NoSQL syntax injection

Detecting syntax injection in MongoDB

Consider a shopping application that displays products in different categories. When the user selects the Fizzy drinks category, their browser requests the following URL:

https://insecure-website.com/product/lookup?category=fizzy

This causes the application to send a JSON query to retrieve relevant products from the product collection in the MongoDB database:

this.category == 'fizzy'

To test whether the input may be vulnerable, submit a fuzz string in the value of the category parameter. An example string for MongoDB is:

'"`{
;$Foo}
$Foo \xYZ

Use this fuzz string to construct the following attack:

https://insecure-website.com/product/lookup?category='%22%60%7b%0d%0a%3b%24Foo%7d%0d%0a%24Foo%20%5cxYZ%00

If this causes a change from the original response, this may indicate that user input isn't filtered or sanitized correctly.

Note

NoSQL injection vulnerabilities can occur in a variety of contexts, and you need to adapt your fuzz strings accordingly. Otherwise, you may simply trigger validation errors that mean the application never executes your query.

In this example, we're injecting the fuzz string via the URL, so the string is URL-encoded. In some applications, you may need to inject your payload via a JSON property instead. In this case, this payload would become '\"`{\r;$Foo}\n$Foo \\xYZ\u0000.

Determining which characters are processed

To determine which characters are interpreted as syntax by the application, you can inject individual characters. For example, you could submit ', which results in the following MongoDB query:

this.category == '''

If this causes a change from the original response, this may indicate that the ' character has broken the query syntax and caused a syntax error. You can confirm this by submitting a valid query string in the input, for example by escaping the quote:

this.category == '\''

If this doesn't cause a syntax error, this may mean that the application is vulnerable to an injection attack.

Confirming conditional behavior

After detecting a vulnerability, the next step is to determine whether you can influence boolean conditions using NoSQL syntax.

To test this, send two requests, one with a false condition and one with a true condition. For example you could use the conditional statements ' && 0 && 'x and ' && 1 && 'x as follows:

https://insecure-website.com/product/lookup?category=fizzy'+%26%26+0+%26%26+'x

https://insecure-website.com/product/lookup?category=fizzy'+%26%26+1+%26%26+'x

If the application behaves differently, this suggests that the false condition impacts the query logic, but the true condition doesn't. This indicates that injecting this style of syntax impacts a server-side query.

Overriding existing conditions

Now that you have identified that you can influence boolean conditions, you can attempt to override existing conditions to exploit the vulnerability. For example, you can inject a JavaScript condition that always evaluates to true, such as '||1||':

https://insecure-website.com/product/lookup?category=fizzy%27%7c%7c%31%7c%7c%27

This results in the following MongoDB query:

this.category == 'fizzy'||'1'=='1'

As the injected condition is always true, the modified query returns all items. This enables you to view all the products in any category, including hidden or unknown categories.

Warning

Take care when injecting a condition that always evaluates to true into a NoSQL query. Although this may be harmless in the initial context you're injecting into, it's common for applications to use data from a single request in multiple different queries. If an application uses it when updating or deleting data, for example, this can result in accidental data loss.

You could also add a null character after the category value. MongoDB may ignore all characters after a null character. This means that any additional conditions on the MongoDB query are ignored. For example, the query may have an additional this.released restriction:

this.category == 'fizzy' && this.released == 1

The restriction this.released == 1 is used to only show products that are released. For unreleased products, presumably this.released == 0.

In this case, an attacker could construct an attack as follows:

https://insecure-website.com/product/lookup?category=fizzy'%00

This results in the following NoSQL query:

this.category == 'fizzy'\u0000' && this.released == 1

If MongoDB ignores all characters after the null character, this removes the requirement for the released field to be set to 1. As a result, all products in the fizzy category are displayed, including unreleased products.

Lab

Lab: Detecting NoSQL injection | Web Security AcademyWebSecAcademy

The product category filter for this lab is powered by a MongoDB NoSQL database. It is vulnerable to NoSQL injection.

To solve the lab, perform a NoSQL injection attack that causes the application to display unreleased products.

Steps

• Open burp suite

• Visit the target website

• Send the https://0a1e006f03df90bf8527f42200420044.web-security-academy.net/filter?category=Accessories URL to repeater

• Enter the payload after filter?category=Accessories %27%7c%7c%31%7c%7c%27

https://0a1e006f03df90bf8527f42200420044.web-security-academy.net/filter?category=Accessories%27%7c%7c%31%7c%7c%27

• $$$