CVE-2026-84942 in OpenSearch Service
Summary
by MITRE • 09/08/2026
Improper input validation in the Vega expression function implementation in OpenSearch Dashboards allows a remote authenticated actor with dashboard write permissions to execute arbitrary JavaScript in the context of other users' browser sessions by saving a crafted Vega visualization. The checkForFunctionProperty validation routine failed to recurse into arrays of objects, allowing a function property nested inside an array to bypass validation.
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Analysis
by VulDB Data Team • 09/09/2026
The vulnerability identified within OpenSearch Dashboards stems from a critical flaw in the input validation logic governing the Vega expression function implementation. This security weakness specifically affects users who possess dashboard write permissions and are authenticated to the system. The core issue lies in the checkForFunctionProperty routine, which is designed to sanitize user inputs by detecting and blocking potentially dangerous JavaScript code embedded within visualization configurations. However, this validation mechanism suffers from a logical deficiency where it fails to perform recursive traversal into arrays of objects. Consequently, an attacker can structure their malicious payload such that the hazardous function property is nested deeply within an array data structure, thereby bypassing the superficial checks performed by the validator and allowing arbitrary JavaScript execution in the context of other users' browser sessions when they view the crafted visualization.
From a technical perspective, this flaw represents a classic case of insufficient validation depth where the security control does not account for complex data structures that can encapsulate malicious code. By exploiting the inability to recurse into arrays, an authenticated user with write access can save a Vega visualization configuration containing obfuscated or nested JavaScript commands. When another user accesses this dashboard, their browser executes the injected script under the victim's session context. This scenario effectively leads to Cross-Site Scripting (XSS), enabling actions such as stealing sensitive data like authentication tokens, manipulating interface elements, or performing unauthorized operations on behalf of the victim. The impact is severe because it compromises the confidentiality and integrity of user sessions within the OpenSearch Dashboards environment, potentially leading to broader system compromise if combined with other vulnerabilities or social engineering tactics.
In terms of industry standard classifications, this vulnerability aligns closely with CWE-20 Improper Input Validation, specifically highlighting failures in handling complex data structures during sanitization processes. It also maps directly to MITRE ATT&CK technique T1059 Command and Scripting Interpreter under the Execution tactic, as it allows for arbitrary code execution via JavaScript within a web application context. Furthermore, given that authentication is required prior to exploitation, this falls under privilege escalation scenarios where lower-privileged users can leverage their access rights to impact higher-value assets or other user sessions. The lack of recursive validation in security filters remains a common pitfall in many enterprise applications, underscoring the need for robust parsing libraries and comprehensive test coverage that includes deeply nested payloads during development cycles.
To mitigate this risk, immediate action should be taken by upgrading OpenSearch Dashboards to a version where the checkForFunctionProperty routine has been patched to correctly traverse all levels of data structures, including arrays and objects. Administrators must ensure that their deployment environments are updated with the latest security patches provided by the vendor. In addition to software updates, organizations should enforce strict least-privilege access controls, ensuring that only trusted personnel have dashboard write permissions. Implementing a Web Application Firewall (WAF) with rules capable of detecting nested script injection patterns can provide an additional layer of defense. Regular security audits and penetration testing focused on input validation logic are also recommended to identify similar weaknesses in other components before they can be exploited by malicious actors seeking to compromise user sessions or exfiltrate sensitive information from the dashboard environment.