Server Side Template Injection in Gin with Mongodb

Server Side Template Injection (SSTI) in a Gin application that uses MongoDB can occur when user-controlled input is interpolated into server-side templates and the resulting values are later passed to MongoDB queries. Gin does not include a built-in template engine, but developers commonly integrate third-party engines such as html/template, text/template, or advanced engines like sprig/tpl. If a template executes user input (for example, via .Username or {{ .SearchQuery }}) and those templates use functions that interact with external data stores, an attacker can inject template code that causes unintended behavior. When the template also constructs MongoDB query documents—either by building raw BSON/bson.M objects or by populating structs that are passed to collection.Find or collection.InsertOne—injected expressions can alter query logic or data flow.

For example, consider a handler that parses a user-supplied template string and then executes it with a data context that includes a database handle or a function that runs a MongoDB operation. An attacker might supply a template like {{ "\" + {"$where": "this.password == \"admin\""} }} if the template engine permits function calls or object construction, potentially turning a read-only template into a query manipulation vector. Even without direct MongoDB function exposure, SSTI can lead to Server-Side Request Forgery (SSRF) or local file inclusion when templates fetch external resources; those side channels can expose internal services that also talk to MongoDB. The risk is compounded when the template executes with elevated privileges or when helper functions abstract MongoDB operations in ways that bypass intended validation.

Real-world patterns that amplify the impact include using template loops or conditional logic to drive batch operations on MongoDB, or exposing template globals that reference database cursors. Because SSTI in Gin often stems from dynamic template loading or user-defined snippets (for example, in a theming or rule-engine feature), the attack surface extends beyond HTTP response rendering to data integrity and confidentiality via MongoDB queries. A security scanner that supports OpenAPI/Swagger 2.0/3.0/3.1 with full $ref resolution can detect template endpoints and flag unsafe usages where user input reaches rendering logic that subsequently influences database commands.

Remediation focuses on strict input validation, avoiding direct template execution of user data, and ensuring MongoDB interactions are parameterized. Do not allow raw user input to define template content; if dynamic templates are required, use a curated set of predefined templates with strongly typed view models. For MongoDB operations, prefer building queries with well-validated structs or the official MongoDB Go driver’s filter builders rather than concatenating user input into bson.M maps.

Example of a vulnerable pattern and a secure alternative in Gin:

// Vulnerable: user input used to build a MongoDB filter via a template-like string
userInput := c.Query("search")
filter := bson.M{"name": userInput} // unsafe if userInput is used in complex ways later
var results []Product
collection.Find(ctx, filter).All(&results)

// Secure: validate and bind user input, then use a fixed query
var params struct {
    Name string `json:"name" binding:"required,max=100"`
}
if err := c.ShouldBindJSON(¶ms); err != nil {
    c.JSON(400, gin.H{"error": "invalid input"})
    return
}
// Use an allowlist or regex for expected values if appropriate
if params.Name != sanitizeName(params.Name) {
    c.JSON(400, gin.H{"error": "invalid name"})
    return
}
filter := bson.M{"name": params.Name}
var products []Product
if err := collection.Find(ctx, filter).Limit(50).All(&products); err != nil {
    c.JSON(500, gin.H{"error": "server error"})
    return
}
c.JSON(200, products)

When you must use server-side templates, treat them as a rendering-only mechanism and never pass database handles or raw query-building functions into the template context. Use Go’s html/template with custom, safe function maps that do not include I/O operations. For MongoDB interactions, keep reads and writes outside the template execution path: compute all filter and update documents in the handler using validated inputs, then pass only those documents to the database driver. If you expose APIs that accept OpenAPI/Swagger specs, ensure $ref resolution is performed and that template-related endpoints are reviewed for data exposure risks.

Additional hardening steps include setting reasonable timeouts on MongoDB operations, using context with cancellation to limit resource usage, and monitoring for unexpected errors that might indicate injection attempts. MiddleBrick’s scans can surface endpoints that accept template input and highlight locations where user data reaches MongoDB-related logic, helping teams prioritize fixes based on severity and framework mappings to OWASP API Top 10 and compliance rules.