Double Free in Fiber with Cockroachdb

A Double Free occurs when a program attempts to free the same block of memory more than once. In a Fiber application using CockroachDB, this typically arises from improper management of request-scoped objects, connection handling, or transaction state across concurrent handlers. Because Fiber is a fast, low-level HTTP framework for Go, developers may directly manage resources such as database client sessions or prepared statements. When these resources are freed or closed in one handler and then inadvertently referenced or freed again in another—especially under concurrent load or when middleware recycles context—the runtime can corrupt memory, leading to crashes or exploitable behavior.

With CockroachDB, the interaction amplifies risk during transaction retries and session handoffs. CockroachDB drivers often maintain internal buffers and state for connection pooling and transaction context. If a Fiber route initiates a transaction, encounters an error, and then calls a cleanup function that releases the transaction or connection object before the driver finishes its internal bookkeeping, a Double Free can occur. This is particularly likely when using raw SQL queries with db.Query() or tx.Exec() without proper defer discipline, or when middleware resets the context between retries without ensuring prior allocations are truly released.

For example, consider a handler that begins a CockroachDB transaction, processes a request, and then calls tx.Commit() or tx.Rollback() multiple times due to flawed error handling. Each call may attempt to free associated memory. If the transaction object is also captured by a closure or stored in a request-scoped context that is reused, a second free may be triggered when the context is garbage-collected or explicitly cleared. The Fiber middleware stack, which often wraps handlers with custom Context reuse, can inadvertently pass a stale reference to a transaction that has already been freed, triggering the vulnerability.

Compounding this, the combination of high-concurrency patterns in Fiber and CockroachDB’s distributed transaction model increases surface area. A slow query or network partition may cause a retry loop where the application logic frees the transaction on the first attempt but the driver still holds references. This mismatch between application-level memory management and CockroachDB’s internal state management creates conditions where the same memory is freed twice—once by the application and once by the driver or runtime—leading to memory corruption.

Detection of this issue is challenging without runtime analysis because the corruption may not manifest immediately. It could surface as intermittent panics, degraded performance, or unexpected behavior under load. Since middleBrick scans API endpoints without authentication and tests input validation and runtime behavior, it can identify anomalies consistent with memory safety issues—such as inconsistent transaction states or unexpected closures—by analyzing how the API handles malformed or repeated requests.

To prevent Double Free in a Fiber application using CockroachDB, ensure strict ownership semantics for database transactions and avoid manual memory management by relying on Go’s idioms. The key is to guarantee that each transaction object is used exactly once and that no references remain after commit or rollback. Below are concrete, safe patterns.

1. Always defer rollback after transaction begin, and commit only on success

This pattern ensures that if an error occurs before commit, the transaction is safely rolled back without double interaction.

package main

import (
    "context"
    "github.com/gofiber/fiber/v2"
    "github.com/jackc/pgx/v5/pgxpool"
)

func main() {
    pool, _ := pgxpool.New(context.Background(), "postgresql://localhost:26257/defaultdb")
    app := fiber.New()

    app.Get("/user/:id", func(c *fiber.Ctx) error {
        ctx := c.Context()
        tx, err := pool.Begin(ctx)
        if err != nil {
            return c.SendStatus(fiber.StatusInternalServerError)
        }
        // Ensure rollback if commit is not called
        defer func() {
            if tx.Rollback(ctx) != nil && !tx.RollbackCalled() {
                // Log only if rollback was unexpected
            }
        }()

        var name string
        err = tx.QueryRow(ctx, "SELECT name FROM users WHERE id = $1", c.Params("id")).Scan(&name)
        if err != nil {
            return c.SendStatus(fiber.StatusNotFound)
        }

        if err := tx.Commit(ctx); err != nil {
            return c.SendStatus(fiber.StatusInternalServerError)
        }
        // Transaction committed; defer will attempt rollback but will be a no-op if already committed
        return c.JSON(fiber.Map{"name": name})
    })

    app.Listen(":3000")
}

2. Avoid reusing transaction objects across middleware or handlers

Ensure each request creates a new transaction. Do not store transactions in context variables that may be reused.

func TransactionMiddleware() fiber.Handler {
    return func(c *fiber.Ctx) error {
        // Do NOT attach transaction to context for reuse
        // Instead, let handlers manage their own transactions
        return c.Next()
    }
}

3. Use connection pooling correctly and limit transaction lifetime

Keep transactions short-lived and rely on the pool to manage connections. CockroachDB drivers handle connection lifecycle; your code should only manage transaction boundaries.

func safeQuery(pool *pgxpool.Pool) {
    ctx := context.Background()
    tx, err := pool.Begin(ctx)
    if err != nil {
        // handle
    }
    defer func() {
        if err != nil {
            tx.Rollback(ctx)
            return
        }
        if err = tx.Commit(ctx); err != nil {
            // handle commit error
        }
    }()
    // execute queries
}

By following these patterns, you eliminate scenarios where the same transaction object is freed multiple times. The Go runtime and CockroachDB driver handle memory correctly when ownership is clear and deferred cleanup is consistent.