qs before 6.10.3, as used in Express before 4.17.3 and other products, allows attackers to cause a Node process hang for an Express application because an __ proto__ key can be used. In many typical Express use cases, an unauthenticated remote attacker can place the attack payload in the query string of the URL that is used to visit the application, such as a[__proto__]=b&a[__proto__]&a[length]=100000000. The fix was backported to qs 6.9.7, 6.8.3, 6.7.3, 6.6.1, 6.5.3, 6.4.1, 6.3.3, and 6.2.4 (and therefore Express 4.17.3, which has deps: qs@6.9.7 in its release description, is not vulnerable).
The product 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.
| Name | Vendor | Start Version | End Version |
|---|---|---|---|
| Qs | Qs_project | * | 6.2.4 (excluding) |
| Qs | Qs_project | 6.3.0 (including) | 6.3.3 (excluding) |
| Qs | Qs_project | 6.5.0 (including) | 6.5.3 (excluding) |
| Qs | Qs_project | 6.7.0 (including) | 6.7.3 (excluding) |
| Qs | Qs_project | 6.8.0 (including) | 6.8.3 (excluding) |
| Qs | Qs_project | 6.9.0 (including) | 6.9.7 (excluding) |
| Qs | Qs_project | 6.10.0 (including) | 6.10.3 (excluding) |
| Qs | Qs_project | 6.4.0 (including) | 6.4.0 (including) |
| Qs | Qs_project | 6.6.0 (including) | 6.6.0 (including) |
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 product 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.