When user input accidentally becomes code your server runs
Most modern applications generate content dynamically, personalized emails, invoices, reports, whatever the page needs at that moment, using a template engine on the server. That’s completely normal. The problem starts the moment user input gets treated as part of the template itself instead of just data plugged into it. When that line blurs, someone can potentially write instructions your server will actually execute, rather than just text it displays.
That’s Server-Side Template Injection, SSTI, and depending on the template engine and how carelessly it’s configured, it can range from “mildly embarrassing information leak” to “someone’s running arbitrary code on your server.” We test to find out which one you’re actually dealing with, and whether the vulnerability is real in the first place.
What this actually is
Template engines exist to generate dynamic content: a page, an email, a document, a report, built from a mix of fixed text and variable data. If application data gets inserted into that template without proper safeguards, specially crafted input can get interpreted as an actual template instruction instead of harmless text sitting in a field.
Depending on the engine, how the application is built, and what restrictions are in place, that can mean information disclosure, unintended access to server-side functionality, or in the more serious cases, actual code execution on your server. Testing tells you which of these your application is genuinely exposed to, rather than leaving you guessing.
Why this is worth taking seriously
Standard vulnerability scanners tend to struggle with SSTI, because it depends heavily on how a specific application actually behaves, not just on matching known signatures. This is exactly the kind of thing that slips through automated checks and sits quietly in production until someone deliberately goes looking.
Testing helps you find where user input is reaching a template engine at all, unsafe dynamic template construction, injected template expressions, server-side expression evaluation, sensitive data leaking through error messages, unintended access to template functionality, weak configuration, and, in the worst cases, a genuine path to server-side code execution. Catching this during development, rather than after launch, saves you from having to fix it under pressure later.
What we actually test
Identifying the template engine. First, we figure out what’s actually running under the hood, Jinja2, Twig, Freemarker, Velocity, Thymeleaf, Handlebars, Mustache, or something custom. We look at application behavior, responses, error messages, and framework patterns for clues.
Testing template expressions. We feed carefully chosen inputs into the application and watch closely: does the app treat this as plain data, or does it get interpreted as an actual template instruction? That distinction is the whole ballgame.
Analyzing input reflection. Not every input that comes back in a response is a vulnerability. We check whether it’s just being reflected as plain text, properly encoded, inserted safely into a template, or actually evaluated by the template engine. This separates genuine SSTI from ordinary, harmless reflection.
Testing template context. Template engines often expose objects, variables, and functions to whatever they’re rendering. We check whether an attacker-controlled value can reach into that context and interact with functionality or data it was never meant to touch.
Reading errors and exceptions. Error messages can be surprisingly chatty. We check whether they’re leaking the template engine’s name, framework details, file paths, variable names, or other backend implementation details that make an attacker’s job easier.
Validating real-world impact. When something looks like genuine SSTI, we validate it carefully to see what it actually enables, information disclosure, access to server-side objects, unauthorized functionality, exposed files or configuration, or in serious cases, actual code execution. All of this happens within the scope we’ve agreed with you.
Testing across the whole application, not just web pages. SSTI isn’t limited to the pages you see in a browser. We test template processing in web applications, REST APIs, admin portals, email generation systems, document generation features, notification systems, reporting platforms, and anywhere else dynamic content gets built server-side.
How we run the testing
Scope and planning. We identify the application components, environments, endpoints, and features that are actually in scope.
Discovery. We map the application’s functionality and look for places where user-controlled input is likely being processed or reflected.
Technology identification. Where possible, we work out the frameworks and template technologies actually involved.
Input testing. We test the relevant parameters and input points to see whether supplied values get interpreted by a template engine.
Behavior validation. We investigate anything that looks suspicious to confirm whether the application is actually evaluating server-side expressions, not just behaving oddly for unrelated reasons.
Impact assessment. For anything confirmed, we work out exactly what information or functionality it could expose.
Risk prioritisation. We rank findings by exploitability, what’s affected, data exposure, and business impact, so you know what to fix first.
Reporting and retesting. You get a detailed breakdown of the vulnerability, the affected areas, the evidence, and how to fix it. Once you’ve made changes, we can confirm the fix actually holds.
What tends to show up
Unsafe server-side template processing. User input being interpreted as template syntax. Dynamic template construction built from untrusted content. Injected template expressions. Weak input validation around template contexts. Unsafe template configuration. Excessive permissions granted to templates. Sensitive server-side information leaking through errors or responses. Exposed backend objects. Information disclosure about the underlying template engine. In the more serious cases, genuine paths to server-side code execution.
