Improperly Controlled Modification of Dynamically-Determined Object Attributes

Description

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