Use After Free in Buffalo with Hmac Signatures

A Use After Free (UAF) vulnerability occurs when memory is freed but references to it remain and are subsequently accessed. In the context of Buffalo, this can arise when buffers that hold data used to compute or verify HMAC signatures are released while still in use, and later reused or accessed. HMAC signatures in Buffalo often rely on byte buffers for keys and payloads; if these buffers are freed prematurely or managed incorrectly, an attacker may be able to influence what data occupies the freed memory, leading to signature validation bypass or information disclosure.

Consider a scenario where a request body or header is parsed into a temporary buffer, an HMAC signature is computed over the payload using that buffer, and then the buffer is freed as part of request cleanup. If the signature verification logic or subsequent processing still references that buffer (a classic UAF), an attacker can potentially supply controlled data that reuses the same memory region. Because HMAC relies on exact byte sequences, altering the memory underlying a payload or key after the signature has been computed but before verification completes can cause the application to validate a modified message as authentic. This can lead to authentication bypass or allow an attacker to craft forgeries that appear valid under the HMAC scheme.

Buffalo applications that implement custom HMAC verification—such as signing tokens, webhook payloads, or API request identifiers—are susceptible if buffers are not pinned or safely copied before verification proceeds. For example, if you compute hmac over a request body stored in a slice that is later modified or released due to garbage collection or manual memory management, the signature check may operate on unpredictable data. Even though Buffalo is built on Go and relies on its runtime memory safety, unsafe patterns—such as passing pointers to stack-allocated buffers to long-lived contexts or storing references to request buffers beyond their intended lifecycle—can reintroduce UAF-like behavior. The risk is compounded when HMAC verification is performed asynchronously or across goroutines without ensuring the underlying data remains valid and unchanged.

Insecure deserialization or improper handling of request-scoped data can exacerbate this issue. If an attacker can cause the same memory to be reused for different purposes between the signature computation and verification phases, they may flip bits in the effective payload or key. Because HMAC is sensitive to any change in input, such memory manipulation can result in a valid signature being computed over malicious data, or a forged signature appearing valid when the underlying bytes have been altered post-computation. The scanner’s checks for unsafe consumption and input validation help surface these risks by flagging missing integrity guarantees around data used in cryptographic operations.

To align with security best practices reflected in the scanner’s checks—particularly around input validation and unsafe consumption—developers must ensure that data used in HMAC operations is immutable for the duration of verification. This means copying buffers into fresh, independent memory before performing cryptographic work and avoiding retaining references to request-scoped objects beyond the request lifecycle. The LLM/AI security checks further highlight risks where prompt or data leakage could expose keys or signatures, underscoring the need to isolate sensitive buffers and treat HMAC inputs as untrusted until verified.

Remediation centers on ensuring that data used for HMAC computation and verification is isolated and remains unchanged. Always copy input bytes into new slices before performing cryptographic operations, and avoid passing references to request buffers directly into verification logic. Below are concrete, idiomatic Go examples tailored for Buffalo that demonstrate safe handling of HMAC signatures.

Example 1: Safe HMAC-SHA256 verification with copied buffers

import (
    "crypto/hmac"
    "crypto/sha256"
    "io"
)

// verifyHMAC checks that the provided signature matches the payload using a constant-time comparison.
func verifyHMAC(payload, receivedSig, key []byte) bool {
    // Copy payload to ensure the underlying bytes cannot be modified after verification starts.
    payloadCopy := make([]byte, len(payload))
    copy(payloadCopy, payload)

    // Copy key to avoid any risk of key material being altered or observed externally.
    keyCopy := make([]byte, len(key))
    copy(keyCopy, key)

    mac := hmac.New(sha256.New, keyCopy)
    mac.Write(payloadCopy)
    expectedSig := mac.Sum(nil)

    // Use hmac.Equal for constant-time comparison to prevent timing attacks.
    return hmac.Equal(expectedSig, receivedSig)
}

// In a Buffalo action, apply the verification like this:
// func VerifyWebhook(c buffalo.Context) error {
//     payload := c.Request().Body
//     // Read body into a fresh slice; do not reuse request buffers directly.
//     bodyBytes, err := io.ReadAll(payload)
//     if err != nil {
//         return c.Error(400, err)
//     }
//     sig := c.Request().Header.Get("X-Hub-Signature-256")
//     // Parse signature hex or base64 as appropriate.
//     receivedSig, err := hex.DecodeString(sig)
//     if err != nil {
//         return c.Error(400, err)
//     }
//     key := []byte(os.Getenv("WEBHOOK_SECRET"))
//     if !verifyHMAC(bodyBytes, receivedSig, key) {
//         return c.Error(401, errors.New("invalid signature"))
//     }
//     // Proceed with safe, verified payload.
//     return nil
// }

Example 2: HMAC signing with isolated buffers for outgoing requests

import (
    "crypto/hmac"
    "crypto/sha256"
    "encoding/hex"
    "io"
)

// signPayload creates an HMAC-SHA256 signature over a copy of the payload and key.
func signPayload(payload, key []byte) string {\n    payloadCopy := make([]byte, len(payload))
    copy(payloadCopy, payload)

    keyCopy := make([]byte, len(key))
    copy(keyCopy, key)

    mac := hmac.New(sha256.New, keyCopy)
    mac.Write(payloadCopy)
    return hex.EncodeToString(mac.Sum(nil))
}

// Example usage in a Buffalo client request:
// func PostSigned(c buffalo.Context, url string, body []byte) (*http.Response, error) {
//     key := []byte(os.Getenv("API_SECRET"))
//     sig := signPayload(body, key)
//     req, err := http.NewRequest(http.MethodPost, url, bytes.NewReader(body))
//     if err != nil {
//         return nil, err
n    //     }
//     req.Header.Set("X-API-Signature", sig)
//     client := &http.Client{}
//     return client.Do(req)
// }

Best practices summary

• Copy inputs before crypto: Always duplicate payload and key bytes to ensure immutability during verification.
• Use constant-time comparison: Employ hmac.Equal to avoid timing side channels.
• Avoid reusing buffers: Do not pass request-scoped slices directly into verification routines; read them into fresh allocations.
• Protect key material: Copy keys and avoid logging or exposing them in error messages or stack traces.
• Validate inputs early: Treat all external data as untrusted and validate format, length, and encoding before cryptographic processing.

By following these patterns, you mitigate the risk that memory reuse or premature freeing can undermine HMAC integrity. These measures complement the scanner’s checks for input validation, unsafe consumption, and LLM/AI security, helping ensure that signatures remain trustworthy and resistant to manipulation.