CVE-2026-102031

Executive Summary

CVE-2026-102031 is an OS Command Injection vulnerability (CWE-78) in the TACACS component of Juniper Networks Junos OS Evolved, rated CVSS 9.8 (Critical). A low-privileged, TACACS+-authenticated user can execute arbitrary commands with root privileges when periodic TACACS+ authorization-refresh is enabled. Successful exploitation yields full compromise of confidentiality, integrity, and availability of the affected network device, making this a high-priority risk for enterprises relying on TACACS+ centralized authentication to manage Juniper Junos OS Evolved infrastructure.

Technical Analysis

The vulnerability resides in the TACACS subsystem of Junos OS Evolved. TACACS+ (Terminal Access Controller Access-Control System Plus) is a AAA protocol commonly used to centralize authentication, authorization, and accounting for network devices. In this deployment model, a central TACACS+ server issues authorization decisions to the device, and the device enforces them.

The NVD description states that the flaw is an OS Command Injection (CWE-78) triggered when periodic TACACS+ authorization-refresh is enabled. The core mechanism is as follows: when the authorization-refresh feature is active, the device periodically re-queries the TACACS+ server for updated authorization data. During this refresh cycle, input received from the TACACS+ server — or data assembled from it — is passed to an operating-system command interpreter without adequate sanitization or neutralization of control characters (shell metacharacters).

The class-level weakness, CWE-78, means that untrusted input is concatenated into or otherwise reaches a shell/command context on the underlying host OS. Proper defenses for this class require the use of argument-based execution APIs (no shell interpolation), strict allowlisting of expected values, and escaping of shell metacharacters. The absence of such defenses allows an attacker who can influence the data retrieved or verified during the authorization-refresh path to inject arbitrary commands.

The critical privilege escalation aspect is that the injected command is executed with root privileges. The authorization-refresh operation is performed by a privileged daemon or process within Junos OS Evolved; therefore, any command string that is executed inherits that elevated context. The attacker does not need to already hold administrative privileges — they only need a valid, low-privileged TACACS+-authenticated session, as stated in the description. This is what drives the CVSS vector's Impact subscore to High across confidentiality, integrity, and availability.

The CVSS vector is network-exploitable (AV:N), low complexity (AC:L), no privileges required beyond the low-privileged authenticated session described (PR:N), and no user interaction (UI:N). Scope is unchanged (S:U), meaning the impact is confined to the vulnerable component's security authority.

No public proof-of-concept, exploit code, or Nuclei template is available for this CVE, and no affected version list or vendor patch has been published in the data provided. The precise wire-level details of how the injected string reaches the command interpreter (for example, which TACACS+ attribute or response field carries the payload) are not specified in the available description and should not be assumed. What is certain from the description is the trigger condition (periodic TACACS+ authorization-refresh enabled) and the execution context (root).

Exploitation Scenarios

No public PoC is available; the attack chain below is inferred strictly from the vulnerability class and the NVD description. No specific function names, API endpoints, or file paths are asserted because they are not provided in the source data.

1. Reconnaissance / Target Identification: The attacker identifies a target running Juniper Networks Junos OS Evolved that enforces AAA via TACACS+ and has the periodic authorization-refresh feature enabled. This can be inferred from device fingerprinting, the presence of the device in a managed network, and by observing that TACACS+ authentication is enforced. The attacker requires only a low-privileged TACACS+-authenticated account — a common scenario for contractors, junior operations staff, or any user granted limited access.

2. Trigger of the Flaw Class: The attacker authenticates via TACACS+ with a low-privileged account. Because periodic authorization-refresh is enabled, the device will periodically re-request authorization data. The attacker's ability to influence the content that flows through this refresh cycle is the delivery vector for the command-injection payload. At the vulnerability-class level, shell metacharacters embedded in the data processed during the authorization-refresh path are interpreted by the underlying OS command interpreter instead of being treated as inert data.

3. Payload / Execution Method: Consistent with CWE-78, the payload consists of OS command syntax combined with shell metacharacters (for example, command separators or substitution operators) appended to or embedded within a value that reaches the command context. Because the executing process runs as root, the injected command runs with root privileges. The specific parameter carrying the payload is not documented in the available data and is not fabricated here.

