Format String in Fiber with Cockroachdb

A format string vulnerability occurs when user-controlled input is passed directly into a formatted output function without proper sanitization. In the Fiber web framework for Go, this typically arises when constructing database queries for CockroachDB using string concatenation or formatting verbs that interpret user input as format directives. CockroachDB, like PostgreSQL, uses a wire protocol that treats placeholders as positional parameters (e.g., $1, $2), but if a developer uses Go’s fmt.Sprintf or similar functions to build queries dynamically, attacker-controlled strings can contain format specifiers such as %s, %x, or %n. These specifiers may cause the program to read from or write to memory, potentially leaking stack contents or causing panics that expose internal state.

Consider a typical handler in Fiber that retrieves a user record by ID:

// Unsafe: building SQL with string formatting
userID := c.Params("id")
query := fmt.Sprintf("SELECT username, email FROM users WHERE id = %s", userID)
rows, err := db.Query(query)

If userID contains format verbs (e.g., %s%s%s%s), the fmt.Sprintf call may consume additional arguments from the stack, leading to information disclosure. In a CockroachDB context, this does not directly alter SQL execution because CockroachDB drivers expect parameterized queries, but the vulnerability lies in the Go code constructing the query string, not in CockroachDB itself. An attacker may exploit this to observe memory contents through abnormal output or application behavior, which can then be used to refine further attacks such as BOLA or data exposure checks that middleBrick monitors as part of its 12 security checks.

Another scenario involves logging or error messages where user input is formatted into output. For example:

username := c.FormValue("username")
log.Printf("Authenticating user: %s", username)

If username includes format specifiers, the logging call may read unintended memory, potentially exposing session tokens or other sensitive data that would be flagged under the Data Exposure check. Because middleBrick tests unauthenticated attack surfaces and includes Input Validation and Data Exposure checks, such patterns would be surfaced as high-severity findings with remediation guidance.

To eliminate format string risks when working with CockroachDB in Fiber, always use parameterized queries provided by the database driver. This ensures that user input is treated strictly as data, not as part of the command syntax. The CockroachDB Go driver supports prepared statements and placeholders that are safe from format interpretation.

Here is a secure version of the earlier example using db.QueryRow with placeholders:

// Safe: using parameterized query
userID := c.Params("id")
var username, email string
err := db.QueryRow("SELECT username, email FROM users WHERE id = $1", userID).Scan(&username, &email)
if err != nil {
    c.Status(fiber.StatusInternalServerError)
    return c.JSON(fiber.Map{"error": "unable to fetch user"})
}

For multiple values or dynamic conditions, use sqlx or explicit argument ordering:

// Safe: multiple parameters with clear ordering
startID := c.QueryParam("start")
limit := c.QueryParam("limit")
rows, err := db.Query("SELECT id, username FROM users WHERE id >= $1 AND id < $2 ORDER BY id LIMIT $3", startID, startID+100, limit)

Additionally, validate and sanitize any input that might be used in non-SQL contexts, such as headers or logging. Use Go’s standard formatting verbs explicitly when constructing log messages, and avoid passing untrusted data directly into format strings:

// Safe: explicit formatting with controlled verbs
log.Printf("User login attempt: username=%s", sanitize(username))

Tools like middleBrick’s Input Validation and Data Exposure checks can help identify remaining unsafe patterns. Its scans include checks aligned with OWASP API Top 10 and can map findings to compliance frameworks such as SOC2 and GDPR, providing prioritized remediation guidance. For ongoing protection, the Pro plan enables continuous monitoring and CI/CD integration via the GitHub Action, which can fail builds if insecure patterns are detected during development.