Format String in Echo Go with Dynamodb

A format string vulnerability occurs when user-controlled input is passed directly into a formatting function such as fmt.Sprintf or fmt.Printf without a explicit format specifier. In an Echo Go service that interacts with DynamoDB, this often happens when constructing request parameters, logging query details, or building error messages using unchecked input.

Consider an endpoint that accepts a user_id path parameter and uses it to build a DynamoDB key condition or a log message:

// Echo handler (vulnerable)
func getUser(c echo.Context) error {
    userID := c.Param("user_id")
    // Unsafe: userID is used directly in a log format string
    fmt.Printf("Fetching user: " + userID + " from DynamoDB")
    key, err := dynamodb.BuildKey("user_id", userID)
    if err != nil {
        return err
    }
    // ... use key with DynamoDB
    return c.JSON(http.StatusOK, key)
}

If an attacker sends user_id as %s %s %s, fmt.Printf will consume additional (unprovided) arguments, potentially causing a panic or reading stack memory. More critically, if the same input is used to build a DynamoDB expression without validation, format-like behavior can alter the intent of the expression (e.g., via crafted placeholders that change how the SDK serializes values). While DynamoDB itself does not use printf-style formatting, the vulnerability arises when Echo Go code mixes unchecked strings with DynamoDB attribute values used in condition expressions or key construction, leading to malformed keys or injection-like behavior in logs and error messages.

Another scenario involves constructing a DynamoDB KeyConditionExpression using string concatenation with user input:

// Vulnerable key condition assembly
func queryItems(c echo.Context) error {
    partition := c.QueryParam("partition")
    // Unsafe: partition is concatenated into an expression string
    expr := "pk = '" + partition + "'"
    out, err := svc.Query(context.TODO(), &dynamodb.QueryInput{
        KeyConditionExpression: aws.String(expr),
        TableName:              aws.String("Items"),
    })
    return c.JSON(http.StatusOK, out)
}

If partition contains format-like sequences (e.g., %s), they do not affect the DynamoDB expression directly, but they illustrate how unchecked input can propagate into structured query strings. The real risk in the Echo + DynamoDB context is that developers may mistakenly treat format strings as safe when building SDK inputs, leading to log injection, stack leaks, or malformed expressions that cause unexpected behavior.

Because middleBrick scans unauthenticated attack surfaces and runs checks in parallel, it can detect endpoints where user input reaches logging or expression-building code paths that intersect with DynamoDB usage. Findings include references to OWASP API Top 10 categories such as Injection and Security Misconfiguration, with remediation guidance focused on input validation and strict separation of data and control flow.

Remediation centers on never allowing user-controlled data to directly shape format strings or expression components. Use explicit parameterization, strict validation, and structured SDK builders instead of string concatenation.

• Use fmt.Sprintf with a fixed format and explicit arguments, never with raw user input as the format specifier:

// Safe logging
func getUser(c echo.Context) error {
    userID := c.Param("user_id")
    // Validate userID format (e.g., UUID, alphanumeric)
    if !isValidUserID(userID) {
        return echo.NewHTTPError(http.StatusBadRequest, "invalid user id")
    }
    fmt.Sprintf("Fetching user: %s from DynamoDB", userID)
    // ... proceed safely
}

• Build DynamoDB expressions using the SDK’s expression builders and never via string concatenation. For key conditions, use placeholders and expression attribute values:

// Safe DynamoDB query with expression attributes
func queryItems(c echo.Context) error {
    partition := c.QueryParam("partition")
    if partition == "" || len(partition) > 255 {
        return echo.NewHTTPError(http.StatusBadRequest, "invalid partition key")
    }
    expr := "pk = :pval"
    av := map[string]types.AttributeValue{
        ":pval": &types.AttributeValueMemberS{Value: partition},
    }
    out, err := svc.Query(context.TODO(), &dynamodb.QueryInput{
        KeyConditionExpression: aws.String(expr),
        ExpressionAttributeValues: av,
        TableName:              aws.String("Items"),
    })
    return c.JSON(http.StatusOK, out)
}

• Leverage middleware in Echo to sanitize and validate inputs before handlers run, ensuring DynamoDB-bound parameters conform to expected patterns:

// Echo middleware for input validation
func ValidateParams(next echo.HandlerFunc) echo.HandlerFunc {
    return func(c echo.Context) error {
        userID := c.Param("user_id")
        if !regexp.MustCompile(`^[A-Za-z0-9_-]{1,64}$`).MatchString(userID) {
            return echo.NewHTTPError(http.StatusBadRequest, "invalid user id")
        }
        return next(c)
    }
}

These practices align with middleBrick’s findings, which map to compliance frameworks such as OWASP API Top 10 and PCI-DSS. The free tier on the Web Dashboard allows quick scanning to identify vulnerable endpoints, while the Pro plan’s continuous monitoring can alert teams if new risky patterns appear in commits that modify DynamoDB interaction code.

For CI/CD enforcement, the GitHub Action can fail builds when a submitted URL (e.g., a staging endpoint) returns a risk score above a chosen threshold, preventing merges that introduce format string or injection risks. The CLI tool supports scripting workflows: middlebrick scan https://staging.example.com/api/users returns JSON output that can be parsed to gate deployments.