4. Post-Exploitation Impact: With root-level command execution on a Junos OS Evolved device, the attacker can read and modify configuration, exfiltrate secrets (including any credentials or keys stored on the device), pivot into the management network, disable logging/auditing, and establish persistence. Given the network position of such devices, this can enable lateral movement to adjacent infrastructure and potential disruption of the routed/forwarded traffic plane (Availability impact is High per CVSS).

Impact Assessment

The CVSS 3.1 base score of 9.8 reflects the maximum-impact profile: AV:N (remotely reachable over the network), AC:L (no special conditions beyond the documented trigger), PR:N (the privilege requirement in the vector is None, consistent with the ability to exploit using a merely low-privileged TACACS+-authenticated account), UI:N (no victim interaction), S:U (impact confined to the vulnerable system's authority domain), and C:H/I:H/A:H (total loss of confidentiality, integrity, and availability).

Business impact is severe. Junos OS Evolved devices typically sit at critical points in enterprise and service-provider networks — routing, switching, and security enforcement. Root compromise of such a device can:

• Expose all configuration and credential material stored on the device.
• Allow persistent, difficult-to-detect modification of network policy and routing.
• Enable the attacker to intercept, redirect, or drop transit traffic (Confidentiality and Availability impact).
• Provide a foothold for lateral movement into the management plane and other managed network elements that trust this device.

The blast radius is proportional to the network role of the compromised device. A core router or a device in a chokepoint can expose a large fraction of an environment's traffic and management trust relationships. Because exploitation requires only a valid low-privileged account, insider threat and compromised-credential scenarios are both directly in scope.

Detection Guidance

Sigma Rule

title: Potential OS Command Injection via Junos OS Evolved TACACS+ Authorization Refresh (CVE-2026-102031)
id: 8b1f4e90-1020-31d1-9a00-juniper0001
status: experimental
description: Detects suspicious OS command execution context originating from TACACS authorization refresh processes on Junos OS Evolved devices. Correlate with Syslog/Audit events.
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-102031
author: Threat Intelligence
date: 2026/01/01
tags:
  - attack.t1190
  - cve.2026-102031
logsource:
  product: juniper
  service: junos
detection:
  selection_authz:
    process|contains:
      - 'tacacs'
      - 'authd'
      - 'aaa'
  selection_shell:
    command_line|contains:
      - ';'
      - '&&'
      - '|'
      - '$('
      - '`'
      - '/bin/sh'
      - '/bin/bash'
  condition: selection_authz and selection_shell
falsepositives:
  - Legitimate administrative scripts invoking shell from AAA-related processes (rare)
level: critical

YARA Rule

rule CVE_2026_102031_TACACS_CMD_INJECTION
{
    meta:
        description = "Detects shell metacharacter injection patterns associated with CVE-2026-102031 (Juniper Junos OS Evolved TACACS OS command injection)"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-102031"
        author = "Threat Intelligence"
        date = "2026-01-01"
        severity = "critical"

    strings:
        $tacacs = "tacacs" ascii wide nocase
        $authz  = "authorization-refresh" ascii wide nocase

        $m1 = ";" ascii
        $m2 = "&&" ascii
        $m3 = "||" ascii
        $m4 = "$(" ascii
        $m5 = "`" ascii
        $sh = "/bin/sh" ascii
        $bash = "/bin/bash" ascii

    condition:
        uint16(0) != 0x5A4D and
        filesize < 2MB and
        $tacacs and
        ($m1 or $m2 or $m3 or $m4 or $m5) and
        ($sh or $bash)
}

Suricata IDS Rule

alert tcp $HOME_NET any -> $HOME_NET 49 (msg:"CVE-2026-102031 Possible TACACS+ Command Injection Attempt"; flow:to_server,established; content:"|3B|"; depth:0; sid:10203101; rev:1; metadata:cve CVE-2026-102031, created_at 2026_01_01; classtype:attempted-admin; reference:cve,CVE-2026-102031;)
alert tcp any 49 -> $HOME_NET any (msg:"CVE-2026-102031 TACACS+ Authz Response with Shell Metacharacters"; flow:to_client,established; content:"|24 28|"; nocase; sid:10203102; rev:1; metadata:cve CVE-2026-102031, created_at 2026_01_01; classtype:attempted-admin; reference:cve,CVE-2026-102031;)

Note: TACACS+ commonly operates on TCP/49. The content matches above look for shell metacharacters (; and $() in raw form) within TACACS+ traffic; tune depth/offset and add additional metacharacter patterns based on your environment.

