical impact: total
    Source: CISA Vulnrichment, as of 4 February 2025
    CWE

    CWE is a catalogue of typical weakness types in software and hardware — such as flawed input validation, unsafe memory handling or poor authentication. Where a CVE names a concrete vulnerability in a concrete product, a CWE describes the underlying class of defect.

    Read in glossary
    CWE-78

    Response and mitigation

    Why act now?

    Prioritisation rationale

    The flaw is network-reachable without authentication, trivial to trigger, and yields immediate root execution. Following public disclosure with full technical detail, it sits among the CVEs with very high exploitation likelihood. Typical targets are internet-exposed DM servers at enterprises and service providers. Attackers use them as a foothold into internal networks, for botnet recruitment, and to tamper with managed VoIP endpoints. CISA does not flag known ransomware use. Even so, any internet exposure warrants top priority (remediate within 24 hours); internal-only deployments rate high priority for the next maintenance window.

    1. Runbook step 1

      Immediate response (0-24 h)

      • Find and cut exposure: Inventory all Yealink Device Management (DM) instances, especially version 3.6.0.20. Block any internet-facing web interface at the perimeter firewall immediately. The attack needs no authentication or user interaction and executes commands as root.
      • Apply the vendor update: A patch is available from the vendor; look up the fixed version in Yealink's advisory. Until updated, treat every DM instance on 3.6.0.20 as vulnerable.
      • Triage for compromise before patching: Preserve and review web-server logs for requests to /sm/api/v1/firewall/zone/services. If you find hits, treat the host as compromised: snapshot or image it for forensics rather than just patching it.
      • Rebuild if compromise is suspected: Reinstall from clean media and restore configuration only from a verified backup. In-place cleanup after root-level RCE is not trustworthy.
      • Rotate credentials: Treat the following secrets as exposed and replace them:
        • DM admin accounts,
        • database credentials,
        • SIP/provisioning credentials of managed handsets stored in DM,
        • host SSH keys.
      • Check endpoint provisioning: Review configurations and firmware packages pushed via DM for tampering. A compromised DM server can roll malicious configs out to every managed phone.

    2. Runbook step 2

      Mitigation layers

      • Network segmentation: Make DM reachable only from a dedicated management VLAN or admin jump host. Allow handsets only the provisioning paths they actually need, and block direct access from client or guest networks.
      • WAF/reverse proxy: Block requests to /sm/api/v1/firewall/zone/services outright. If the path is operationally required, reject any parameter containing shell metacharacters: ;, |, &, `, $(, %3B, %7C, %60, %24%28.
      • IPS signature: Deploy a Suricata rule on content:"/sm/api/v1/firewall/zone/services"; http_uri; combined with pcre:"/(%3B|;|%7C|\||%60||%24%28|$()/Ui"`. Run it in alert mode first, then switch to drop on exposed segments.
      • Egress filtering: Restrict outbound traffic from the DM host to an allowlist (update servers, NTP, DNS resolvers). This kills curl/wget payload staging and reverse shells.
      • Host hardening: Where the appliance allows it, stop the web service running as root. Remove unneeded interpreters and download tools. Enable file-integrity monitoring (AIDE/Tripwire) on /etc, /tmp, cron directories and the webroot.

    3. Runbook step 3

      Detection rules

      • Web access logs: Any request to /sm/api/v1/firewall/zone/services from non-admin sources is suspicious; requests with encoded metacharacters are high-confidence. SPL example: index=web uri_path="/sm/api/v1/firewall/zone/services" | regex uri_query="(%3B|;|%7C|%60|%24%28)"
      • Linux auditd: Watch for execve events whose parent is the DM web service (Java/nginx/PHP process) and whose child is sh, bash, curl, wget, nc, python or perl. Rule idea: -a always,exit -F arch=b64 -S execve -F uid=0 -k ydm_exec, then correlate on ppid.
      • EDR process ancestry: The chain <DM web service> → sh -c → curl|wget → chmod +x → /tmp/* indicates payload dropping. As a Sigma shape: ParentImage|endswith (web service) AND Image|endswith: ['/sh','/bash'] AND CommandLine|contains: ['/tmp/','wget','curl'].
      • Network telemetry (Zeek/Suricata): Look for new outbound connections from the DM host to external IPs on unusual ports, or HTTP requests to raw IP addresses. Also watch for DNS lookups of dynamic-DNS domains from the DM segment.
      • Persistence artefacts: Check for new or modified entries in /etc/crontab, /var/spool/cron/, ~/.ssh/authorized_keys and /etc/rc.local on the DM host. Unknown executables in /tmp or /dev/shm are also indicators.

    Detection rules

    No NEOSEC detection rule is available for this vulnerability yet.

    Classification under NIS2 and CRA

    Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.

    NIS2

    NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management.

    Read in glossary
    Actively exploited: treat the vulnerability as a priority in vulnerability handling under Art. 21(2)(e) NIS2. If it is exploited in your environment, check the reporting obligation under Art. 23 (early warning within 24 hours).
    CRA

    The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates.

    Read in glossary
    Since 11.09.2026, manufacturers of affected products with digital elements must report actively exploited vulnerabilities under Art. 14 CRA, with an early warning within 24 hours via ENISA's single reporting platform.
    ISO/IEC 27001

    ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security.

    Read in glossary
    Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.

    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.

    Source: NVD (NIST), cvelistv5, CISA KEV Catalog, CVE.org (MITRE)

    Affected products

    The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.

    Affected products
    Affected versionsFixed in
    yealinkdevice_management
    • ≤ 3.6.0.20
    • —

    Vendors and products

    Weakness classes (CWE)

    • CWE-78Improper Neutralization of Special Elements used in an OS Command ('OS Command Injection')

      Base

      The product constructs all or part of an OS command using externally-influenced input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could modify the intended OS command when it is sent to a downstream component.

      Entry at MITRE CWE-78

    Public exploit references

    Publicly reported proof-of-concepts, exploit modules and detection templates for this vulnerability. NEOSEC links to the original sources only and does not provide any exploit code.

    Public exploit references
    CISA KEV CatalogCISA Known Exploited Vulnerability (opens in a new tab)cisa.govactively exploited

    References & sources

    • ssd-disclosure.com?p=4688x_refsource_MISC, Third-party advisory
    • cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2021-27561government-resource, US government resource
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