CVE-2026-77539 Overview
CVE-2026-77539 is a command injection vulnerability in Ubiquiti UniFi OS Server caused by improper input validation (CWE-20). An authenticated attacker with high privileges and network access can inject operating system commands that execute on the host device. Successful exploitation yields code execution in the context of the UniFi OS Server process, with a scope change that impacts adjacent components. Ubiquiti disclosed the issue in Security Advisory Bulletin 067.
Critical Impact
A network-reachable attacker holding high-privilege credentials can execute arbitrary commands on UniFi OS Server hosts, compromising confidentiality, integrity, and availability of the managed network infrastructure.
Affected Products
- Ubiquiti UniFi OS Server
- Refer to UI Security Advisory Bulletin 067 for version details
- Environments running UniFi OS Server management software
Discovery Timeline
- 2026-08-26 - CVE-2026-77539 published to NVD
- 2026-08-27 - Last updated in NVD database
Technical Details for CVE-2026-77539
Vulnerability Analysis
The flaw resides in UniFi OS Server input handling paths that pass user-controllable data to underlying system command execution routines. Because the server fails to properly validate or sanitize input, an authenticated administrator can supply crafted values that break out of the intended argument context. The resulting commands run with the privileges of the UniFi OS Server process on the host device.
The scope change reflected in the CVSS vector indicates that exploitation affects resources beyond the vulnerable component itself. In a UniFi deployment, that typically means adjacent managed devices, controller data, and host operating system resources. The vulnerability is classified under CWE-20: Improper Input Validation.
Root Cause
UniFi OS Server accepts input from privileged administrative interfaces and forwards portions of that input to shell or process execution primitives without enforcing strict validation. Neither allowlisting of expected values nor safe argument passing is applied consistently, which allows metacharacters and command separators to alter command semantics.
Attack Vector
Exploitation requires network reachability to the UniFi OS Server management interface and valid high-privilege credentials. An attacker submits crafted parameters through an administrative feature that invokes host commands. The injected payload is concatenated into a command line and executed on the host. See the UI Security Advisory Bulletin 067 for the affected functionality and vendor technical detail.
No verified public proof-of-concept code is currently available.
Detection Methods for CVE-2026-77539
Indicators of Compromise
- Unexpected child processes spawned by the UniFi OS Server service, such as sh, bash, curl, wget, or nc.
- Administrative API or web requests containing shell metacharacters (;, |, &, backticks, $()) in input fields.
- New or modified files in UniFi OS Server directories, cron entries, or systemd units on the controller host.
- Outbound connections from the UniFi host to unfamiliar external IP addresses shortly after administrative activity.
Detection Strategies
- Alert on process lineage where the UniFi OS Server parent process launches interactive shells or network utilities.
- Inspect HTTP request logs for injection patterns targeting administrative endpoints.
- Correlate high-privilege administrator logins with subsequent host command execution events.
Monitoring Recommendations
- Forward UniFi OS Server logs and host process telemetry to a centralized analytics platform.
- Baseline normal administrative behavior and flag deviations in command execution frequency or type.
- Monitor authentication logs for use of privileged accounts from unexpected source networks.
How to Mitigate CVE-2026-77539
Immediate Actions Required
- Apply the fixed UniFi OS Server release referenced in UI Security Advisory Bulletin 067.
- Restrict network access to the UniFi OS Server management interface to trusted administrative networks only.
- Rotate credentials for all high-privilege UniFi accounts and enforce multi-factor authentication.
- Audit recent administrator activity for signs of command injection attempts.
Patch Information
Ubiquiti has published fixed builds through Security Advisory Bulletin 067. Consult the advisory for the specific fixed version corresponding to your UniFi OS Server deployment and upgrade path.
Workarounds
- Block external access to the UniFi OS Server administrative interface at the network perimeter.
- Limit the number of accounts assigned high-privilege roles until patching is complete.
- Place the UniFi OS Server host behind a jump host or VPN that enforces strong authentication.
# Example: restrict management access with host firewall (iptables)
iptables -A INPUT -p tcp --dport 443 -s 10.10.0.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 443 -j DROP
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
