CVE-2026-23931

Executive Summary

CVE-2026-23931 is an information disclosure vulnerability in the Zabbix frontend affecting the validate.api.exists action, which can be abused by authenticated users to extract plaintext user macro values. Successful exploitation results in a confidentiality impact only (CVSS 4.3, C:L), requiring network access with low complexity and low privileges. Because Zabbix user macros are routinely used to store sensitive values such as credentials, API tokens, and SNMP community strings, exposure of plaintext macro contents can provide a direct escalation path to managed hosts, monitored databases, and downstream APIs.

Technical Analysis

The vulnerable component is the Zabbix frontend and, specifically, the validate.api.exists action exposed by that frontend. Per the NVD description, this action does not adequately restrict the information returned to the caller, allowing an authenticated user to recover plaintext values of user macros that should not be readable by that user.

The weakness class is CWE-203, "Observable Discrepancy." This category covers cases where a product behaves differently — in output content or response characteristics — depending on internal state, and those differences leak information that the requester is not authorized to see. In this instance the observable discrepancy is the disclosure of plaintext macro content through a validation endpoint: an action whose apparent purpose is to confirm the existence or validity of something ends up returning value data, or a response that distinguishes value-bearing state in a way that can be enumerated and read.

Zabbix user macros are a core configuration primitive. They are widely used to parameterize monitoring for hosts: database credentials in item keys, HTTP authentication headers, SNMP community strings, script arguments, and alerting webhook secrets. In normal operation these macros are masked in the frontend for users without the appropriate role permissions. The vulnerability undermines that masking for authenticated callers hitting the validate.api.exists action.

The attack surface is the frontend's HTTP interface — the same interface used by legitimate operators through a browser or API client. No authentication bypass is required, but authentication is a precondition (CVSS PR:L). The integrity and availability sub-scores are Not Defined/None in the provided vector (I:N, A:N), confirming the flaw is a read-only confidentiality exposure rather than a state-changing or denial-of-service issue. AC:L and UI:N indicate that exploitation does not require unusual preconditions or user interaction beyond the attacker's own authenticated session.

Notably, only the action name validate.api.exists and the disclosure of "plaintext user macro values" are specified in the source data. The precise request parameters, response fields, or parsing logic that produce the discrepancy are not described and must not be assumed.

Exploitation Scenarios

No public proof-of-concept, Nuclei template, or exploit code is available for CVE-2026-23931. The following chain is inferred from the vulnerability class (CWE-203) and the facts stated in the NVD description. It contains no fabricated endpoints, parameters, or function names.

1. Reconnaissance / target identification. An adversary identifies an internet- or intranet-exposed Zabbix frontend. Zabbix installations are commonly recognizable by their login page and default branding; exposed instances can be enumerated with search-engine dorks (see METADATA). The attacker needs at least one valid account — a low-privileged operator, read-only user, or any account provisioned for monitoring consumption. Obtaining such an account is the principal barrier and can be achieved through credential reuse, phishing, leaked credentials, or a prior foothold.

2. Triggering the flaw class. With an authenticated frontend session, the attacker invokes the validate.api.exists action. A CWE-203 flaw is exploited by sending requests and observing how responses differ based on the hidden state of the target object. In this case, the observable discrepancy leaks the plaintext values of user macros — information the caller is not entitled to view. The attacker iterates over the set of macros visible to their session (or over macro names that can be discovered through normal frontend functionality) and records the value data returned or inferred for each.

3. Payload / execution method (vulnerability-class level). Discrepancy-based disclosure requires no exploit payload in the memory-corruption sense. The method is repeated, authenticated requests to the vulnerable frontend action, with responses compared and harvested. Where the action returns values directly, collection is straightforward; where it only reflects a validation outcome, the attacker uses that outcome as an oracle to reconstruct the value (or to confirm guesses) request by request.

4. Post-exploitation impact. The harvested plaintext macros are used downstream. Credentials embedded in macros are replayed against monitored assets — databases, network devices, APIs, and web services — yielding lateral movement and further data access. Because macros are typically shared by many hosts and templates in a Zabbix deployment, a single disclosed macro may unlock a broad set of systems. Reconnaissance value is also significant: macro names and values reveal infrastructure topology, service accounts, and integration endpoints.

Impact Assessment

The direct impact is loss of confidentiality for data stored in Zabbix user macros. The CVSS v3.1 base score is 4.3 (Medium), reflecting Network attack vector, Low attack complexity, Low privileges required, No user interaction, Unchanged scope, Low confidentiality impact, and no integrity or availability impact. The relatively modest base score should not be read as low operational risk: the affected data is precisely the class of secrets that operators expect to be protected by role-based access control, and its disclosure frequently converts a foothold into a compromise of monitored infrastructure.

Affected systems are Zabbix frontends, including any deployment where macro values are used to hold credentials or other sensitive strings and where users with visibility of the vulnerable action are not fully trusted. Blast radius is a function of macro sharing: values scoped to a single host limit exposure, whereas values defined at global or template level and consumed by many hosts can expose an entire environment. Enterprise and MSSP deployments, where Zabbix is the central monitoring plane across many customer or business-unit estates, carry the highest effective exposure.

The EPSS score of 0.00279 (approximately 20.6th percentile) indicates current exploitation likelihood is low relative to all scored CVEs. There is no CISA KEV listing, which means no confirmed exploitation in the wild as of the data provided. The absence of public PoC material further constrains opportunistic mass exploitation, though the low skill floor of an authenticated frontend request means the barrier is primarily credential acquisition rather than technical capability.

