COMMON WEAKNESS ENUMERATIONIncomplete

CWE-639

Authorization Bypass Through User-Controlled Key

The system's authorization functionality does not prevent one user from gaining access to another user's data or record by modifying the key value identifying the data.

Abstraction

Base

Structure

Simple

Likelihood of Exploit

High

Understand the problem

What is the problem?

The system's authorization functionality does not prevent one user from gaining access to another user's data or record by modifying the key value identifying the data.

Extended description

Retrieval of a user record occurs in the system based on some key value that is under user control. The key would typically identify a user-related record stored in the system and would be used to lookup that record for presentation to the user. It is likely that an attacker would have to be an authenticated user in the system. However, the authorization process would not properly check the data access operation to ensure that the authenticated user performing the operation has sufficient entitlements to perform the requested data access, hence bypassing any other authorization checks present in the system. For example, attackers can look at places where user specific data is retrieved (e.g. search screens) and determine whether the key for the item being looked up is controllable externally. The key may be a hidden field in the HTML form field, might be passed as a URL parameter or as an unencrypted cookie variable, then in each of these cases it will be possible to tamper with the key value. One manifestation of this weakness is when a system uses sequential or otherwise easily-guessable session IDs that would allow one user to easily switch to another user's session and read/modify their data.

Terminology

Alternate terms and usages

Insecure Direct Object Reference / IDOR

The "Insecure Direct Object Reference" term, as described in the OWASP Top Ten, is broader than this CWE because it also covers path traversal (CWE-22). Within the context of vulnerability theory, there is a similarity between the OWASP concept and CWE-706: Use of Incorrectly-Resolved Name or Reference.

Broken Object Level Authorization / BOLA

BOLA is used in the 2019 OWASP API Security Top 10 and appears to have some overlap, but BOLA may be applicable to other weaknesses in addition to this one.

Horizontal Authorization

"Horizontal Authorization" is used to describe situations in which two users have the same privilege level, but must be prevented from accessing each other's resources. This is fairly common when using key-based access to resources in a multi-user context.

Root cause

How and when does the problem arise?

Lifecycle phase

Architecture and Design

REALIZATION: This weakness is caused during implementation of an architectural security tactic.

Risk

What does successful exploitation lead to?

Bypass Protection Mechanism

Affected security scopes: Access Control

Access control checks for specific user data or functionality can be bypassed.
Gain Privileges or Assume Identity

Affected security scopes: Access Control

Horizontal escalation of privilege is possible (one user can view/modify information of another user).
Gain Privileges or Assume Identity

Affected security scopes: Access Control

Vertical escalation of privilege is possible if the user-controlled key is actually a flag that indicates administrator status, allowing the attacker to gain administrative access.

Defence

How is it prevented and fixed?

Architecture and Design

Architecture and design approach

Detail
For each and every data access, ensure that the user has sufficient privilege to access the record that is being requested.

Architecture and Design · Implementation

Defence to apply across several lifecycle phases

Detail
Make sure that the key that is used in the lookup of a specific user's record is not controllable externally by the user or that any tampering can be detected.

Architecture and Design

Architecture and design approach

Detail
Use encryption in order to make it more difficult to guess other legitimate values of the key or associate a digital signature with the key so that the server can verify that there has been no tampering.

Verification

How is it detected?

Automated Static Analysis

High
Automated static analysis, commonly referred to as Static Application Security Testing (SAST), can find some instances of this weakness by analyzing source code (or binary/compiled code) without having to execute it. Typically, this is done by building a model of data flow and control flow, then searching for potentially-vulnerable patterns that connect "sources" (origins of input) with "sinks" (destinations where the data interacts with external components, a lower layer such as the OS, etc.)