CVE-2021-25948

Overview

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.

Details

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

Remediation

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”.

Prevention

No Fix