Format String in Chi with Cockroachdb

Chi is a minimal, composable router for building HTTP services in Go. When developers use string formatting to construct SQL queries for Cockroachdb, a Format String vulnerability can occur if user-controlled input is passed directly into formatting verbs without proper sanitization. Cockroachdb, like other SQL databases, expects strictly typed queries; when format strings are built dynamically using functions such as fmt.Sprintf, attacker-controlled values can alter the structure of the resulting query.

For example, consider a handler that builds a SELECT statement using fmt.Sprintf to inject a table or column name. An attacker could supply a path parameter like ?table=users%27;%20DROP%20TABLE%20users;%20--, which, if passed directly into the format string, may lead to unexpected SQL structure or injection. While this is primarily a SQL injection concern, the format string layer amplifies risk by allowing additional control over the final query string beyond simple value substitution.

In Chi, routes often use middleware and URL parameters that feed into query construction. If a developer writes code such as query := fmt.Sprintf("SELECT * FROM %s WHERE id = $1", tableName) and tableName comes from user input, the format string becomes mutable by the attacker. This can lead to UNION-based data exposure, schema probing, or privilege escalation depending on the database permissions. The interaction between Chi’s routing and Cockroachdb’s SQL engine means that format string weaknesses can translate directly into unauthorized data access or modification.

Unlike prepared statements, which separate SQL logic from data, format-based query construction embeds control characters and syntax directly into the command string. This increases the attack surface because format verbs such as %s, %v, or %x can be leveraged to inject additional formatting instructions. Cockroachdb does not sanitize these inputs, so the database receives the fully formed string and executes it as intended. This is especially dangerous when combined with verbose error messages, which may reveal table names or query structure to an attacker.

Although middleBrick does not fix or block these issues, its scans can detect signs of unsafe format usage in API endpoints that interact with Cockroachdb. By analyzing OpenAPI specs and runtime behavior, the tool can highlight endpoints where user input flows into string formatting routines that ultimately reach the database. This supports compliance mapping to OWASP API Top 10 and helps developers prioritize remediation efforts focused on input validation and query construction discipline.

To prevent Format String vulnerabilities when working with Cockroachdb in Chi, always use parameterized queries or prepared statements instead of string formatting for SQL construction. Never pass user input directly into format verbs that influence SQL structure. Below are concrete, safe patterns using Go’s database/sql package with Cockroachdb.

Safe Query Building with placeholders

Use db.Query or db.Exec with $1, $2 placeholders. Table or column names cannot be parameterized, so they must be validated against an allowlist.

import (
    "context"
    "database/sql"
    _ "github.com/lib/pq"
)

func getUserByID(db *sql.DB, userID int) (string, error) {
    var name string
    // Safe: value placeholder used for data
    err := db.QueryRowContext(context.Background(), "SELECT name FROM users WHERE id = $1", userID).Scan(&name)
    if err != nil {
        return "", err
    }
    return name, nil
}

Allowlist Validation for Identifiers

When dynamic identifiers such as table or column names are required, validate them against a strict allowlist before inclusion in the query string. This prevents format string manipulation at the SQL layer.

func queryTable(db *sql.DB, table string, id int) (string, error) {
    allowedTables := map[string]bool{
        "users": true,
        "logs":  true,
    }
    if !allowedTables[table] {
        return "", sql.ErrNoRows
    }
    // Safe: table name is validated, only value uses placeholder
    query := "SELECT data FROM " + table + " WHERE id = $1"
    var result string
    err := db.QueryRowContext(context.Background(), query, id).Scan(&result)
    return result, err
}

Avoiding Sprintf for SQL Construction

Do not use fmt.Sprintf to construct SQL strings, even with seemingly harmless inputs. If logging is required, use structured logging separately from query execution.

// Unsafe — do not use
// query := fmt.Sprintf("SELECT * FROM %s WHERE id = %d", table, id)

// Safe alternative
stmt, err := db.Prepare("SELECT * FROM users WHERE id = $1")
if err != nil {
    // handle error
}
defer stmt.Close()
var email string
err = stmt.QueryRowContext(context.Background(), id).Scan(&email)

These practices reduce the risk of format string–induced SQL manipulation and align with secure coding standards for database interaction. middleBrick can support this process by scanning API definitions and detecting endpoints that may pass unchecked user input into database-facing functions, enabling teams to focus remediation efforts where it matters most.