CVE-2025-66648: Vega-Functions XSS Vulnerability

CVE-2025-66648 Overview

CVE-2025-66648 is a Cross-Site Scripting (XSS) vulnerability affecting vega-functions, which provides function implementations for the Vega expression language. Prior to version 6.1.1, sites that allow users to supply untrusted user input are vulnerable to malicious exploitation of an internal function (not part of the public API) that could be leveraged to execute unintentional JavaScript in the context of a victim's browser.

Critical Impact

This XSS vulnerability allows attackers to execute arbitrary JavaScript code in victim browsers when untrusted user input is processed through the Vega expression language, potentially leading to session hijacking, data theft, or further compromise of affected web applications.

Affected Products

• vega-functions versions prior to 6.1.1
• Applications using Vega visualization grammar with untrusted user input
• Web applications leveraging vega.expressionInterpreter (CSP safe mode does not prevent this issue)

Discovery Timeline

• 2026-01-05 - CVE CVE-2025-66648 published to NVD
• 2026-01-08 - Last updated in NVD database

Technical Details for CVE-2025-66648

Vulnerability Analysis

This vulnerability is classified as CWE-79 (Improper Neutralization of Input During Web Page Generation), commonly known as Cross-Site Scripting. The flaw exists within an internal function of vega-functions that was not part of the intended public API. When web applications accept untrusted user input and process it through the Vega expression language, attackers can craft malicious input that exploits this internal function to execute arbitrary JavaScript code.

The vulnerability is network-accessible and requires no authentication or user interaction to exploit. The scope is changed, meaning the vulnerability can affect resources beyond the security scope of the vulnerable component, impacting both confidentiality and integrity of user sessions and data.

Root Cause

The root cause of this vulnerability lies in insufficient input sanitization within an internal function of the vega-functions library. Although this function was not intended for public use, it remained accessible and could be invoked through crafted expressions. The internal function failed to properly validate and sanitize input before processing, allowing malicious JavaScript to be injected and executed in the browser context.

Notably, the vulnerability persists even when using vega.expressionInterpreter as described in CSP safe mode, indicating that the security bypass occurs at a level where Content Security Policy mitigations are ineffective.

Attack Vector

The attack vector for CVE-2025-66648 involves network-based exploitation where an attacker supplies malicious input to a vulnerable web application that processes data through the Vega expression language. The attacker crafts input that invokes the vulnerable internal function with JavaScript payload, which then executes in the victim's browser when the Vega visualization or expression is rendered.

Since the vulnerability bypasses CSP safe mode protections, traditional XSS mitigations may not be effective. The attack does not require authentication or privileges, and no user interaction is needed beyond the victim viewing content containing the malicious expression.

For detailed technical information about the vulnerability mechanism, refer to the GitHub Security Advisory.

Detection Methods for CVE-2025-66648

Indicators of Compromise

• Unexpected JavaScript execution originating from Vega visualization components
• Anomalous network requests to external domains from pages containing Vega visualizations
• User reports of suspicious behavior when interacting with data visualization features
• Evidence of session token exfiltration or unauthorized data access following visualization rendering

Detection Strategies

• Monitor application logs for unusual expression patterns or payloads in Vega-related input fields
• Implement Web Application Firewall (WAF) rules to detect potential XSS payloads targeting Vega expressions
• Deploy browser-based security monitoring to detect unexpected script execution contexts
• Audit client-side JavaScript execution for calls to internal vega-functions methods

Monitoring Recommendations

• Enable verbose logging for all user-supplied input processed by Vega components
• Configure SIEM alerts for XSS-related indicators in web application traffic
• Monitor Content Security Policy violation reports if implemented alongside vega.expressionInterpreter
• Track version information of deployed vega-functions packages across environments

How to Mitigate CVE-2025-66648

Immediate Actions Required

• Upgrade vega-functions to version 6.1.1 or later immediately
• Audit all applications using Vega visualization libraries for potentially vulnerable versions
• Review and restrict sources of untrusted user input that flow into Vega expressions
• Deploy additional input validation layers at the application level pending upgrade

Patch Information

The vulnerability has been fixed in vega-functions version 6.1.1. Organizations should upgrade to this version or later to remediate the vulnerability. The fix addresses the vulnerable internal function to prevent JavaScript execution through malicious expression input. For additional details, see the GitHub Security Advisory.

Workarounds

• There is no workaround available besides upgrading to version 6.1.1 or later
• Using vega.expressionInterpreter as described in CSP safe mode does NOT prevent this issue
• As a temporary measure, restrict or disable user-supplied input to Vega visualizations until upgrade is complete
• Consider implementing strict input validation at the application layer to filter potentially malicious expressions

# Upgrade vega-functions to the patched version
npm update vega-functions@6.1.1

# Verify installed version
npm list vega-functions

# For yarn users
yarn upgrade vega-functions@6.1.1