CVE-2021-0223 in Junos
Summary
by MITRE • 01/16/2021
A local privilege escalation vulnerability in telnetd.real of Juniper Networks Junos OS may allow a locally authenticated shell user to escalate privileges and execute arbitrary commands as root. telnetd.real is shipped with setuid permissions enabled and is owned by the root user, allowing local users to run telnetd.real with root privileges. This issue affects Juniper Networks Junos OS: all versions prior to 15.1R7-S9; 17.3 versions prior to 17.3R3-S11; 17.4 versions prior to 17.4R2-S12, 17.4R3-S3; 18.1 versions prior to 18.1R3-S11; 18.2 versions prior to 18.2R3-S6; 18.3 versions prior to 18.3R2-S4, 18.3R3-S4; 18.4 versions prior to 18.4R2-S7, 18.4R3-S6; 19.1 versions prior to 19.1R2-S2, 19.1R3-S4; 19.2 versions prior to 19.2R1-S6, 19.2R3-S1; 19.3 versions prior to 19.3R3-S1; 19.4 versions prior to 19.4R2-S2, 19.4R3; 20.1 versions prior to 20.1R1-S4, 20.1R2; 20.2 versions prior to 20.2R2.
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Analysis
by VulDB Data Team • 02/15/2021
This vulnerability represents a critical local privilege escalation flaw in Juniper Networks Junos OS operating systems where the telnetd.real binary exhibits dangerous setuid permissions. The vulnerability stems from the telnetd.real component being installed with root ownership and setuid bit enabled, creating an inherent security risk that allows authenticated local users to exploit this configuration. The flaw exists because the system permits local users to execute this privileged binary with elevated privileges, effectively bypassing normal access controls that should prevent non-root users from gaining administrative access. This issue falls under the category of privilege escalation vulnerabilities as defined by CWE-269, specifically CWE-269: "Improper Privilege Management" which occurs when a system fails to properly enforce access controls on privileged operations.
The technical execution of this vulnerability relies on the fundamental flaw in how the telnetd.real binary handles authentication and privilege management. When a local user authenticates through the telnet service and the system invokes telnetd.real with root privileges due to its setuid nature, the attacker can manipulate the execution environment to escalate their privileges. The vulnerability is particularly concerning because it requires only local authentication and does not depend on network connectivity or external attack vectors, making it easier to exploit in environments where local access is possible. The exploitation process typically involves leveraging the setuid binary to execute arbitrary commands with root privileges, which aligns with ATT&CK technique T1068: "Local Privilege Escalation" and specifically targets the privilege escalation path through setuid binaries.
The operational impact of this vulnerability extends beyond simple privilege escalation to encompass complete system compromise. An attacker who gains access to a local shell account can leverage this vulnerability to execute arbitrary code with root privileges, potentially leading to full system control, data exfiltration, or persistence mechanisms establishment. The affected versions span multiple major releases of Junos OS, indicating this was a widespread issue that affected numerous network devices running Juniper's operating system. Network infrastructure devices running vulnerable versions of Junos OS become particularly attractive targets for attackers seeking to establish persistent access to network environments, as these devices often serve as critical points of control within enterprise networks. The vulnerability's presence in telnetd.real specifically impacts devices where telnet services are enabled, which historically were common in network device management but have been largely deprecated in favor of more secure protocols like SSH.
Mitigation strategies for this vulnerability require immediate patching of affected Junos OS versions to the recommended secure releases, as outlined in the CVE description. Organizations should implement comprehensive vulnerability management procedures to identify and remediate all instances of affected software versions across their network infrastructure. Security teams should also conduct thorough audits of setuid binaries on network devices to identify other potential privilege escalation vectors beyond telnetd.real. Additional mitigations include disabling unnecessary telnet services and ensuring that only authorized users have local shell access to network devices. The remediation process should incorporate monitoring for unauthorized privilege escalation attempts and implementation of least privilege principles for local user accounts. Network segmentation and access control measures should be strengthened to limit local access to critical network infrastructure devices. This vulnerability demonstrates the importance of proper privilege management in system design and highlights how seemingly benign security configurations can create significant attack surfaces when not properly secured. The remediation efforts should also include updating security policies to address the identification and handling of setuid binaries in network operating systems, as this vulnerability represents a fundamental design flaw in privilege management that could affect other similar components within the system.