CWE-838: Inappropriate Encoding for Output Context

Description

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.

This weakness can cause the downstream component to use a decoding method that produces different data than what the product intended to send. When the wrong encoding is used - even if closely related - the downstream component could decode the data incorrectly. 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. While using output encoding is essential for ensuring that communications between components are accurate, the use of the wrong encoding - even if closely related - could cause the downstream component to misinterpret the output. 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 for 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.

Potential Impact

Integrity, Confidentiality, Availability

Modify Application Data, Execute Unauthorized Code or Commands

Demonstrative Examples

This code dynamically builds an HTML page using POST data:

Bad

$username = $_POST['username'];$picSource = $_POST['picsource'];$picAltText = $_POST['picalttext'];
                     ...
                     
                     echo "<title>Welcome, " . htmlentities($username) ."</title>";echo "<img src='". htmlentities($picSource) ." ' alt='". htmlentities($picAltText) . '" />';
                     ...

The programmer attempts to avoid XSS exploits (CWE-79) by encoding the POST values so they will not be interpreted as valid HTML. However, the htmlentities() encoding is not appropriate when the data are used as HTML attributes, allowing more attributes to be injected.

For example, an attacker can set picAltText to:

Attack

"altTextHere' onload='alert(document.cookie)"

This will result in the generated HTML image tag:

Result

<img src='pic.jpg' alt='altTextHere' onload='alert(document.cookie)' />

The attacker can inject arbitrary javascript into the tag due to this incorrect encoding.

Mitigations & Prevention

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.

Detection Methods

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 sea

Real-World CVE Examples

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

Taxonomy Mappings

The CERT Oracle Secure Coding Standard for Java (2011): IDS13-J — Use compatible encodings on both sides of file or network IO

Frequently Asked Questions

What is CWE-838?

CWE-838 (Inappropriate Encoding for Output Context) is a software weakness identified by MITRE's Common Weakness Enumeration. It is classified as a Base-level weakness. 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.

How can CWE-838 be exploited?

Attackers can exploit CWE-838 (Inappropriate Encoding for Output Context) to modify application data, execute unauthorized code or commands. This weakness is typically introduced during the Implementation phase of software development.

How do I prevent CWE-838?

Key mitigations include: 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 assu

What is the severity of CWE-838?

CWE-838 is classified as a Base-level weakness (Medium abstraction). It has been observed in 1 real-world CVEs.