CVE-2026-42044: Axios Privilege Escalation Vulnerability

CVE-2026-42044 Overview

CVE-2026-42044 is a Prototype Pollution vulnerability affecting Axios, a popular promise-based HTTP client library for browsers and Node.js. This vulnerability allows attackers to exploit prototype pollution elsewhere in an application's dependency tree to achieve surgical, invisible modification of all JSON API responses. The attack can lead to privilege escalation, balance manipulation, and authorization bypass through the manipulation of Axios's response transformation mechanism.

Critical Impact

Attackers can manipulate any JSON API response processed by Axios, enabling privilege escalation, financial balance manipulation, and authorization bypass without detection.

Affected Products

• Axios versions 1.0.0 to 1.15.1 (Node.js)
• Axios for browser environments (versions 1.0.0 to 1.15.1)
• Applications using Axios with vulnerable dependency chains

Discovery Timeline

• 2026-04-24 - CVE CVE-2026-42044 published to NVD
• 2026-04-27 - Last updated in NVD database

Technical Details for CVE-2026-42044

Vulnerability Analysis

This vulnerability represents a Prototype Pollution "Gadget" attack that transforms existing prototype pollution vulnerabilities in an application's dependency chain into a powerful exploitation primitive. The attack targets the default transformResponse function in Axios, which processes all incoming JSON data from API calls.

The core issue resides in the lib/defaults/index.js file at line 124, where Axios calls JSON.parse(data, this.parseReviver). The this context refers to the merged configuration object. Because parseReviver is not defined in Axios defaults, not validated by assertOptions, and not subject to any constraints, the function is susceptible to prototype pollution.

When an attacker pollutes Object.prototype.parseReviver with a malicious function, that function is invoked for every key-value pair in every JSON response processed by Axios. This gives the attacker the ability to selectively modify individual values while leaving the rest of the response intact, making the attack extremely difficult to detect.

Root Cause

The root cause is the lack of proper initialization and validation of the parseReviver property in the Axios configuration object. The JSON.parse reviver function parameter is designed to transform parsing results, but Axios does not explicitly set this to undefined or validate its presence/type. This allows prototype pollution attacks to inject a malicious reviver function through Object.prototype.parseReviver, which then gets inherited by all configuration objects.

Attack Vector

The attack is network-accessible and requires no authentication or user interaction. An attacker must first establish prototype pollution somewhere in the application's dependency tree—this could be through a separate vulnerability in any npm package. Once Object.prototype.parseReviver is polluted with a malicious function, every subsequent JSON response parsed by Axios will pass through this function.

The malicious reviver function receives the key and value for each parsed JSON property, allowing the attacker to:

• Modify user role fields to escalate privileges (e.g., changing "role": "user" to "role": "admin")
• Alter financial values (e.g., account balances, transaction amounts)
• Bypass authorization checks by manipulating permission flags
• Inject malicious payloads into response data

The surgical nature of this attack means only targeted fields are modified while the rest of the response remains unchanged, making detection extremely challenging.

Detection Methods for CVE-2026-42044

Indicators of Compromise

• Unexpected modifications to Object.prototype properties, particularly parseReviver
• Anomalous behavior in API responses where server logs don't match client-side data
• Privilege escalation events without corresponding authentication changes
• Financial discrepancies between server-side calculations and client-displayed values

Detection Strategies

• Implement runtime monitoring for Object.prototype modifications using Object.freeze(Object.prototype) in development environments
• Deploy application-level integrity checks comparing server response hashes with received data
• Use Content Security Policy and Subresource Integrity to detect tampered responses
• Monitor for unusual property access patterns on configuration objects

Monitoring Recommendations

• Enable detailed logging for Axios request/response cycles in production environments
• Implement server-side validation for all critical operations rather than relying on client-side data
• Set up alerts for unexpected prototype modifications using security monitoring tools
• Review npm audit reports regularly for prototype pollution vulnerabilities in dependencies

How to Mitigate CVE-2026-42044

Immediate Actions Required

• Upgrade Axios to version 1.15.2 or later immediately across all projects
• Audit your application's dependency tree for known prototype pollution vulnerabilities
• Review recent security incidents for potential exploitation of this vulnerability
• Implement server-side validation for all authorization and financial operations

Patch Information

The vulnerability is fixed in Axios version 1.15.2. The patch ensures that the parseReviver property is properly handled and cannot be inherited from Object.prototype. Refer to the GitHub Security Advisory for complete details.

To update Axios in your project:

# Using npm
npm update axios

# Using yarn
yarn upgrade axios

# Verify installed version
npm list axios

Workarounds

• Explicitly set transformResponse in your Axios configuration to override the default behavior
• Freeze Object.prototype early in your application initialization (note: may cause compatibility issues)
• Implement custom response interceptors that validate response integrity before processing
• Use a wrapper around Axios that sanitizes the configuration object before each request

# Configuration example
# Explicitly define transformResponse in your Axios instance to prevent prototype pollution
# This ensures parseReviver cannot be inherited from Object.prototype

# In your axios configuration file:
# const axiosInstance = axios.create({
#   transformResponse: [(data) => {
#     if (typeof data === 'string') {
#       try {
#         return JSON.parse(data);
#       } catch (e) {
#         return data;
#       }
#     }
#     return data;
#   }]
# });