Out Of Bounds Write in Buffalo with Hmac Signatures

An Out Of Bounds Write in a Buffalo application combined with Hmac Signatures can occur when user-controlled data influences buffer boundaries or memory layout before signature verification is applied. In Go, this typically surfaces through unsafe use of slices, byte arrays, or Cgo-backed buffers where length checks are missing or misaligned with the expected signature format.

Buffalo applications often handle request payloads that include both data and an Hmac Signature header or query parameter. If the application parses or copies payload elements into fixed-size buffers without validating lengths, an attacker can supply oversized data that shifts memory regions. When the application later computes or verifies the Hmac Signature, the altered memory can cause the signature comparison to use corrupted state, potentially bypassing intended integrity checks or leading to crashes that expose internal behavior.

Consider a scenario where a Buffalo handler reads raw bytes into a fixed-size buffer: the developer may assume the payload size matches the buffer, but an attacker can send more bytes. This out-of-bounds write can overwrite adjacent stack variables, including parts of the Hmac Signature or keys stored in memory. Because Hmac Signature verification depends on consistent key and message states, memory corruption can cause the verification logic to operate on tainted data, either producing false negatives or exposing runtime details that aid further exploitation.

Real-world patterns include using copy(dst[:], src) without ensuring len(src) <= len(dst), or binding request form values directly into fixed-length byte arrays. In frameworks like Buffalo, which encourage rapid prototyping, such unsafe operations are more likely when developers prioritize convenience over strict bounds checking. The interaction between unchecked input sizes and cryptographic operations like Hmac Signature generation is especially dangerous because it can undermine the very integrity mechanisms the application relies on.

Another vector involves JSON or multipart form parsing where nested fields are mapped to byte buffers. If the mapping does not enforce size limits, an attacker can send deeply nested or large string fields that trigger out-of-bounds writes during buffer assembly. When the application subsequently generates an Hmac Signature over the processed data, the memory corruption may alter the signature bytes or key material in memory, leading to inconsistent verification outcomes that are hard to detect without careful auditing.

Because Buffalo applications frequently rely on middleware for signature validation, an out-of-bounds write occurring earlier in the request lifecycle can persist into the Hmac Signature stage. This makes it critical to validate input lengths before any cryptographic operation and to ensure that buffers used for signature computation are explicitly sized and checked. Tools like middleBrick can help by scanning for these input validation and data exposure risks, highlighting where user data flows into fixed-size structures before signature verification.

Remediation centers on strict length validation, using Go’s built-in types and safe copying, and isolating cryptographic operations from unchecked buffers. Always compare lengths before copying data into fixed-size structures, and use slices with dynamic sizing wherever possible to avoid fixed buffer pitfalls.

Safe Hmac Signature Handling Example

// Safe: use dynamic slices and explicit length checks
package main

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

func verifySignature(payload []byte, receivedSig string, secret string) bool {
    // Ensure payload fits within expected operational limits
    const maxPayloadSize = 65536
    if len(payload) > maxPayloadSize {
        return false
    }

    key := []byte(secret)
    mac := hmac.New(sha256.New, key)
    mac.Write(payload)
    expected := mac.Sum(nil)

    // Use constant-time comparison to avoid timing leaks
    return hmac.Equal(expected, parseHex(receivedSig))
}

func parseHex(s string) []byte {
    // implement hex decoding with error handling
    b, err := hex.DecodeString(s)
    if err != nil || len(b) != sha256.Size {
        return nil
    }
    return b
}

func handler(w http.ResponseWriter, r *http.Request) {
    // Read body with a reasonable limit
    r.Body = http.MaxBytesReader(w, r.Body, 65536)
    payload, err := io.ReadAll(r.Body)
    if err != nil {
        http.Error(w, "payload too large", http.StatusRequestEntityTooLarge)
        return
    }

    sig := r.Header.Get("X-Hmac-Signature")
    if !verifySignature(payload, sig, "my-secret-key") {
        http.Error(w, "invalid signature", http.StatusUnauthorized)
        return
    }

    // Process verified payload
    fmt.Fprintf(w, "OK")
}

In this example, the payload size is explicitly capped before reading, and the Hmac Signature is computed over a dynamically sized slice rather than a fixed buffer. The hmac.Equal function provides constant-time comparison to mitigate timing attacks, which is essential when handling cryptographic signatures.

Struct-Based Mapping with Bounds Checks

// Safe: struct mapping with length validation
package main

import (
    "encoding/json"
    "fmt"
    "net/http"
)

type RequestData struct {
    Data  []byte `json:"data"`
    Extra []byte `json:"extra,omitempty"`
}

func safeHandler(w http.ResponseWriter, r *http.Request) {
    const maxSize = 32768
    dec := json.NewDecoder(io.LimitReader(r.Body, maxSize))
    var req RequestData
    if err := dec.Decode(&req); err != nil {
        http.Error(w, "invalid request", http.StatusBadRequest)
        return
    }
    if len(req.Data) == 0 || len(req.Data) > maxSize {
        http.Error(w, "data length invalid", http.StatusBadRequest)
        return
    }
    // Compute Hmac over validated data
    sig := computeHmac(req.Data, "secret-key")
    fmt.Fprintf(w, "sig=%x", sig)
}

func computeHmac(data []byte, keyStr string) []byte {
    key := []byte(keyStr)
    mac := hmac.New(sha256.New, key)
    mac.Write(data)
    return mac.Sum(nil)
}

This second example demonstrates safe struct unmarshaling with an explicit size limit on the reader and additional length checks on critical fields. By keeping buffers dynamic and validating sizes before cryptographic processing, you eliminate the conditions that enable out-of-bounds writes and protect the integrity of Hmac Signatures.

Operational Practices

• Set http.MaxBytesReader or equivalent middleware to enforce request body limits early.
• Avoid fixed-size byte arrays for payloads; prefer slices with validated lengths.
• Isolate Hmac computation in dedicated functions with clear input contracts.
• Use hmac.Equal for signature comparisons to prevent timing side channels.
• Run scans with middleBrick to detect input validation gaps and data exposure risks before deploying handlers.