CWE-838 Inappropriate Encoding for Output Context

The product uses or specifies an encoding when generating output to a downstream component, but the specified encoding is not the same as the encoding that is expected by the downstream component.

Using the correct output encoding is essential for ensuring that communications between components are accurate. For example, HTML entity encoding is used for elements in the HTML body of a web page. However, a programmer might use entity encoding when generating output that is used within an attribute of an HTML tag, which could contain functional Javascript that is not affected by the HTML encoding. While web applications have received the most attention for this problem, this weakness could potentially apply to any type of product that uses a communications stream that could support multiple encodings.

• Integrity, Confidentiality, Availability: Modify Application Data, Execute Unauthorized Code or Commands — An attacker could modify the structure of the message or data being sent to the downstream component. When the wrong encoding is used - even if closely related - the downstream component could decode the data incorrectly and misinterpret the output. This can have security consequences when the provided boundaries between control and data are inadvertently broken, because the resulting data could introduce control characters or special elements that were not sent by the product. The resulting data could then be used to bypass protection mechanisms such as input validation, and enable injection attacks.

• [Implementation] Use context-aware encoding. That is, understand which encoding is being used by the downstream component, and ensure that this encoding is used. If an encoding can be specified, do so, instead of assuming that the default encoding is the same as the default being assumed by the downstream component. • [Architecture and Design] Where possible, use communications protocols or data formats that provide strict boundaries between control and data. If this is not feasible, ensure that the protocols or formats allow the communicating components to explicitly state which encoding/decoding method is being used. Some template frameworks provide built-in support. • [Architecture and Design] Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid. For example, consider using the ESAPI Encoding control [REF-45] or a similar tool, library, or framework. These will help the programmer encode outputs in a manner less prone to error. Note that some template mechanisms provide built-in support for the appropriate encoding.

• Automated Static Analysis: 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.)

• CVE-2009-2814: Server does not properly handle requests that do not contain UTF-8 data; browser assumes UTF-8, allowing XSS.