CVE-2026-59782

Executive Summary

CVE-2026-59782 is a memory disclosure vulnerability in the JavaScript preprocessing engine (Duktape) on the Zabbix server. An attacker holding limited administrative privileges on a Zabbix deployment can cause the engine to return raw heap data, potentially exposing information belonging to other concurrently running preprocessors that the attacker is not authorized to view. The impact is confidentiality-only: unauthorized cross-tenant or cross-job data exposure within a shared processing context, not code execution or data integrity loss.

Technical Analysis

The vulnerability is classified as CWE-125 (Out-of-bounds Read). The affected component is the JavaScript preprocessing subsystem on the Zabbix server, which uses the Duktape embedded JavaScript engine to evaluate preprocessing scripts attached to items. Preprocessing is a server-side feature: item values collected by the Zabbix server are passed through a configured preprocessing pipeline, which may include a "JavaScript" step executed by Duktape inside the server process. Because multiple preprocessors can run within the same server process (and therefore the same or adjacent heap allocations), a flaw that causes Duktape-mediated JavaScript execution to return memory outside the bounds of the intended buffer can disclose residual heap contents belonging to other preprocessing jobs.

The core mechanism implied by the description is that a limited administrator — that is, a user with administrative rights scoped below super-admin, or with permission to define/configure preprocessing — is able to influence a JavaScript preprocessing step such that the engine surfaces raw heap bytes back through the value pipeline or an observable error/output channel. The result is an out-of-bounds read that leaks data from neighboring allocations. The "other running preprocessors" wording indicates the leaked data originates from concurrent or previously executed preprocessing work belonging to items the limited administrator is not entitled to access — an isolation failure at the memory level rather than an authorization-check failure at the application level.

Several characteristics follow directly from the description:

• Attack surface: the Zabbix server's JavaScript preprocessing path (Duktape execution context). It is server-side, not agent-side.
• Privilege requirement: the flaw is reachable by a "limited administrator," implying authenticated access with elevated-but-not-full privileges. Pre-auth exploitation is not indicated.
• Isolation boundary crossed: heap data from other preprocessors, i.e., data the attacker lacks authorization to read through normal Zabbix RBAC.
• No integrity or availability impact is described; the exposure is read-only.

Insufficient technical detail is available to identify specific Duktape API functions, the exact JavaScript construct that triggers the out-of-bounds read, the size of the over-read, or the precise output channel through which leaked bytes become visible. Analysts should not assume a particular function name or code path until upstream advisory or patch details are published. The description supports only the vulnerability class (improper bounds checking leading to heap over-read within an embedded scripting engine), not implementation specifics.

Exploitation Scenarios

No public proof-of-concept exists and no exploit code or Nuclei template is tracked. The attack chain below is described at the vulnerability-class level and is inferred from the CWE-125 classification and the NVD description; it does not rely on any fabricated function, parameter, or endpoint names.

1. Reconnaissance / target identification. The attacker identifies a Zabbix server exposing the frontend and confirms they hold a limited administrative account (either compromised credentials for a scoped admin role, or legitimate low-tier admin access abused for lateral data access). The Zabbix web frontend and server API port (server-side functionality; the canonical Zabbix server port is 10051) are the relevant surfaces. The attacker maps which items have JavaScript preprocessing configured, since those items define where Duktape executes.

2. Triggering the flaw class. With the ability to configure or supply JavaScript preprocessing input, the attacker crafts input designed to induce an out-of-bounds read in the preprocessing engine — i.e., input that causes the engine to read past the end of an intended buffer. Per CWE-125 this is a missing bounds check on a read operation. Because the engine processes data within the shared Zabbix server heap, the over-read returns bytes that do not belong to the attacker's job.

3. Payload / execution method. At the vulnerability-class level, the "payload" is crafted preprocessing input rather than machine code. Successful triggering causes the engine to surface raw heap contents through whatever output channel is available to the limited administrator (a returned preprocessed value, a diagnostic string, or an analogous observable result). Repeated execution with varied input can be used to sample different heap regions and reconstruct fragments of memory. The attacker does not gain control-flow hijack: this is disclosure only.

4. Post-exploitation impact. Leaked heap fragments may contain data produced by other, concurrently or previously running preprocessors — including values from items the limited administrator cannot access, and potentially secrets or sensitive strings that happened to reside in memory. The attacker can aggregate leaked fragments over time to build an unauthorized view of other tenants'/teams' data, or to harvest credentials or tokens for follow-on abuse. No public PoC is available; the attack chain is inferred from the vulnerability class.

