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Date: May 25, 2021


Prototype pollution vulnerability in 'deep-defaults' versions 1.0.0 through 1.0.5 allows attacker to cause a denial of service and may lead to remote code execution.


The NPM module `deep-defaults` can be abused by Prototype Pollution vulnerability since the function `_deepDefaults()` does not check for the type of object before assigning value to the property. Due to this flaw an attacker could create a non-existent property or able to manipulate the property which leads to Denial of Service or potentially Remote code execution.

PoC Details

The `_deepDefaults ()` function accepts `dest`, `src` as arguments. Due to the absence of validation on the values passed into the `src` argument, an attacker can supply a malicious value by adjusting the value to include the `__proto__` property. Since there is no validation before assigning the property to check whether the assigned argument is the Object's own property or not, the property `polluted` will be directly be assigned to the new object thereby polluting the Object prototype. Later in the code, if there is a check to validate `polluted` the valued would be substituted as "Yes! Its Polluted" as it had been polluted.

PoC Code

var deepDefaults = require("deep-defaults")

var malicious_payload = '{"__proto__":{"polluted":"Yes! Its Polluted"}}';

var obj ={};

console.log("Before : " + {}.polluted);

deepDefaults(obj, JSON.parse(malicious_payload));

console.log("After : " + {}.polluted);

Affected Environments

1.0.0 through 1.0.5


There are a couple of ways to mitigate prototype pollution vulnerabilities, for example: Most of the cases can be solved by freezing an object which doesn’t allow to add, remove, or change its properties. Validating the JSON input with shcema validation, this guarantees that the JSON input contains only predefined attributes. We can change the objects, so they won’t have any prototype association by using “Object.create”.


No Fix

Language: JS

Good to know:


Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution')


Upgrade Version

No fix version available

Base Score:
Attack Vector (AV): Network
Attack Complexity (AC): Low
Privileges Required (PR): None
User Interaction (UI): None
Scope (S): Unchanged
Confidentiality (C): High
Integrity (I): High
Availability (A): High
Base Score:
Access Vector (AV): Network
Access Complexity (AC): Low
Authentication (AU): None
Confidentiality (C): Partial
Integrity (I): Partial
Availability (A): Partial
Additional information: