COMMON WEAKNESS ENUMERATIONIncomplete

CWE-942

Permissive Cross-domain Security Policy with Untrusted Domains

The product uses a web-client protection mechanism such as a Content Security Policy (CSP) or cross-domain policy file, but the policy includes untrusted domains with which the web client is allowed to communicate.

Abstraction

Variant

Structure

Simple

Likelihood of Exploit

Medium

Understand the problem

What is the problem?

The product uses a web-client protection mechanism such as a Content Security Policy (CSP) or cross-domain policy file, but the policy includes untrusted domains with which the web client is allowed to communicate.

Extended description

If a cross-domain policy file includes domains that should not be trusted, such as when using wildcards under a high-level domain, then the application could be attacked by these untrusted domains. In many cases, the attack can be launched without the victim even being aware of it.

Root cause

How and when does the problem arise?

Lifecycle phase

Implementation

Lifecycle phase

Architecture and Design

COMMISSION: This weakness refers to an incorrect design related to an architectural security tactic.
In HTTP/HTTPS, policies such as the Same Origin Policy prevent web clients from loading resources from (or making requests to) domains that did not match the web site's own domain, e.g., Javascript or other code hosted on third-party servers. These policies are strictly enforced by browsers and other products. However, these restrictions can be reduced using mechanisms that specify other domains that are allowed to be contacted from the original site, such as Content Security Policy (CSP) or cross-domain policy files (e.g., "crossdomain.xml" in Adobe Flash or Reader, "clientaccesspolicy.xml" in Silverlight, etc.). These mechanisms define a list of domains from which a client is allowed to make cross-domain requests. When making a cross-domain request, the client will first look for the policy file on the target server. If it is found, and the domain hosting the application is explicitly allowed to make requests, the request is made.

Risk

What does successful exploitation lead to?

Execute Unauthorized Code or CommandsBypass Protection MechanismRead Application DataVaries by Context

Affected security scopes: Confidentiality, Integrity, Availability, Access Control

With an overly permissive policy file, an attacker may be able to bypass the web browser's same-origin policy and conduct many of the same attacks seen in Cross-Site Scripting (CWE-79). An attacker can exploit the weakness to transfer private information from the victim's machine to the attacker, manipulate or steal cookies that may include session information, create malicious requests to a web site on behalf of the victim, or execute malicious code on the end user systems. Other damaging attacks include the disclosure of end user files, installation of Trojan horse programs, redirecting the user to some other page or site, running ActiveX controls (under Microsoft Internet Explorer) from sites that a user perceives as trustworthy, and modifying presentation of content.

Defence

How is it prevented and fixed?

Architecture and Design · Operation

Attack Surface Reduction

Detail
Define a restrictive Content Security Policy [REF-1486] or cross-domain policy file.

Architecture and Design · Operation

Attack Surface Reduction

Detail
Avoid using wildcards in the CSP / cross-domain policy file. Any domain matching the wildcard expression will be implicitly trusted, and can perform two-way interaction with the target server.

Architecture and Design · Operation

Environment Hardening

Detail
For Flash, modify crossdomain.xml to use meta-policy options such as 'master-only' or 'none' to reduce the possibility of an attacker planting extraneous cross-domain policy files on a server.

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.)