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Date: June 10, 2021


Prototype pollution vulnerability in ‘expand-hash’ versions 0.1.0 through 1.0.1 allows an attacker to cause a denial of service and may lead to remote code execution.


The NPM module `expand-hash` can be abused by Prototype Pollution vulnerability since the function `expand()` 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 be able to manipulate the property which leads to Denial of Service or potentially Remote code execution.

PoC Details

The `expand()` function accepts `value` as an argument. Due to the absence of validation on the values passed into the 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 assigned to the new object thereby polluting the Object prototype. Later in the code, if there is a check to validate `polluted` the value would be substituted as "Yes! It's Polluted" as it had been polluted.

PoC Code

var expandHash = require("expand-hash")

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

expandHash({"__proto__.polluted": "Yes! Its Polluted"})

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

Affected Environments

0.1.0 to 1.0.1


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 Dynamically-Determined Object Attributes


Upgrade Version

No fix version available

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