Impact Assessment

The vulnerability is confidentiality-impacting only. In CVSS v3.1 terms, the characteristics implied by the description map to: Attack Vector Network (the Zabbix server frontend/API is remotely reachable), Attack Complexity Low (no unusual preconditions beyond the configuration ability), Privileges Required High (a limited administrator), User Interaction None, Scope Unchanged (the leak occurs within the same server security authority), Confidentiality High (raw heap data can include arbitrary sensitive values from other jobs), Integrity None, and Availability None. This yields the analyst-inferred vector CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:H/I:N/A:N. Note that NVD has assigned no official score; the above is a structural estimate derived solely from the description and must not be cited as an authoritative CVSS value.

Business impact centers on multi-tenant and role-segmented Zabbix deployments. Organizations that run shared Zabbix servers for multiple teams, customers, or business units rely on Zabbix RBAC to enforce data boundaries. This flaw undermines those boundaries at the memory layer, meaning the RBAC model cannot be trusted to fully protect preprocessing-derived data while the flaw is unpatched. Leaked heap data could include item values, credentials embedded in preprocessing scripts, API tokens, or other secrets. Blast radius is proportional to the number of concurrent JavaScript preprocessors and the sensitivity of the data they handle. Because exploitation requires an authenticated limited-admin account, the primary threat model is the malicious or compromised insider, a lateral-moving attacker who has already obtained a scoped Zabbix admin account, or a customer of a managed Zabbix service abusing their tier. EPSS has not scored this CVE, and it is not listed in CISA KEV; no in-the-wild exploitation is currently evidenced.

Detection Guidance

Detection should focus on the Zabbix server process, its JavaScript/Duktape preprocessing activity, and administrative actions that create or modify JavaScript preprocessing. Because no official signature set exists, the following rules are behavioral heuristics keyed to the vulnerability class and must be tuned to the environment.

Sigma Rule

title: Zabbix JavaScript Preprocessing Heap Over-Read Attempt (CVE-2026-59782)
id: 4f1c9b2e-7d3a-4c8e-9a51-cve202659782
status: experimental
description: Detects suspicious or anomalous JavaScript preprocessing activity on a Zabbix server consistent with attempts to trigger an out-of-bounds heap read in the Duktape engine.
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-59782
author: valtersit.com
date: 2026-01-01
logsource:
  product: zabbix
  service: server
detection:
  selection_process:
    Image|endswith:
      - '/zabbix_server'
      - '\\zabbix_server.exe'
  selection_keywords:
    - 'zabbix preprocessor'
    - 'js preprocessing'
    - 'duktape'
  selection_behavior:
    - 'preprocessing heap'
    - 'preprocess error'
    - 'javascript preprocess'
  condition: selection_process and (selection_keywords or selection_behavior)
falsepositives:
  - Legitimate custom preprocessing scripts that log verbose diagnostics
level: medium
tags:
  - attack.t1552
  - cve.2026.59782

YARA Rule

rule CVE_2026_59782_Zabbix_Duktape_Overread
{
    meta:
        description = "Heuristic detection of anomalous JavaScript preprocessing payloads targeting the Zabbix Duktape engine (CVE-2026-59782)"
        author = "valtersit.com"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-59782"
        cve = "CVE-2026-59782"
        date = "2026-01-01"

    strings:
        $js1 = "preprocessing" ascii nocase
        $js2 = "JavaScript" ascii nocase
        $js3 = "zbx_" ascii nocase
        $duk1 = "duk_" ascii nocase
        $b1 = "heap" ascii nocase
        $b2 = "length" ascii nocase

    condition:
        uint16(0) == 0x5A4D or filesize < 5MB and
        (2 of ($js1, $js2, $js3, $duk1) and any of ($b1, $b2))
}

Suricata IDS Rule

alert tcp $HOME_NET any -> $HOME_NET 10051 (msg:"CVE-2026-59782 Zabbix JavaScript Preprocessing Heap Over-Read Attempt"; flow:established,to_server; content:"preprocessing"; nocase; content:"JavaScript"; nocase; content:"condition"; nocase; threshold:type limit, track by_src, count 10, seconds 60; classtype:attempted-recon; reference:cve,2026-59782; reference:url,nvd.nist.gov/vuln/detail/CVE-2026-59782; sid:202659782; rev:1;)

Elastic Detection

sequence by host.id, process.entity_id with maxspan=30s
  [process where process.name == "zabbix_server" and
    process.args : "*preprocessing*"]
  [process where process.name == "zabbix_server" and
    (process.args : "*javascript*" or process.args : "*duktape*")]

Splunk SPL Query

index=zabbix OR index=os sourcetype=zabbix:server OR sourcetype=syslog
("preprocessing" AND ("javascript" OR "duktape" OR "heap" OR "preprocess error"))
| stats count values(_raw) as events by host, src_user, src_ip
| where count >= 5
| sort - count

Wazuh OSSEC Rule

<group name="zabbix,cve-2026-59782,">
  <rule id="2026597" level="8">
    <decoded_as>zabbix</decoded_as>
    <match>preprocess</match>
    <regex>javascript|duktape|heap</regex>
    <description>CVE-2026-59782: Possible Zabbix JavaScript preprocessing heap over-read activity</description>
    <mitre>
      <id>T1552</id>
    </mitre>
    <group>cve_2026_59782,</group>
  </rule>
</group>

Analysts should supplement the above with alerting on any administrative action that creates or modifies JavaScript preprocessing on items (Zabbix audit log), and on Zabbix server process anomalies (unexpected memory growth or crashes) that can accompany repeated over-read attempts.

Mitigation & Patch Guidance

Patch confidence: 0/4 (UNPATCHED — no patch evidence from any tracked source). No vendor fix, workaround, or fixed version is currently identified. Until an upstream Zabbix patch is published, treat all Zabbix servers offering JavaScript preprocessing as affected.

1. Immediate mitigations.

   • Restrict who can create or modify JavaScript preprocessing. Limit the "limited administrator" and preprocessing permissions to the smallest possible set of trusted users; enumerate and review all accounts with even partial administrative rights.
   • Where JavaScript preprocessing is not strictly required, disable or remove those preprocessing steps to eliminate the attack surface.
   • Run preview/testing of JavaScript preprocessing only on non-production, non-sensitive, isolated Zabbix instances.
   • Segment Zabbix servers so that a single server does not process data spanning strong trust or tenant boundaries. Reduce the number of concurrent JavaScript preprocessors handling mixed-sensitivity data.
   • Enforce strict network controls on the Zabbix frontend and API (limit source IPs, require MFA at the reverse proxy, disable unused API access).
   • Treat any data returned by preprocessing as untrusted and log access to preprocessing configuration changes for review.

2. Patch / upgrade steps.

   • Monitor the official Zabbix security advisories and the NVD entry for CVE-2026-59782. No fixed version is known at this time; do not assume a specific release contains a fix.
   • When a vendor patch is released, test it in staging and roll out to all Zabbix servers as an emergency change. Verify the fix by regression-testing preprocessing behavior and access controls.
   • Subscribe to the Zabbix security mailing list and CVE feeds for authoritative notification.

3. Long-term hardening.

   • Apply least privilege to all Zabbix administrative roles; separate duties so that the ability to define preprocessing is not bundled with general administration.
   • Isolate preprocessing workloads (e.g., dedicated Zabbix servers per trust boundary/tenant) to limit the blast radius of any memory-disclosure flaw in the shared server heap.
   • Rotate secrets, tokens, and credentials that may have been present in Zabbix server memory or preprocessing scripts as a precaution.
   • Implement continuous monitoring (see Detection Guidance) and periodic review of preprocessing definitions.
   • Maintain an inventory of Zabbix servers and versions, and track CVE exposure for the platform.

References

• NVD — CVE-2026-59782: https://nvd.nist.gov/vuln/detail/CVE-2026-59782 (no CVSS score assigned; classified CWE-125)
• Zabbix security advisory portal (monitor for an official advisory and fixed version): https://www.zabbix.com/security_advisories
• Zabbix documentation — Item value preprocessing (JavaScript preprocessing): https://www.zabbix.com/documentation/current/en/manual/config/items/preprocessing
• Duktape embedded JavaScript engine (upstream project used by Zabbix preprocessing): https://duktape.org/
• CISA Known Exploited Vulnerabilities catalog (checked; CVE-2026-59782 is NOT listed as of this report): https://www.cisa.gov/known-exploited-vulnerabilities-catalog

No threat actor campaign or exploit-in-the-wild evidence is associated with CVE-2026-59782 at the time of writing. No public PoC, exploit code, or Nuclei template is available.