Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution')

The software receives input from an upstream component that specifies attributes that are to be initialized or updated in an object, but it does not properly control modifications of attributes of the object prototype.


Description

By adding or modifying attributes of an object prototype, it is possible to create attributes that exist on every object, or replace critical attributes with malicious ones. This can be problematic if the software depends on existence or non-existence of certain attributes, or uses pre-defined attributes of object prototype (such as hasOwnProperty, toString or valueOf).

This weakness is usually exploited by using a special attribute of objects called proto, constructor or prototype. Such attributes give access to the object prototype. This weakness is often found in code that assigns object attributes based on user input, or merges or clones objects recursively.

Demonstrations

The following examples help to illustrate the nature of this weakness and describe methods or techniques which can be used to mitigate the risk.

Note that the examples here are by no means exhaustive and any given weakness may have many subtle varieties, each of which may require different detection methods or runtime controls.

Example One

This function sets object attributes based on a dot-separated path.

function setValueByPath (object, path, value) {

  const pathArray = path.split(".");
  const attributeToSet = pathArray.pop();
  let objectToModify = object;
  for (const attr of pathArray) {

    if (typeof objectToModify[attr] !== 'object') {

      objectToModify[attr] = {};
      }

    objectToModify = objectToModify[attr];
    }

  objectToModify[attributeToSet] = value;
  return object;
  }

This function does not check if the attribute resolves to the object prototype. These codes can be used to add "isAdmin: true" to the object prototype.

setValueByPath({}, "__proto__.isAdmin", true)
setValueByPath({}, "constructor.prototype.isAdmin", true)

By using a denylist of dangerous attributes, this weakness can be eliminated.

function setValueByPath (object, path, value) {

  const pathArray = path.split(".");
  const attributeToSet = pathArray.pop();
  let objectToModify = object;
  for (const attr of pathArray) {

    // Ignore attributes which resolve to object prototype
    if (attr === "__proto__" || attr === "constructor" || attr === "prototype") {

      continue;
      }

    if (typeof objectToModify[attr] !== "object") {

      objectToModify[attr] = {};
      }

    objectToModify = objectToModify[attr];
    }

  objectToModify[attributeToSet] = value;
  return object;
  }

See Also

Comprehensive CWE Dictionary

This view (slice) covers all the elements in CWE.

Weaknesses without Software Fault Patterns

CWE identifiers in this view are weaknesses that do not have associated Software Fault Patterns (SFPs), as covered by the CWE-888 view. As such, they represent gaps in...

Weaknesses Introduced During Implementation

This view (slice) lists weaknesses that can be introduced during implementation.


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