CVE-2026-27837: Dottie Auth Bypass Vulnerability

CVE-2026-27837 Overview

CVE-2026-27837 is a prototype pollution vulnerability in Dottie.js, a JavaScript library that provides nested object access and manipulation. This vulnerability exists due to an incomplete fix for the earlier CVE-2023-26132, where the prototype pollution guard introduced in commit 7d3aee1 only validates the first segment of a dot-separated path. This allows attackers to bypass the protection by placing __proto__ at any position other than the first in the path, affecting both dottie.set() and dottie.transform() functions.

Critical Impact

Attackers can exploit this prototype pollution bypass to inject malicious properties into JavaScript object prototypes, potentially leading to denial of service, data manipulation, or in some cases, remote code execution depending on how the polluted objects are subsequently used by the application.

Affected Products

• Dottie.js versions 2.0.4 through 2.0.6

Discovery Timeline

• 2026-02-26 - CVE-2026-27837 published to NVD
• 2026-02-26 - Last updated in NVD database

Technical Details for CVE-2026-27837

Vulnerability Analysis

This vulnerability is classified under CWE-1321 (Improperly Controlled Modification of Object Prototype Attributes, also known as Prototype Pollution). The root issue stems from the incomplete security patch that was meant to address the original prototype pollution vulnerability CVE-2023-26132.

The original fix only checked if the first path segment was __proto__, leaving the application vulnerable when __proto__ appeared in any subsequent position within a dot-separated path string. This allows attackers to craft payloads like "foo.__proto__.bar" to bypass the protection entirely and pollute the JavaScript Object prototype.

The vulnerability requires user interaction via a network attack vector, as an attacker would need to supply malicious input that gets processed by the dottie.set() or dottie.transform() functions. Successful exploitation can impact confidentiality, integrity, and availability of the affected application.

Root Cause

The root cause is inadequate input validation in the path parsing logic. The security guard implemented in commit 7d3aee1 performed validation only on pieces[0], the first segment of the dot-separated path array. This left a critical gap where dangerous property keys (__proto__, constructor, prototype) could be placed at any index greater than zero to bypass the protection mechanism and achieve prototype pollution.

Attack Vector

The attack requires network access to an application using vulnerable versions of Dottie.js where user-controlled input is passed to the dottie.set() or dottie.transform() functions. An attacker constructs a path string with __proto__ positioned at any segment other than the first (e.g., "a.__proto__.polluted") to inject arbitrary properties into the Object prototype. This can lead to application-wide behavioral changes, denial of service, or potentially code execution depending on how the polluted properties are consumed by downstream code.

   // Set nested value
   Dottie.set = function(object, path, value, options) {
     var pieces = Array.isArray(path) ? path : path.split('.'), current = object, piece, length = pieces.length;
-    if (pieces[0] === '__proto__') return;
 
+    // Guard against prototype pollution at ANY position in the path
+    // Covers __proto__, constructor, and prototype to prevent all known vectors
+    var DANGEROUS_KEYS = ['__proto__', 'constructor', 'prototype'];
+    if (pieces.some(function(p) { return DANGEROUS_KEYS.indexOf(p) !== -1; })) return;
+    
     if (typeof current !== 'object') {
         throw new Error('Parent is not an object.');
     }

Source: GitHub Commit

Detection Methods for CVE-2026-27837

Indicators of Compromise

• Unexpected properties appearing in JavaScript Object prototypes during application runtime
• Application crashes or unexpected behavior due to polluted prototype chains
• Log entries showing unusual dot-separated paths containing __proto__, constructor, or prototype at non-first positions
• API requests containing suspicious nested object path strings targeting prototype manipulation

Detection Strategies

• Implement runtime monitoring for prototype pollution attempts by detecting modifications to Object.prototype
• Use static analysis tools to identify usage of dottie.set() and dottie.transform() with user-controlled input
• Deploy web application firewalls (WAF) with rules to detect and block requests containing __proto__, constructor, or prototype in nested path parameters
• Conduct dependency audits to identify Dottie.js versions 2.0.4 through 2.0.6 in your package-lock.json or yarn.lock files

Monitoring Recommendations

• Enable verbose logging for input validation failures in applications using Dottie.js
• Monitor npm/yarn audit reports for known vulnerabilities in your JavaScript dependencies
• Set up alerts for any runtime prototype modifications using tools like Object.freeze(Object.prototype) in development environments
• Implement application performance monitoring (APM) to detect anomalous behavior patterns that may indicate exploitation attempts

How to Mitigate CVE-2026-27837

Immediate Actions Required

• Upgrade Dottie.js to version 2.0.7 or later which contains the complete fix for this vulnerability
• Audit your application code for any usage of dottie.set() or dottie.transform() with user-controlled input
• Implement input validation to reject any paths containing dangerous keys before passing them to Dottie.js functions
• Consider using Object.freeze(Object.prototype) as a defense-in-depth measure in sensitive application contexts

Patch Information

The vulnerability has been addressed in Dottie.js version 2.0.7. The fix expands the security guard to check all path segments (not just the first) against a list of dangerous keys including __proto__, constructor, and prototype. The patch is available in commit 7e8fa1345a4b46325f0eab8d7aeb1c4deaefdb14. For detailed information, refer to the GitHub Security Advisory.

Workarounds

• If immediate upgrade is not possible, implement a wrapper function that validates all path segments before calling Dottie.js methods
• Use strict content-type validation and input sanitization to prevent malicious path strings from reaching vulnerable functions
• Deploy prototype pollution protection middleware that freezes critical prototype objects at application startup
• Consider replacing Dottie.js with alternative libraries that have built-in comprehensive prototype pollution protections

# Configuration example
# Upgrade Dottie.js to patched version
npm update dottie@2.0.7

# Or explicitly install the fixed version
npm install dottie@^2.0.7

# Verify the installed version
npm list dottie