CVE-2026-23920: Authentication Bypass Vulnerability

CVE-2026-23920 Overview

CVE-2026-23920 is a command injection vulnerability affecting Zabbix host and event action script input validation. The vulnerability stems from regex validation running in multiline mode, which allows authenticated users to bypass input checks by injecting newline characters. When administrators configure input validation using ^ and $ anchors, attackers can exploit this behavior to inject arbitrary shell commands.

Critical Impact

Authenticated users can bypass input validation and execute arbitrary shell commands on the Zabbix server, potentially leading to complete system compromise.

Affected Products

• Zabbix Server (specific versions not disclosed in advisory)

Discovery Timeline

• 2026-03-24 - CVE-2026-23920 published to NVD
• 2026-03-25 - Last updated in NVD database

Technical Details for CVE-2026-23920

Vulnerability Analysis

This vulnerability is classified as CWE-78 (Improper Neutralization of Special Elements used in an OS Command), commonly known as OS Command Injection. The core issue lies in how Zabbix validates user-supplied input for host and event action scripts.

When administrators define regex patterns using ^ (start of string) and $ (end of string) anchors to validate user input, they typically expect these anchors to match the beginning and end of the entire input string. However, because the validation runs in multiline mode, these anchors instead match the beginning and end of each line within the input.

This semantic difference creates a security gap where an attacker can craft input that passes validation on one line while containing malicious shell commands on subsequent lines. The validated portion satisfies the administrator's regex pattern, while the injected commands after a newline character escape validation entirely and are subsequently executed by the shell.

Root Cause

The root cause is the improper configuration of the regex engine's multiline mode during input validation. In multiline mode, the ^ anchor matches at the start of each line (after newline characters), and $ matches at the end of each line (before newline characters). This differs from single-line mode where these anchors only match the absolute beginning and end of the input string.

When user input containing embedded newline characters (\n) is validated, only the first line may be checked against the administrator's regex pattern, allowing subsequent lines containing shell metacharacters and commands to pass through unvalidated.

Attack Vector

The attack requires network access and authenticated user privileges. An attacker with valid credentials can exploit this vulnerability through the following mechanism:

1. Identify a host or event action script that accepts user input
2. Craft input containing a legitimate value on the first line that satisfies the regex validation
3. Append a newline character followed by shell command injection payload
4. Submit the malicious input, which passes validation and executes the injected commands

The vulnerability mechanism can be understood through this example scenario: If an administrator configures a regex pattern like ^[a-zA-Z0-9]+$ to allow only alphanumeric input, an attacker could submit input such as validinput\n; malicious_command. The first line validinput matches the regex, while the second line containing shell commands bypasses validation entirely due to multiline mode behavior.

For detailed technical information, refer to the Zabbix Issue Tracker Entry.

Detection Methods for CVE-2026-23920

Indicators of Compromise

• Unexpected newline characters (\n, %0a, or URL-encoded variants) in action script input parameters
• Shell metacharacters (;, |, &, backticks) appearing in input fields following newline sequences
• Anomalous process execution spawned from Zabbix server processes
• Unusual network connections or file system modifications originating from the Zabbix server

Detection Strategies

• Monitor Zabbix server logs for input containing newline characters in action script parameters
• Implement application-layer inspection to detect multiline input submissions to action script endpoints
• Deploy endpoint detection rules to identify shell command execution chains spawned by Zabbix processes
• Review audit logs for authenticated users submitting suspicious input patterns

Monitoring Recommendations

• Enable detailed logging for all host and event action script executions
• Configure alerting for process creation events from Zabbix server with unusual command-line arguments
• Implement network traffic analysis to detect command-and-control communications from Zabbix infrastructure
• Establish baseline behavior for Zabbix server processes and alert on deviations

How to Mitigate CVE-2026-23920

Immediate Actions Required

• Review all configured action scripts and their input validation regex patterns for use of ^ and $ anchors
• Restrict access to action script configuration to trusted administrators only
• Audit recent action script executions for signs of exploitation
• Consider temporarily disabling user-configurable action scripts until patches are applied

Patch Information

Refer to the Zabbix Issue Tracker Entry for official patch information and updated versions. Organizations should apply vendor-provided security updates as soon as they become available.

Workarounds

• Modify regex patterns to use \A (absolute start) and \z (absolute end) anchors instead of ^ and $ where supported
• Implement additional input sanitization to strip or reject newline characters before regex validation
• Apply network segmentation to limit exposure of Zabbix server interfaces to trusted networks only
• Enable multi-factor authentication for Zabbix user accounts to reduce risk of credential compromise

Organizations should implement defense-in-depth by combining input validation improvements with network-level controls and monitoring until official patches can be deployed.