What steps should be taken to safely sanitize untrusted JSON data before parsing it with JSON.parse method

Question

What steps should be taken to safely sanitize untrusted JSON data before parsing it with JSON.parse method

How can we properly sanitize a user-provided JSON string to prevent any vulnerabilities before executing JSON.parse(untrustedString)?

My main concern revolves around prototype pollution, but I'm also interested in knowing about any other potential risks to watch out for. If prototype pollution is the only issue to worry about, I assume we can address it using regular expressions. However, I suspect there may be other security concerns as well?

For instance, I came across an article discussing the risks associated with parsing untrusted JSON and then creating a duplicate of the object.:

Imagine receiving some malicious JSON data via an endpoint.
{
  "user": {
    "__proto__": {
      "admin": true
    }
  }
} 
If this JSON payload is processed by JSON.parse, it will generate an object with a __proto__ property. If the copying mechanism operates as described in the example, it will transfer the admin property onto the prototype of req.session.user!

javascript json security parsing sanitization

Answer 1

Answer №1

My main concern revolves around prototype pollution

It's important to note that JSON.parse does not pollute any prototype objects. If the JSON string contains a "__proto__" key, it will simply create that key like any other key, with the corresponding value being a property value and not affecting the prototype object (Object.prototype).

The real risk comes into play when interacting with the object afterwards. If you perform a (deep) copy using property assignments or Object.assign, then you could potentially mutate the prototype object.

How can we sanitize it before executing JSON.parse(untrustedString)? ... Perhaps through the use of regex?

Avoid using regular expressions for sanitization in this case. Instead, utilize the second argument of JSON.parse:

const cleaner = (key, value) => key === "__proto__" ? undefined : value;

// demonstration
let json = '{"user":{"__proto__":{"admin": true}}}';

console.log(JSON.parse(json));
console.log(JSON.parse(json, cleaner));

Answer 2

My main concern revolves around prototype pollution

It's important to note that JSON.parse does not pollute any prototype objects. If the JSON string contains a "__proto__" key, it will simply create that key like any other key, with the corresponding value being a property value and not affecting the prototype object (Object.prototype).

The real risk comes into play when interacting with the object afterwards. If you perform a (deep) copy using property assignments or Object.assign, then you could potentially mutate the prototype object.

How can we sanitize it before executing JSON.parse(untrustedString)? ... Perhaps through the use of regex?

Avoid using regular expressions for sanitization in this case. Instead, utilize the second argument of JSON.parse:

const cleaner = (key, value) => key === "__proto__" ? undefined : value;

// demonstration
let json = '{"user":{"__proto__":{"admin": true}}}';

console.log(JSON.parse(json));
console.log(JSON.parse(json, cleaner));

Answer 3

Answer №2

Initially, when dealing with the variable userString, it's crucial to recognize that it's merely a string. In isolation, the string itself poses no threat to a system unless the system unknowingly facilitates harm by processing it in an unsafe manner.

One solution to safely handle such data is by utilizing JSON.parse().

JSON.parse() essentially serves as a tool for converting formats. It refrains from executing any functions present within the data (a vulnerability exploited in proto pollution), or delving deep into the actual content of the stringified object itself, focusing on the structural syntax it adheres to along with JavaScript reserved words for validation purposes (referencing JSON syntax, and an example of a MDN polyfill). Similarly, akin to handling the string, as long as precautions are taken with the output object, any potential risks to the system can be mitigated.

In essence, effective prevention against abuse revolves around the principles of validation and safe data management techniques:

Thoroughly inspect the object derived from parsing the string, and validate it within strict boundaries while disregarding prototype modifications.
Employ
```
Object.prototype.hasOwnProperty.call()
```
and reinforce these strategies within your codebase utilizing tools like eslint (through the no-prototype-builtins rule).
It's unrealistic to assume that users will consistently submit flawless, secure data.

Referencing the linked article, the writer emphasizes this very concept:

...data coming in from users should always be filtered and sanitized.

Answer 4

Initially, when dealing with the variable userString, it's crucial to recognize that it's merely a string. In isolation, the string itself poses no threat to a system unless the system unknowingly facilitates harm by processing it in an unsafe manner.

One solution to safely handle such data is by utilizing JSON.parse().

JSON.parse() essentially serves as a tool for converting formats. It refrains from executing any functions present within the data (a vulnerability exploited in proto pollution), or delving deep into the actual content of the stringified object itself, focusing on the structural syntax it adheres to along with JavaScript reserved words for validation purposes (referencing JSON syntax, and an example of a MDN polyfill). Similarly, akin to handling the string, as long as precautions are taken with the output object, any potential risks to the system can be mitigated.

In essence, effective prevention against abuse revolves around the principles of validation and safe data management techniques:

Thoroughly inspect the object derived from parsing the string, and validate it within strict boundaries while disregarding prototype modifications.
Employ
```
Object.prototype.hasOwnProperty.call()
```
and reinforce these strategies within your codebase utilizing tools like eslint (through the no-prototype-builtins rule).
It's unrealistic to assume that users will consistently submit flawless, secure data.

Referencing the linked article, the writer emphasizes this very concept:

...data coming in from users should always be filtered and sanitized.

Answer 5

Answer №3

One crucial consideration is to always restrict the size of data. Exceeding memory limits can potentially breach the secure confines of your application.

Equally significant is the need to define a specific character set and sanitize any unnecessary elements to minimize risk.

If you do not require comprehensive Unicode compatibility, it may be wise to filter out such complexities and simplify to a more manageable system like ASCII to avoid potential code corruption.

Lastly, maintain a clear list of supported keys and validate the input values for each to ensure the integrity of your application's functionality.

Answer 6

One crucial consideration is to always restrict the size of data. Exceeding memory limits can potentially breach the secure confines of your application.

Equally significant is the need to define a specific character set and sanitize any unnecessary elements to minimize risk.

If you do not require comprehensive Unicode compatibility, it may be wise to filter out such complexities and simplify to a more manageable system like ASCII to avoid potential code corruption.

Lastly, maintain a clear list of supported keys and validate the input values for each to ensure the integrity of your application's functionality.

What steps should be taken to safely sanitize untrusted JSON data before parsing it with JSON.parse method

Answer №1

Answer №2

...data coming in from users should always be filtered and sanitized.

Answer №3

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