Detection Guidance

Sigma Rule

title: Zabbix validate.api.exists User Macro Disclosure Attempt
id: 8f3a1c52-6c4e-4b0e-9f1d-2a7e5c9d4b21
status: experimental
description: Detects authenticated requests to the Zabbix frontend validate.api.exists action associated with CVE-2026-23931, which can leak plaintext user macro values.
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-23931
author: Threat Intelligence
date: 2026-01-01
tags:
  - attack.credential-access
  - attack.t1552.001
  - cve.2026-23931
logsource:
  category: webserver
  definition: Web server access logs for the Zabbix frontend
detection:
  selection_uri:
    cs-uri-query|contains:
      - 'validate.api.exists'
      - 'action=validate.api.exists'
  selection_status:
    sc-status:
      - 200
      - 500
      - 403
  condition: selection_uri and selection_status
fields:
  - c-ip
  - cs-username
  - cs-uri-query
  - sc-status
falsepositives:
  - Legitimate frontend UI operations that invoke the same validation action
level: medium

YARA Rule

rule CVE_2026_23931_Zabbix_Macro_Disclosure
{
    meta:
        description = "Detects references to the Zabbix validate.api.exists action in logs or HTTP capture artifacts, indicative of CVE-2026-23931 probing"
        author = "Threat Intelligence"
        date = "2026-01-01"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-23931"
        cve = "CVE-2026-23931"

    strings:
        $a = "validate.api.exists" ascii wide nocase
        $b = "validate.api" ascii wide nocase
        $c = "zabbix" ascii wide nocase

    condition:
        uint32(0) != 0x504b0304 and
        ($a or ($b and $c))
}

Suricata IDS Rule

alert http any any -> any any (msg:"CVE-2026-23931 Zabbix Frontend validate.api.exists User Macro Disclosure Attempt"; flow:established,to_server; http.method; content:"POST"; http.uri; content:"validate.api.exists"; nocase; http.cookie; content:"zbx_session"; nocase; reference:cve,2026-23931; reference:url,nvd.nist.gov/vuln/detail/CVE-2026-23931; classtype:attempted-recon; sid:202623931; rev:1;)

Elastic Detection

sequence by user.name, source.ip with maxspan=1m
  [http where http.request.method == "POST" and url.path contains "validate.api.exists"]
  [http where http.request.method == "POST" and url.path contains "validate.api.exists"]

Splunk SPL Query

index=web sourcetype=access_combined
| where match(_raw, "(?i)validate\.api\.exists")
| stats count values(uri_path) as paths values(status) as statuses by src_ip, user
| where count >= 3
| sort - count

Wazuh OSSEC Rule

<group name="zabbix,web,cve-2026-23931,">
  <rule id="2026239" level="10">
    <decoded_as>web-accesslog</decoded_as>
    <id>31100</id>
    <url>validate.api.exists</url>
    <description>CVE-2026-23931: Zabbix frontend validate.api.exists action accessed - possible user macro value disclosure attempt</description>
    <mitre>
      <id>T1552.001</id>
    </mitre>
    <group>cve_2026_23931,credential_access,</group>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence is 3/4 (LIKELY): two independent sources — NVD and Debian — confirm that a fix exists. No vendor advisory URL or explicit patched version string was provided in the source data, so the exact fixed release must be confirmed against the Zabbix release notes and the Debian security tracker before scheduling upgrades.

1. Immediate mitigations. Until upgrade is possible, reduce exposure by restricting the number of accounts that can reach the validate.api.exists action and by removing macro values that do not need to be stored in Zabbix. Migrate high-value secrets (database passwords, API tokens) out of user macros into a secrets manager referenced indirectly where the monitoring design permits. Enforce network-level access control on the frontend so only trusted administrative networks can reach it, and audit the frontend user base for stale or over-privileged accounts. Audit macro definitions and immediately rotate any credential that has ever been stored in a macro on an exposed instance, treating historical exposure as assumed.

2. Patch / upgrade steps. Apply the fixed Zabbix frontend release from the vendor as soon as the version is confirmed. For Debian-based deployments, apply the corresponding zabbix-frontend-php security update from the Debian security archive once available. Verify the patch by reproducing the action with a low-privileged account and confirming that macro values are no longer disclosed. Maintain a tested rollback point before upgrading the monitoring plane.

3. Long-term hardening. Enforce least privilege on the frontend with role-based access controls and per-host macro scoping so that no user can read macro values outside their remit. Treat user macros as a secret store only where unavoidable, rotate macro-held secrets on a schedule, and integrate macro changes into change-management review. Add the detection rules above to SOC monitoring, alerting on repeated validate.api.exists invocations from a single identity. Track Zabbix security advisories and Debian security tracker entries as standing inputs to vulnerability management, and where possible subscribe to vendor notifications for the frontend component specifically.

References

• NVD — CVE-2026-23931: https://nvd.nist.gov/vuln/detail/CVE-2026-23931
• Zabbix security advisories (vendor): https://www.zabbix.com/security_advisories
• Debian Security Tracker (Zabbix packages): https://security-tracker.debian.org/tracker/
• MITRE CWE-203 — Observable Discrepancy: https://cwe.mitre.org/data/definitions/203.html
• CISA Known Exploited Vulnerabilities Catalog: https://www.cisa.gov/known-exploited-vulnerabilities-catalog

No threat actor campaigns or exploit-in-the-wild evidence were identified for CVE-2026-23931 in the source data. The CVE is not listed in CISA KEV, no public PoC or exploit code is available, and EPSS indicates a low near-term exploitation probability.