CVE-2026-23931 Overview
CVE-2026-23931 is an information disclosure vulnerability in the Zabbix frontend validate.api.exists action. Authenticated users can abuse this action to extract plaintext user macro values that should remain confidential. The flaw is tracked in the Zabbix bug tracker as ZBX-28070 and is categorized under CWE-203: Observable Discrepancy.
The vulnerability requires low-privilege authentication and can be triggered over the network without user interaction. Successful exploitation leads to loss of confidentiality of macro data, which frequently stores secrets such as credentials, API tokens, and connection strings used by Zabbix monitoring workflows.
Critical Impact
Authenticated attackers can enumerate and extract plaintext user macro values, exposing sensitive configuration data used across the Zabbix monitoring environment.
Affected Products
- Zabbix frontend (see ZBX-28070 for affected versions)
- Deployments exposing the Zabbix web UI to authenticated users
- Environments storing secrets in Zabbix user macros
Discovery Timeline
- 2026-08-18 - CVE-2026-23931 published to NVD
- 2026-08-18 - Last updated in NVD database
Technical Details for CVE-2026-23931
Vulnerability Analysis
The validate.api.exists action in the Zabbix frontend is designed to check whether a given entity or value exists in the backend. It is not intended to disclose stored values. According to the advisory, the action's response behavior differs in observable ways depending on the value being validated. An authenticated user can leverage those differences to infer, and ultimately reconstruct, the plaintext contents of user macros.
User macros in Zabbix commonly hold sensitive values such as database passwords, SNMP community strings, and integration API keys. Recovering them undermines the segregation between low-privileged Zabbix users and the credentials that support host monitoring.
Root Cause
The root cause is an observable discrepancy [CWE-203] in the response of a frontend validation endpoint. The action leaks information about protected values through side-channel signals such as differing response content, status, or timing based on submitted input. This allows candidate values to be confirmed or rejected one at a time.
Attack Vector
An authenticated attacker with access to the Zabbix web interface issues repeated requests to the validate.api.exists action, supplying crafted inputs that target user macro values. By comparing responses, the attacker distinguishes matching values from non-matching values and iteratively recovers the plaintext macro contents. No elevated role or user interaction is required beyond a valid authenticated session. Refer to ZBX-28070 for technical details.
Detection Methods for CVE-2026-23931
Indicators of Compromise
- High-volume requests from a single authenticated Zabbix user to the validate.api.exists frontend action.
- Repeated validation requests that iterate over character sets or dictionary-like input patterns.
- Authenticated sessions generating anomalous request rates against the Zabbix frontend outside normal administrative windows.
Detection Strategies
- Enable Zabbix frontend audit and web server access logging, then baseline normal usage of the validate.api.exists endpoint.
- Alert on non-administrator accounts invoking validation endpoints repeatedly or in tight loops.
- Correlate frontend request patterns with subsequent authentication failures or lateral movement attempts using credentials stored in macros.
Monitoring Recommendations
- Forward Zabbix frontend, web server, and database logs to a centralized analytics pipeline for query-based detection.
- Monitor for anomalous outbound activity from hosts whose credentials are stored in Zabbix user macros.
- Review the Zabbix user role assignments and reduce the number of accounts able to reach the frontend API.
How to Mitigate CVE-2026-23931
Immediate Actions Required
- Apply the Zabbix security update referenced in ZBX-28070 as soon as it is available for your deployed version.
- Rotate any secrets previously stored in Zabbix user macros, including database passwords, API tokens, and SNMP community strings.
- Audit Zabbix accounts and revoke access for unused or over-privileged users.
Patch Information
Zabbix tracks the fix under ZBX-28070. Consult the Zabbix Bug Tracker Entry for the list of fixed versions and upgrade instructions. Upgrade the frontend to the patched release that corresponds to your major version branch.
Workarounds
- Restrict network access to the Zabbix frontend using firewall rules, VPN, or a reverse proxy allowlist.
- Limit the number of accounts with authenticated access to the Zabbix frontend and enforce strong authentication.
- Where feasible, move highly sensitive credentials out of user macros and into an external secrets manager referenced by Zabbix agents.
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
