SSTI: server-side template injection
Less well known than other vulnerabilities, server-side template injection (SSTI) still has a high impact: it often leads to remote code execution on the server. A vulnerability that must not be underestimated.
What is an SSTI vulnerability?
Many applications use a template engine to separate presentation (HTML, CSS) from logic (PHP, Python, etc.). The template combines fixed data (the layout) and dynamic data (variables); the engine replaces variables with their values to produce the final page.
An SSTI occurs when user-provided data is inserted directly into the template and then interpreted by the engine, instead of being handled as a simple value. The attacker can then inject expressions interpreted by the engine, up to, in some cases, taking control of the server. It is a classic topic in penetration tests application-focused.
Where the risk appears
Advanced personalization
Sites offering advanced personalization features (wikis, blogs, marketing applications, CMS) where the user can modify templates.
Email templates
An application that lets the user edit the template for an automatic email: if its expressions are evaluated, the feature becomes vulnerable.
Up to code execution
Depending on the context, SSTI can lead to remote code execution (RCE) and server compromise, the maximum impact.
Other possible attacks
Even without reaching RCE, it can enable file reading, information leaks, or privilege escalation.
A discreet flaw with serious consequences
SSTI is less frequently looked for than XSS or SQL injection, simply because it is less well known. This is exactly what makes it dangerous: it often goes unnoticed during superficial checks, while its impact can be maximal. As soon as a feature lets a user influence a template, the issue deserves careful review as part of an application security approach.
The defense principle: never make the user the template's “author”
The golden rule: user data must be passed as values to the template, never concatenated into the model itself. When template customization is necessary, rely on an engine in “ sandbox ” mode (sandbox), severely restrict the accessible functions, and rigorously validate inputs. Detection requires injecting template-engine-specific special characters, typically during a penetration test.
Prevent SSTI, step by step
Separate data and template
Pass user data as variables, without ever inserting it into the model structure.
Avoid template editing
Limit as much as possible any feature that lets a user modify a template; reserve it for trusted profiles.
Isolate the engine
Use an engine in sandbox mode and disable dangerous or unnecessary functions.
Validate inputs
Filter template-engine-specific characters and expressions in the relevant fields.
Apply least privilege
Restrict the server process rights to limit the impact of a successful exploitation.
Test specifically
Have SSTI specifically checked during a pentest, because it escapes generic controls.
Why work with BCIT?
Specialized testing
We specifically test for SSTI, where automated controls would miss it.
Tailored fixes
Recommendations aligned with your template engine and your real use cases.
Comprehensive coverage
From application security to security audits, we cover the whole chain.
Not ready to talk yet? Discover our cybersecurity diagnostic →
Are your template engines under control?
Let's take 15 minutes to assess your applications' exposure to SSTI and define the right prevention measures.
SSTI: server-side template injection
Less well known than other vulnerabilities, server-side template injection (SSTI) still has a high impact: it often leads to remote code execution on the server. A vulnerability that must not be underestimated.
What is an SSTI vulnerability?
Many applications use a template engine to separate presentation (HTML, CSS) from logic (PHP, Python, etc.). The template combines fixed data (the layout) and dynamic data (variables); the engine replaces variables with their values to produce the final page.
An SSTI occurs when user-provided data is inserted directly into the template and then interpreted by the engine, instead of being handled as a simple value. The attacker can then inject expressions interpreted by the engine, up to, in some cases, taking control of the server. It is a classic topic in penetration tests application-focused.
Where the risk appears
Advanced personalization
Sites offering advanced personalization features (wikis, blogs, marketing applications, CMS) where the user can modify templates.
Email templates
An application that lets the user edit the template for an automatic email: if its expressions are evaluated, the feature becomes vulnerable.
Up to code execution
Depending on the context, SSTI can lead to remote code execution (RCE) and server compromise, the maximum impact.
Other possible attacks
Even without reaching RCE, it can enable file reading, information leaks, or privilege escalation.
A discreet flaw with serious consequences
SSTI is less frequently looked for than XSS or SQL injection, simply because it is less well known. This is exactly what makes it dangerous: it often goes unnoticed during superficial checks, while its impact can be maximal. As soon as a feature lets a user influence a template, the issue deserves careful review as part of an application security approach.
The defense principle: never make the user the template's “author”
The golden rule: user data must be passed as values to the template, never concatenated into the model itself. When template customization is necessary, rely on an engine in “ sandbox ” mode (sandbox), severely restrict the accessible functions, and rigorously validate inputs. Detection requires injecting template-engine-specific special characters, typically during a penetration test.
Prevent SSTI, step by step
Separate data and template
Pass user data as variables, without ever inserting it into the model structure.
Avoid template editing
Limit as much as possible any feature that lets a user modify a template; reserve it for trusted profiles.
Isolate the engine
Use an engine in sandbox mode and disable dangerous or unnecessary functions.
Validate inputs
Filter template-engine-specific characters and expressions in the relevant fields.
Apply least privilege
Restrict the server process rights to limit the impact of a successful exploitation.
Test specifically
Have SSTI specifically checked during a pentest, because it escapes generic controls.
Why work with BCIT?
Specialized testing
We specifically test for SSTI, where automated controls would miss it.
Tailored fixes
Recommendations aligned with your template engine and your real use cases.
Comprehensive coverage
From application security to security audits, we cover the whole chain.
Not ready to talk yet? Discover our cybersecurity diagnostic →
Are your template engines under control?
Let's take 15 minutes to assess your applications' exposure to SSTI and define the right prevention measures.