Server Side Template Injection in Gorilla Mux with Dynamodb

Server Side Template Injection (SSTI) occurs when an attacker can inject template directives that are later interpreted by a templating engine. In a Go API using Gorilla Mux as the router and DynamoDB as the data store, risk arises when user-controlled input is passed into a response template without proper escaping or validation. Gorilla Mux does not provide templating itself, so SSTI typically manifests when a handler passes request parameters to a third-party template engine (for example html/template or text/template) that processes dynamic content. If the template evaluates injected strings as logic rather than data, an attacker can execute arbitrary template code.

DynamoDB data can become part of this chain when a handler retrieves an item by key and directly inserts a user-supplied identifier (such as a path variable) into a query or into a field used to render a template. Consider a route like /user/{username} where username is used both as a DynamoDB key and as a variable injected into a template. If the handler does not sanitize the value, an attacker could craft a payload that leverages template syntax to execute unwanted behavior when the rendered output is later processed elsewhere (for example, in an email or a downstream system that interprets templates). While DynamoDB itself does not execute templates, the combination of untrusted data from DynamoDB records and insufficient output encoding in templates can amplify impact by enabling stored or reflected injection scenarios.

Another vector involves the use of DynamoDB metadata (such as attribute names or values containing user-controlled content) that is rendered without escaping. For instance, if a handler pulls an item containing user-generated fields and feeds them into a template context without escaping, an attacker who can control those fields may attempt template injection if the downstream consumer reuses that data in a templating context. This is less about DynamoDB executing code and more about ensuring that data retrieved from DynamoDB is never implicitly trusted when used in any subsequent templating layer.

Because middleBrick scans unauthenticated attack surfaces and tests input validation and output encoding through its OWASP API Top 10 checks, it can surface indicators of improper escaping or missing context-aware encoding when DynamoDB-driven responses are involved. The scanner does not assume trust in data sources and emphasizes rigorous input validation and strict output encoding as primary mitigations.

Secure handling of DynamoDB data in Gorilla Mux requires strict separation between data and logic, explicit escaping for any context in which data is rendered, and disciplined use of the AWS SDK. Below are concrete patterns and examples to reduce risk.

• Use parameterized queries and avoid string concatenation: When querying DynamoDB using the AWS SDK for Go (v2), prefer the expression package to build key conditions rather than interpolating user input into attribute names or values.

import (
    "context"
    "github.com/aws/aws-sdk-go-v2/aws"
    "github.com/aws/aws-sdk-go-v2/feature/dynamodb/expression"
    "github.com/aws/aws-sdk-go-v2/service/dynamodb"
)

func getUserByEmail(ctx context.Context, client *dynamodb.Client, email string) (*dynamodb.GetItemOutput, error) {
    // Safe: using expression builder to avoid injection at the DynamoDB level
    keyExpr, err := expression.NewBuilder().
        WithKey(expression.Key{"Email": expression.Value(email)}).
        Build()
    if err != nil {
        return nil, err
    }
    input := &dynamodb.GetItemInput{
        Key: keyExpr.Key(),
        TableName: aws.String("Users"),
    }
    return client.GetItem(ctx, input)
}

• Validate and encode output in templates: When passing DynamoDB item attributes to an html/template, ensure the context is correctly specified (HTML, JS, URL) and that autoescaping is enabled. Do not pass raw strings into JavaScript or style contexts without appropriate escaping.

import (
    "html/template"
    "net/http"
)

type PageData struct {
    Username html.Template // explicitly typed as HTML to enforce escaping
    Bio      string
}

func profileHandler(client *dynamodb.Client) http.HandlerFunc {
    return func(w http.ResponseWriter, r *http.Request) {
        vars := mux.Vars(r)
        username := vars["username"]
        // Assume fetchUserFromDynamo returns validated data
        user, err := fetchUserFromDynamo(client, username)
        if err != nil {
            http.Error(w, "not found", http.StatusNotFound)
            return
        }
        // Proper escaping by using template.HTMLEscapeString or by letting html/template handle it via context-aware escaping
        data := PageData{
            Username: template.HTML(username), // only if you trust and intend HTML; otherwise use plain string and let autoescape handle it
            Bio:      user.Bio,
        }
        tmpl, parseErr := template.New("profile").Parse(`
            <h1>{{.Username}}</h1>
            <p>{{.Bio}}</p>
        `)
        if parseErr != nil {
            http.Error(w, "internal error", http.StatusInternalServerError)
            return
        }
        renderErr := tmpl.Execute(w, data)
        if renderErr != nil {
            http.Error(w, "render error", http.StatusInternalServerError)
        }
    }
}

• Treat metadata as untrusted: If you expose DynamoDB attribute names or construct selectors based on user input, validate against a strict allowlist and avoid direct concatenation into any template or redirect target. Use middleBrick’s input validation and property authorization checks to surface risky patterns where user data influences behavior beyond data retrieval.

By combining parameterized DynamoDB access, context-aware template escaping, and rigorous validation of any user-influenced values, you reduce the likelihood of SSTI and related injection issues when using Gorilla Mux.