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ID

CWE-1336: Improper Neutralization of Special Elements Used in a Template Engine

Weakness ID: 1336
Abstraction: Base
Structure: Simple
Status: Incomplete
Presentation Filter:
+ Description
The product uses a template engine to insert or process externally-influenced input, but it does not neutralize or incorrectly neutralizes special elements or syntax that can be interpreted as template expressions or other code directives when processed by the engine.
+ Extended Description

Many web applications use template engines that allow developers to insert externally-influenced values into free text or messages in order to generate a full web page, document, message, etc. Such engines include Twig, Jinja2, Pug, Java Server Pages, FreeMarker, Velocity, ColdFusion, Smarty, and many others - including PHP itself. Some CMS (Content Management Systems) also use templates.

Template engines often have their own custom command or expression language. If an attacker can influence input into a template before it is processed, then the attacker can invoke arbitrary expressions, i.e. perform injection attacks. For example, in some template languages, an attacker could inject the expression "{{7*7}}" and determine if the output returns "49" instead. The syntax varies depending on the language.

In some cases, XSS-style attacks can work, which can obscure the root cause if the developer does not closely investigate the root cause of the error.

Template engines can be used on the server or client, so both "sides" could be affected by injection. The mechanisms of attack or the affected technologies might be different, but the mistake is fundamentally the same.

+ Alternate Terms
Server-Side Template Injection / SSTI:
This term is used for injection into template engines being used by a server.
Client-Side Template Injection / CSTI:
This term is used for injection into template engines being used by a client.
+ Relationships
Section HelpThis table shows the weaknesses and high level categories that are related to this weakness. These relationships are defined as ChildOf, ParentOf, MemberOf and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as PeerOf and CanAlsoBe are defined to show similar weaknesses that the user may want to explore.
+ Relevant to the view "Research Concepts" (CWE-1000)
NatureTypeIDName
ChildOfBaseBase - a weakness that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource.94Improper Control of Generation of Code ('Code Injection')
+ Modes Of Introduction
Section HelpThe different Modes of Introduction provide information about how and when this weakness may be introduced. The Phase identifies a point in the life cycle at which introduction may occur, while the Note provides a typical scenario related to introduction during the given phase.
PhaseNote
Architecture and DesignThe developer might choose a template engine that makes it easier for programmers to write vulnerable code.
ImplementationThe programmer might not use engine's built-in sandboxes or other capabilities to escape or otherwise prevent template injection from untrusted input.
+ Applicable Platforms
Section HelpThis listing shows possible areas for which the given weakness could appear. These may be for specific named Languages, Operating Systems, Architectures, Paradigms, Technologies, or a class of such platforms. The platform is listed along with how frequently the given weakness appears for that instance.

Languages

Java (Undetermined Prevalence)

PHP (Undetermined Prevalence)

Python (Undetermined Prevalence)

JavaScript (Undetermined Prevalence)

Class: Interpreted (Undetermined Prevalence)

Operating Systems

Class: OS-Independent (Undetermined Prevalence)

Technologies

Class: Client Server (Undetermined Prevalence)

+ Common Consequences
Section HelpThis table specifies different individual consequences associated with the weakness. The Scope identifies the application security area that is violated, while the Impact describes the negative technical impact that arises if an adversary succeeds in exploiting this weakness. The Likelihood provides information about how likely the specific consequence is expected to be seen relative to the other consequences in the list. For example, there may be high likelihood that a weakness will be exploited to achieve a certain impact, but a low likelihood that it will be exploited to achieve a different impact.
ScopeImpactLikelihood
Integrity

Technical Impact: Execute Unauthorized Code or Commands

+ Observed Examples
ReferenceDescription
server-side template injection in content management server
authentication / identity management product has client-side template injection
Server-Side Template Injection using a Twig template
devops platform allows SSTI
bypass of Server-Side Template Injection protection mechanism with macros in Velocity templates
web browser proxy server allows Java EL expressions from Server-Side Template Injection
SSTI involving mail templates and JEXL expressions
product does not use a "safe" setting for a FreeMarker configuration, allowing SSTI
product allows read of sensitive database username/password variables using server-side template injection
+ Potential Mitigations

Phase: Architecture and Design

Choose a template engine that offers a sandbox or restricted mode, or at least limits the power of any available expressions, function calls, or commands.

Phase: Implementation

Use the template engine's sandbox or restricted mode, if available.
+ Notes

Relationship

Since expression languages are often used in templating languages, there may be some overlap with CWE-917 (Expression Language Injection). XSS (CWE-79) is also co-located with template injection.
+ References
[REF-1193] James Kettle. "Server-Side Template Injection". 2015-08-05. <https://portswigger.net/research/server-side-template-injection>.
[REF-1194] James Kettle. "Server-Side Template Injection: RCE For The Modern Web App". 2015-12-27. <https://www.youtube.com/watch?v=3cT0uE7Y87s>.
+ Content History
+ Submissions
Submission DateSubmitterOrganization
2021-07-19CWE Content TeamMITRE
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Page Last Updated: July 20, 2021