Server Side Template Injection in Buffalo with Dynamodb

Server Side Template Injection (SSTI) in a Buffalo application that uses Amazon DynamoDB can occur when user-controlled input is reflected into server-side templates and then passed to DynamoDB operations in a way that allows attacker-controlled data to influence query construction or be rendered back in unsafe contexts. Buffalo uses the standard Go html/template package by default, which provides auto-escaping; however, if developers use .HTML or call template.HTML on user input, or use third-party template engines that do not escape by default, reflected values can lead to SSTI.

When a compromised template executes, injected template code can trigger unintended behavior in the application layer. In a DynamoDB context, this typically does not mean direct NoSQL injection via template syntax, but it can lead to unsafe usage patterns: for example, concatenating user input into a DynamoDB expression attribute names or values without proper validation, or exposing sensitive data in rendered responses. Consider a handler that fetches an item by an identifier taken from the URL and then renders a template with the returned map:

// Example unsafe pattern in a Buffalo handler
func ItemHandler(c buffalo.Context) error {
    id := c.Param("id")
    out, err := db.Query("SELECT * FROM Items WHERE id = ?", id)
    if err != nil {
        return c.Error(500, err)
    }
    // Danger: passing raw database result to template without sanitization
    return c.Render(200, r.H{"item": out})
}

If the template directly interpolates {{ .item.SomeField }} and SomeField contains malicious template syntax (e.g., from a crafted DynamoDB attribute that includes template-like content), SSTI can manifest during rendering. Additionally, if the handler builds DynamoDB expression attribute names using string concatenation with user input, such as "#status": ? where the placeholder key is user-influenced, this can lead to misuse of the DynamoDB API even though the root cause is insufficient input validation and unsafe reflection. Attackers may leverage SSTI to probe environment variables, trigger server-side request forgery (SSRF) via helper methods in the template, or exfiltrate data through side channels when the injected template causes the application to make unintended calls.

Because DynamoDB is often used for storing structured metadata or configuration, templates might render item properties dynamically. If those properties are user-controlled and are rendered without escaping, the combination of a dynamic rendering path and a schemalNoSQL store increases the impact of SSTI: a single malicious item can compromise multiple users who render that item. This makes input validation, strict type assertions, and output encoding critical when working with DynamoDB results in Buffalo handlers.

To mitigate SSTI when using DynamoDB in Buffalo, ensure that all data retrieved from DynamoDB is treated as untrusted and is never directly injected into templates without proper escaping. Use Go’s html/template package correctly and avoid marking content as safe unless it has been explicitly sanitized. Below are concrete, DynamoDB-aware code fixes and patterns.

1. Parameter validation and type-safe structs

Define concrete structs for DynamoDB results and validate identifiers before using them in queries. Avoid using raw map[string]interface{} for rendering.

// Safe: using a typed struct and validated input
type Item struct {
    ID    string `json:"id"`
    Name  string `json:"name"`
    Email string `json:"email"`
}

func ItemHandler(c buffalo.Context) error {
    id := c.Param("id")
    if !isValidID(id) {
        return c.Error(400, errors.New("invalid item id"))
    }

    var item Item
    err := db.Get(&item, "Items", map[string]interface{}{"id": id})
    if err != nil {
        return c.Error(404, err)
    }
    return c.Render(200, r.H{"item": item})
}

func isValidID(id string) bool {
    // Basic alphanumeric and length check; tighten per your schema
    matched, _ := regexp.MatchString(`^[a-zA-Z0-9_-]{1,64}$`, id)
    return matched
}

2. Safe DynamoDB query construction and expression attribute names

Never concatenate user input into expression attribute names or expression strings. Use placeholders and pass attribute values separately.

// Safe: using placeholders for attribute names and values
func GetItemByAttr(c buffalo.Context) error {
    attrName := "status"
    attrValue := c.Param("status")

    // Validate attrName against a whitelist to prevent injection via DynamoDB expression
    if attrName != "status" && attrName != "category" {
        return c.Error(400, errors.New("invalid attribute"))
    }

    out, err := db.Query("Items", map[string]interface{}{
        "FilterExpression": aws.String("#status = :val"),
        "ExpressionAttributeNames": map[string]interface{}{
            "#status": attrName,
        },
        "ExpressionAttributeValues": map[string]interface{}{
            ":val": attrValue,
        },
    })
    if err != nil {
        return c.Error(500, err)
    }
    // Render using a safe, non-template-executing context or serialize as JSON
    return c.Render(200, r.H{"items": out})
}

3. Template rendering discipline

Ensure templates escape content by default. Do not use template.HTML with data from DynamoDB unless you have sanitized it with a robust library.

{{/* Safe: auto-escaping in html/template */}}
<div>Name: {{ .item.Name }}</div>
<div>Email: {{ .item.Email }}</div>

{{/* Unsafe and discouraged: only use with fully trusted content */}}
{{/* <div>Raw: {{ .item.RawHTML }}</div> */}}

4. Middleware and content-type handling

Use Buffalo’s before/after hooks to enforce safe rendering defaults and to reject responses that may include unintended template fragments. Log and monitor suspicious input patterns that could indicate probing for SSTI.

5. Least-privilege IAM and secret handling

Ensure the DynamoDB credentials used by Buffalo have minimal permissions (e.g., read-only for specific tables). Do not embed secrets in templates or pass them via context that may be exposed through error messages or logs.