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Example server-side prototype pollution check

Last updated: April 18, 2024
Read time: 3 Minutes

This BCheck tests for server-side prototype pollution. It works by attempting to pollute the status property of Object.prototype, changing the value of that property to a specific value.


metadata:
    language: v2-beta
    name: "Server-side prototype pollution"
    description: "Server-side prototype pollution using the status technique"
    author: "Gareth Heyes"
    tags: "Server-Side Prototype Pollution"

define:
    payload = `,"__proto__":\{"status\":510\}\}`
    nullify = `,"__proto__":\{"status\":0\}\}`
    issueDetail = "Server-Side Prototype Pollution was found on this web site."
    issueRemediation = "Ensure that property keys, such as __proto__, constructor, and prototype are correctly filtered when merging objects. When creating objects, we recommend using the Object.create(null) API to ensure that your object does not inherit
from the Object.prototype and therefore won't be vulnerable to prototype pollution."

given request then
    if {base.request.body} matches "^[{]" then
        send request called inject_sspp:
            method: "POST"
            body: {regex_replace({base.request.body}, "[}]$", {payload})}

        send request called check_sspp:
            method: "POST"
            body: "{,}"

        if {latest.response.body} matches "\"statusCode\":510" or {latest.response.status_code} is "510" then
            send request called clean_sspp:
                method: "POST"
                body: {regex_replace({base.request.body}, "[}]$", {nullify})}

            send request called followup_check_sspp:
                method: "POST"
                body: "{,}"

            if not({followup_check_sspp.response.body} matches "\"statusCode\":510" or {followup_check_sspp.response.status_code} is "510") then
                report issue:
                    severity: high
                    confidence: firm
                    detail: `{issueDetail}`
                    remediation: `{issueRemediation}`
            end if
        end if
    end if

Step 1: Add metadata


metadata:
    language: v2-beta
    name: "Server-side prototype pollution"
    description: "Server-side prototype pollution using the status technique"
    author: "Gareth Heyes"
    tags: "Server-Side Prototype Pollution"

The definition starts with a metadata block. For more information on available metadata properties, see the reference documentation.

Step 2: Declare variables


define:
    payload = `,"__proto__":\{"status\":510\}\}`
    nullify = `,"__proto__":\{"status\":0\}\}`
    issueDetail = "Server-Side Prototype Pollution was found on this web site."
    issueRemediation = "Ensure that property keys, such as __proto__, constructor, and prototype are correctly filtered when merging objects. When creating objects, we recommend using the Object.create(null) API to ensure that your object does not inherit
from the Object.prototype and therefore won't be vulnerable to prototype pollution."

The next step is to declare the following variables:

payload attempts to pollute the prototype by changing its error status code to 510.
nullify changes the prototype's error status code to 0.
issueDetail defines informational text that can be called when required.
issueRemediation defines remediation text that can be called when required.

Step 3: Attempt to inject SSPP


given request then
    if {base.request.body} matches "^[{]" then
        send request called inject_sspp:
            method: "POST"
            body: {regex_replace({base.request.body}, "[}]$", {payload})}

The next step is to send the initial polluting request.

Burp Scanner first uses an if statement to check for JSON requests. Where it finds a JSON request (identified by an opening curly brace character), Burp Scanner sends a request that uses the regex_replace function to add the contents of the payload variable to the end of the JSON object.

Step 4: Force an error to check for SSPP


send request called check_sspp:
    method: "POST"
    body: "{,}"

The next step is to send a second request to test whether the prototype was successfully polluted. Burp Scanner does this by using an invalid JSON object to force an error.

Step 5: Evaluate results and send follow-up request


if {latest.response.body} matches "\"statusCode\":510" or {latest.response.status_code} is "510" then
    send request called clean_sspp:
        method: "POST"
        body: {regex_replace({base.request.body}, "[}]$", {nullify})}

The next step is to evaluate the error response sent and potentially send a follow-up request.

Burp Scanner uses an if statement to check whether the response's status is now 510. If this is the case, then it sends a follow up request using the nullify variable to reset the status code to 0.

Step 6: Do a second check for SSPP


send request called followup_check_sspp:
    method: "POST"
    body: "{,}"

The next step is to send another request to check the object's status code. Again, Burp Scanner does this by using an invalid JSON object to force an error.

Step 7: Report issues


if not({followup_check_sspp.response.body} matches "\"statusCode\":510" or {followup_check_sspp.response.status_code} is "510") then
    report issue:
        severity: high
        confidence: firm
        detail: `{issueDetail}`
        remediation: `{issueRemediation}`

The final step is to report issues where required.

Burp Scanner uses an if statement to check whether the object's status code is still 510. If it is not, it is likely that the prototype can be polluted. Burp Scanner reports an issue with firm confidence.

Test this BCheck

You can test this BCheck out on the Privilege escalation via server-side prototype pollution Web Security Academy lab.

BCheck definition reference.