By default qs protects against attacks that attempt to overwrite an object's existing prototype properties, such as toString(), hasOwnProperty(),etc.

Overwriting these properties can impact application logic, potentially allowing attackers to work around security controls, modify data, make the application unstable and more.

In versions of the package affected by this vulnerability, it is possible to circumvent this protection and overwrite prototype properties and functions by prefixing the name of the parameter with [ or ]. e.g. qs.parse("]=toString") will return {toString = true}, as a result, calling toString() on the object will throw an exception.

References:

https://snyk.io/vuln/npm:qs:20170213

Upstream patches:

https://github.com/ljharb/qs/commit/beade029171b8cef9cee0d03ebe577e2dd84976d https://github.com/ljharb/qs/commit/12152db9