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The vulnerability, identified as CVE-2026-30226, resides in the `devalue.parse` and `devalue.unflatten` functions of the devalue library versions 5.6.3 and earlier . devalue is a library used to serialize JavaScript data structures into strings, going beyond standard JSON capabilities . The core issue is a failure to properly sanitize input during the parsing process. A maliciously crafted payload can manipulate the object’s prototype (__proto__) . Specifically, by including a `__proto__` key in a serialized object, an attacker can pollute the base Object.prototype. This adds or overwrites properties on all objects in the application. Furthermore, the parser does not validate that array indices are numeric, allowing an attacker to assign array prototype methods, like push, to object properties, leading to unexpected behavior . Successful exploitation can result in Denial of Service (DoS) by causing exceptions or type confusion, which might alter application logic . The vulnerability is fixed in version 5.6.4 .
Platform: devalue (npm)
Version: 5.6.3, earlier
Vulnerability : Prototype Pollution
Severity: Moderate (CVSS:6.3)
date: March 11, 2026
Prediction: Patched (2026-03-11)
What Undercode Say:
Analytics:
This vulnerability in devalue highlights a critical class of issues in JavaScript serialization libraries. The library is designed to handle complex data structures, but its `parse` and `unflatten` methods lack essential input validation. By injecting a `__proto__` key or non-numeric indices, an attacker can perform a prototype pollution attack. This is particularly dangerous because it can be triggered remotely without authentication. The moderate severity rating (CVSS 6.3) reflects the potential for Denial of Service (DoS) and type confusion, which can lead to application instability or unexpected logical flaws . While not directly leading to remote code execution in this specific vector, the type confusion can be a stepping stone for further exploitation. The fix in version 5.6.4 involves implementing proper checks to reject such malicious payloads during the parsing phase .
Bash and Code Examples:
The following commands and code snippets demonstrate how to identify the vulnerability and understand its exploitation.
1. Check Current Version:
npm list devalue
2. Update to Patched Version:
npm install [email protected]
Or, if using yarn:
yarn add [email protected]
3. Vulnerable Code Example (Conceptual):
This shows how a malicious payload could be used.
const devalue = require('devalue');
// Malicious payload attempting to pollute the prototype
const maliciousPayload = '[{"<strong>proto</strong>":"polluted"}, "data"]';
try {
// If using devalue v5.6.3 or earlier, this line could cause prototype pollution
const result = devalue.parse(maliciousPayload);
console.log("Parsed result:", result);
} catch (e) {
console.error("Parsing failed:", e);
}
// Check if the prototype was polluted
console.log("Polluted property:", {}.polluted); // In vulnerable versions, this might log a value.
4. Safer Handling with Validation (Example):
While the fix is in the library, applications can add a validation layer.
function safeDevalueParse(payload) {
// Basic check for common pollution patterns (not foolproof, update library instead)
if (payload.includes('<strong>proto</strong>') || payload.includes('constructor')) {
throw new Error('Potentially malicious payload detected.');
}
return devalue.parse(payload);
}
How Exploit:
The exploit leverages a crafted string passed to the parsing function.
`__proto__` Pollution: The attacker creates a payload where a property key is __proto__. When parsed, this attempts to assign a value to the base object’s prototype, affecting all objects.
// Example of a payload that pollutes Object.prototype.toString
const payload = '[{"<strong>proto</strong>":{"toString":"polluted"}}, "data"]';
// After parsing with a vulnerable version, {}.toString becomes "polluted".
Array Method Assignment: The attacker provides a non-numeric index (like a method name) to assign an array’s prototype method to a regular object.
// This payload tries to assign Array.prototype.push to an object's toString property.
const object = devalue.parse('[{"toString":"push"}]');
// object.toString is now the push method. Calling object.toString() would execute push.
Protection from this CVE:
Immediate Patching: The primary and most effective protection is to update the `devalue` library to version 5.6.4 or later .
Input Validation: If updating immediately is not possible, implement strict input validation for any data passed to `devalue.parse` or devalue.unflatten. Reject any strings containing __proto__, constructor, or `prototype` keys. However, this is a stop-gap measure and not a complete solution.
Freeze Prototype: As a defense-in-depth measure, you can freeze the base object prototype to prevent any modifications, though this may have side effects.
Object.freeze(Object.prototype);
Impact:
Successful exploitation can lead to:
Denial of Service (DoS): By polluting critical properties, the attacker can cause the application to throw exceptions or behave unexpectedly, leading to crashes or unavailability.
Type Confusion: By assigning array methods to regular objects, an attacker can alter the intended behavior of the application. For example, a property expected to be a string might become a function, leading to logic bypasses or other unintended consequences . This could potentially be chained with other vulnerabilities to achieve more severe impacts like privilege escalation.
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Sources:
Reported By: github.com
Extra Source Hub:
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