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.

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.

Primary

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.

Architecture and Design
The developer might choose a template engine that makes it easier for programmers to write vulnerable code.

Implementation
The programmer might not use engine's built-in sandboxes or other capabilities to escape or otherwise prevent template injection from untrusted input.

Integrity
Execute Unauthorized Code or Commands

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.)
High

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.

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

CVE-2024-34359
Chain: Python bindings for LLM
library do not use a sandboxed environment when
parsing a template and constructing a prompt,
allowing jinja2 Server Side Template Injection and
code execution - one variant of a "prompt
injection" attack.
https://www.cve.org/CVERecord?id=CVE-2024-34359

CVE-2017-16783
server-side template injection in content management server
https://www.cve.org/CVERecord?id=CVE-2017-16783

CVE-2020-9437
authentication / identity management product has client-side template injection
https://www.cve.org/CVERecord?id=CVE-2020-9437

CVE-2020-12790
Server-Side Template Injection using a Twig template
https://www.cve.org/CVERecord?id=CVE-2020-12790

CVE-2021-21244
devops platform allows SSTI
https://www.cve.org/CVERecord?id=CVE-2021-21244

CVE-2020-4027
bypass of Server-Side Template Injection protection mechanism with macros in Velocity templates
https://www.cve.org/CVERecord?id=CVE-2020-4027

CVE-2020-26282
web browser proxy server allows Java EL expressions from Server-Side Template Injection
https://www.cve.org/CVERecord?id=CVE-2020-26282

CVE-2020-1961
SSTI involving mail templates and JEXL expressions
https://www.cve.org/CVERecord?id=CVE-2020-1961

CVE-2019-19999
product does not use a "safe" setting for a FreeMarker configuration, allowing SSTI
https://www.cve.org/CVERecord?id=CVE-2019-19999

CVE-2018-20465
product allows read of sensitive database username/password variables using server-side template injection
https://www.cve.org/CVERecord?id=CVE-2018-20465

Allowed
This CWE entry is at the Base level of abstraction, which is a preferred level of abstraction for mapping to the root causes of vulnerabilities.
Carefully read both the name and description to ensure that this mapping is an appropriate fit. Do not try to 'force' a mapping to a lower-level Base/Variant simply to comply with this preferred level of abstraction.

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.

The interrelationships and differences between CWE-917 and CWE-1336 need to be further clarified.

CWE Content Team
MITRE
2021-07-19
4.5
2021-07-20

CWE Content Team
MITRE
2022-06-28
updated Maintenance_Notes, Relationships

CWE Content Team
MITRE
2023-04-27
updated Relationships

CWE Content Team
MITRE
2023-06-29
updated Mapping_Notes

CWE Content Team
MITRE
2024-07-16
4.15
2024-07-16
updated Applicable_Platforms, Observed_Examples

CWE Content Team
MITRE
2025-12-11
4.19
2025-12-11
updated Applicable_Platforms, Detection_Factors, Weakness_Ordinalities