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| 1 | +<!DOCTYPE qhelp PUBLIC |
| 2 | +"-//Semmle//qhelp//EN" |
| 3 | +"qhelp.dtd"> |
| 4 | +<qhelp> |
| 5 | + |
| 6 | +<overview> |
| 7 | + <p> |
| 8 | + Most JavaScript objects inherit the properties of the built-in <code>Object.prototype</code> object. |
| 9 | + Prototype pollution is a type of vulnerability in which an attacker is able to modify <code>Object.prototype</code>. |
| 10 | + Since most objects inherit from the compromised <code>Object.prototype</code>, the attacker can use this |
| 11 | + to tamper with the application logic, and often escalate to remote code execution or cross-site scripting. |
| 12 | + </p> |
| 13 | + |
| 14 | + <p> |
| 15 | + One way to cause prototype pollution is through use of an unsafe <em>merge</em> or <em>extend</em> function |
| 16 | + to recursively copy properties from one object to another. |
| 17 | + Such a function has the potential to modify any object reachable from the destination object, and |
| 18 | + the built-in <code>Object.prototype</code> is usually reachable through the special properties |
| 19 | + <code>__proto__</code> and <code>constructor.prototype</code>. |
| 20 | + </p> |
| 21 | +</overview> |
| 22 | + |
| 23 | +<recommendation> |
| 24 | + <p> |
| 25 | + The most effective place to guard against this is in the function that performs |
| 26 | + the recursive copy. |
| 27 | + </p> |
| 28 | + |
| 29 | + <p> |
| 30 | + Only merge a property recursively when it is an own property of the <em>destination</em> object. |
| 31 | + Alternatively, blacklist the property names <code>__proto__</code> and <code>constructor</code> |
| 32 | + from being merged. |
| 33 | + </p> |
| 34 | +</recommendation> |
| 35 | + |
| 36 | +<example> |
| 37 | + <p> |
| 38 | + This function recursively copies properties from <code>src</code> to <code>dst</code>: |
| 39 | + </p> |
| 40 | + |
| 41 | + <sample src="examples/PrototypePollutionUtility.js"/> |
| 42 | + |
| 43 | + <p> |
| 44 | + However, if <code>src</code> is the object <code>{"__proto__": {"xxx": true}}</code>, |
| 45 | + it will inject the property <code>xxx: true</code> in in <code>Object.prototype</code>. |
| 46 | + </p> |
| 47 | + |
| 48 | + <p> |
| 49 | + The issue can be fixed by ensuring that only own properties of the destination object |
| 50 | + are merged recursively: |
| 51 | + </p> |
| 52 | + |
| 53 | + <sample src="examples/PrototypePollutionUtility_fixed.js"/> |
| 54 | + |
| 55 | + <p> |
| 56 | + Alternatively, blacklist the <code>__proto__</code> and <code>constructor</code> properties: |
| 57 | + </p> |
| 58 | + |
| 59 | + <sample src="examples/PrototypePollutionUtility_fixed2.js"/> |
| 60 | +</example> |
| 61 | + |
| 62 | +<references> |
| 63 | + <li>Prototype pollution attacks: |
| 64 | + <a href="https://hackerone.com/reports/380873">lodash</a>, |
| 65 | + <a href="https://hackerone.com/reports/454365">jQuery</a>, |
| 66 | + <a href="https://hackerone.com/reports/381185">extend</a>, |
| 67 | + <a href="https://hackerone.com/reports/430291">just-extend</a>, |
| 68 | + <a href="https://hackerone.com/reports/381194">merge.recursive</a>. |
| 69 | + </li> |
| 70 | +</references> |
| 71 | +</qhelp> |
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