SSTI vs. client-side template injection
These sound alike but happen in completely different places. SSTI occurs when untrusted input is interpreted by a template engine sitting on your server. Client-side template injection happens in the user’s browser, inside client-side JavaScript. The testing approach and the potential damage look quite different between the two, so it matters which one you’re actually dealing with.
SSTI vs. SQL injection
Both involve unsafe handling of attacker-controlled input, but they hit different targets entirely. SQL injection goes after your database, affecting confidentiality, integrity, or availability of the data sitting there. SSTI goes after server-side template processing, and depending on the engine and architecture, it can expose application data or backend functionality in a very different way. Both need testing that’s specific to how your application actually works, because generic checks miss the nuance.
SSTI vs. cross-site scripting
Cross-site scripting runs in the victim’s browser. SSTI runs on your server. That distinction matters a lot, because SSTI can potentially affect backend systems directly, not just whatever a user happens to see on their screen. Same general family of “unsafe input handling,” very different blast radius.
Who this is for
Dynamic web applications, SaaS platforms, customer portals, e-commerce applications, enterprise applications, API-driven applications, email template systems, document generation platforms, reporting systems, notification platforms, CMS platforms, and anything using server-side rendering.
It matters most wherever users or administrators can influence content that later gets processed by a template engine, which is more common than most teams realize, custom email templates and report generators being the classic culprits.
When to test
Before launching a new application. During regular application security testing. After major application changes. When introducing a new template engine or switching server-side frameworks. When adding dynamic email or document generation. When implementing custom templates. During regular penetration testing. After major backend architecture changes. Before exposing new functionality publicly.
Testing during development is the ideal time, catching unsafe template handling before it ever reaches production is a lot cheaper than fixing it after the fact.
How to actually prevent this
Treat user input as data, never as template code. Avoid building templates dynamically from untrusted content. Use safe, restricted template APIs and configurations. Apply proper escaping and output encoding. Limit what functionality templates are actually allowed to reach. Keep template engines and frameworks updated. Keep trusted templates clearly separated from anything user-controlled. Review custom template implementations carefully. Back all of it up with strong application-level authorization. And keep testing regularly, because configurations drift and new features get added without anyone rechecking the old assumptions.
Why work with us
We treat SSTI as an application-level problem, not something you solve by pointing a scanner at it and calling it done. We trace how input actually moves through your application and where template processing genuinely happens. Anything that looks like a real vulnerability gets carefully validated to understand its actual impact, not just flagged and left ambiguous. Findings are prioritised by real exposure, explained clearly, and paired with fixes your team can act on. Once you’ve made changes, we retest to confirm the underlying issue is actually gone.
Bottom line
Server-side templates make dynamic applications possible, but they need careful handling the moment user-controlled input gets anywhere near them. If untrusted input can become part of a server-side template, you may well have an SSTI vulnerability sitting there, waiting. This kind of testing finds unsafe template processing, validates what it actually enables, and gives your team a genuinely clear picture of how input is being handled on the server, not just a guess based on what looks suspicious.
FAQs
What is Server-Side Template Injection?
A vulnerability where attacker-controlled input gets interpreted by a server-side template engine as template instructions, rather than treated as ordinary data.
Is SSTI actually serious?
It depends entirely on the application and what the template engine allows. Some cases are limited to information disclosure. Others can expose much more significant server-side functionality.
Which applications can be affected?
Web applications, APIs, email systems, document generators, reporting platforms, CMS solutions, and anything else using a server-side template engine.
How do you actually detect it?
By testing how the application handles specific inputs and examining its responses to see whether user-controlled values are being interpreted by a template engine, then carefully validating anything that looks real.
Is this the same as XSS?
No. SSTI happens during server-side template processing. XSS is about script execution in a user’s browser. Different location, different risk.
Is this the same as SQL injection?
No. SQL injection targets your database. SSTI targets server-side template processing. Related idea, completely different attack path.
Can automated tools catch this?
Some cases, yes. But because the real impact depends so heavily on how your specific application is built, manual testing tends to matter a lot more here than with most vulnerability classes.
How do you fix it?
Stop treating untrusted input as template code, use secure template configurations, restrict what templates can access, keep frameworks updated, and keep trusted templates separate from anything users control.
How often should this be tested?
During development, after major technology changes, as part of regular penetration testing, and before any significant new functionality goes live.
Identify SSTI vulnerabilities in your application
Talk to Sunvak Boreal about your applications and APIs. Let’s find out whether your templates are quietly running instructions they were never meant to receive.