Heap Overflow in Buffalo with Hmac Signatures

A heap-based buffer overflow can occur in a Buffalo application when Hmac signatures are handled without strict length validation. Buffalo provides convenient helpers for signing and verifying HMAC, but if the application reads untrusted input into a fixed-size buffer on the heap and then passes it to Hmac signing or verification routines, an attacker can supply data that exceeds the buffer, corrupting adjacent memory. This typically manifests when developers use fixed-size byte arrays or slices derived from untrusted sources (e.g., raw request body or query parameters) and then compute Hmac signatures on that data without verifying its length first.

For example, consider a route that parses a JSON payload containing a data field intended for signing. If the server copies this field into a stack-allocated or heap-allocated buffer sized with an assumed maximum, and the actual payload is larger, a heap overflow occurs. When the application then calls hmac.New with the oversized buffer, the overflow may lead to memory corruption or information leakage. Although Go’s runtime includes bounds checks that mitigate classic C-style overflows, misuse of byte slices—such as slicing beyond capacity or passing unsafe pointers—can still expose heap memory to corruption when combined with unchecked external input.

In the context of API security scanning, middleBrick tests such scenarios by submitting payloads designed to exceed expected buffer sizes and observing whether the application crashes, leaks memory, or behaves inconsistently. Because Hmac signatures often protect integrity, an overflow in this path can undermine the entire security model, allowing an attacker to bypass signature checks or execute unintended logic. The scanner checks for missing length validation, improper use of byte buffers, and unsafe operations around Hmac signing inputs as part of its Input Validation and Unsafe Consumption checks.

Real-world examples include CVE-2021-29425, where improper bounds handling in HTTP request parsing led to memory corruption, and patterns flagged by OWASP API Top 10 A05:2023 (Security Misconfiguration) and A03:2023 (Injection). These underscore the importance of validating input length before using it in cryptographic operations, especially when the data influences Hmac key derivation or message authentication.

To remediate heap overflow risks specific to Hmac signatures in Buffalo, always validate input lengths before using them in cryptographic operations and avoid fixed-size buffers for untrusted data. Use Go’s built-in slices safely by checking lengths and capacities, and prefer high-level APIs that handle memory automatically.

Below are concrete code examples for safely computing and verifying Hmac signatures in a Buffalo application.

// Safe Hmac signing with length checks
package controllers

import (
    "crypto/hmac"
    "crypto/sha256"
    "fmt"
    "net/http"

    "github.com/gobuffalo/buffalo"
)

func SignHandler(c buffalo.Context) error {
    // Read the request body safely
    body := c.Request().Body
    data := make([]byte, c.Request().ContentLength)
    _, err := body.Read(data)
    if err != nil {
        return c.Render(400, r.JSON(map[string]string{"error": "failed to read body"}))
    }

    // Enforce a reasonable maximum length to prevent resource exhaustion
    const maxSize = 1024 * 1024 // 1 MB
    if int64(len(data)) > maxSize {
        return c.Render(400, r.JSON(map[string]string{"error": "payload too large"}))
    }

    key := []byte("super-secret-key")
    mac := hmac.New(sha256.New, key)
    mac.Write(data)
    signature := mac.Sum(nil)

    return c.Render(200, r.JSON(map[string]interface{}{
        "signature": fmt.Sprintf("%x", signature),
    }))
}

// Safe Hmac verification with constant-time comparison
func VerifyHandler(c buffalo.Context) error {
    providedSig := c.Param("signature")
    body := c.Request().Body
    data := make([]byte, c.Request().ContentLength)
    _, err := body.Read(data)
    if err != nil {
        return c.Render(400, r.JSON(map[string]string{"error": "failed to read body"}))
    }

    const maxSize = 1024 * 1024
    if int64(len(data)) > maxSize {
        return c.Render(400, r.JSON(map[string]string{"error": "payload too large"}))
    }

    key := []byte("super-secret-key")
    mac := hmac.New(sha256.New, key)
    mac.Write(data)
    expected := mac.Sum(nil)

    // Use hmac.Equal for constant-time comparison to avoid timing attacks
    if !hmac.Equal([]byte(providedSig), expected) {
        return c.Render(401, r.JSON(map[string]string{"error": "invalid signature"}))
    }

    return c.Render(200, r.JSON(map[string]string{"status": "ok"}))
}

Additional best practices include:

• Never slice or copy untrusted input into fixed-size arrays without explicit length checks.
• Use hmac.Equal for signature comparison to prevent timing side-channels.
• Set explicit size limits on request bodies using middleware or framework configuration.
• Leverage Buffalo’s built-in parameter parsing and binding, ensuring that bound fields are validated for length and type.

By combining input validation with safe memory practices, developers can prevent heap overflow conditions that might otherwise be triggered through Hmac signature pathways. middleBrick’s scans can help identify missing length checks and unsafe buffer handling as part of its Input Validation and Unsafe Consumption checks.