Elastic Detection

sequence by host.name with maxspan=5m
 [process where process.name like~ ("*tacacs*", "*authd*", "*aaa*")]
 [process where event.type == "start" and
    process.parent.name like~ ("*tacacs*", "*authd*", "*aaa*") and
    process.command_line regex~ """.*(;|&&|\|\||\$\(|`).*""" and
    process.args in ("/bin/sh", "/bin/bash", "sh", "bash")]

Splunk SPL Query

index=juniper sourcetype=junos:*
| eval suspicious=if(match(_raw, "(tacacs|authd|aaa).*(;|&&|\|\||\$\(|`)"), 1, 0)
| where suspicious=1
| stats count values(_raw) as events by host, user
| where count > 0
| sort - count

Tune the regex and sourcetype to your normalized Junos feed. High-value correlation: flag any instance where a TACACS/AAA process spawns a shell during a known authorization-refresh window.

Wazuh OSSEC Rule

<group name="juniper,cve-2026-102031,">
  <rule id="102031" level="14">
    <decoded_as>json</decoded_as>
    <field name="process">\.*(tacacs|authd|aaa)\.*</field>
    <match>;|\&amp;\&amp;|\|\||\$\(|`</match>
    <description>CVE-2026-102031: Possible OS command injection via TACACS authorization refresh on Junos OS Evolved</description>
    <mitre>
      <id>T1190</id>
    </mitre>
    <group>command_injection,critical,</group>
  </rule>

  <rule id="1020312" level="12">
    <if_sid>102031</if_sid>
    <field name="user">\.+</field>
    <description>CVE-2026-102031: Command injection correlated with low-privileged TACACS+ user session</description>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 0/4 (UNPATCHED — no patch evidence from any tracked source).

1. Immediate Mitigations

• Disable periodic TACACS+ authorization-refresh on affected Junos OS Evolved devices if operationally feasible. The NVD description identifies this feature as the precondition for exploitation; disabling it removes the trigger surface.
• Restrict TACACS+ server trust: ensure only hardened, monitored TACACS+ servers can communicate with devices, and enforce mutual authentication/channel integrity where supported.
• Apply strict AAA account hygiene: remove dormant and unnecessary low-privileged TACACS+-authenticated accounts, enforce least privilege, and monitor for anomalous low-privileged logins followed by privileged activity.
• Network segmentation: restrict management-plane access to TACACS+ (TCP/49) and device management interfaces to dedicated, monitored administrative VLANs.

2. Patch / Upgrade Steps

• No vendor patch is currently available per tracked sources (Patch Confidence 0/4). Monitor the Juniper Networks security advisories page for an official Junos OS Evolved fix.
• When a patched release is published, upgrade to the fixed Junos OS Evolved version and verify the TACACS authorization-refresh code path is remediated.
• Until a patch exists, treat affected devices as compromised-able and apply compensating controls above.

3. Long-Term Hardening

• Build a TACACS+/AAA input validation layer: never pass AAA-derived data to shell interpreters; use argument-based execution APIs with strict allowlisting.
• Deploy continuous monitoring for command execution spawned by AAA/authz processes, correlating with TACACS+ session events.
• Institute change control and integrity monitoring on device configurations and running processes to detect post-exploitation persistence.
• Adopt defense-in-depth for management-plane traffic: out-of-band management, jump hosts with logging, and MFA at the TACACS+ layer to make low-privileged account abuse harder.

References

• NVD — CVE-2026-102031: https://nvd.nist.gov/vuln/detail/CVE-2026-102031
• CWE-78 — Improper Neutralization of Special Elements used in an OS Command ('OS Command Injection'): https://cwe.mitre.org/data/definitions/78.html
• MITRE ATT&CK T1190 — Exploit Public-Facing Application: https://attack.mitre.org/techniques/T1190/
• Juniper Networks Security Advisories (monitor for patch): https://supportportal.juniper.net/s/global-search/%40uri
• CISA Known Exploited Vulnerabilities Catalog (not currently listed; verify for updates): https://www.cisa.gov/known-exploited-vulnerabilities-catalog

No public PoC, exploit code, Nuclei template, threat-actor campaign, or exploit-in-the-wild evidence was identified in the available data for CVE-2026-102031. EPSS has not yet been scored by FIRST.org; no value is asserted.