Heap Overflow in Buffalo with Basic Auth

A heap-based buffer overflow in a Buffalo application that uses HTTP Basic Authentication can occur when user-controlled input is copied into a fixed-size buffer on the runtime stack or heap without proper length checks. In Go, this typically manifests as an unsafe use of C bindings via cgo or as unchecked byte slices passed to functions that assume a maximum size. When combined with Basic Auth, the Authorization header value (e.g., Basic dXNlcjpwYXNz) is base64-decoded and parsed into a username and password. If the decoded password is written into a fixed-length byte array on the heap and the attacker supplies a longer string, the extra bytes can overflow adjacent memory, potentially leading to arbitrary code execution or application crash.

Consider a handler that decodes the Basic Auth credential and passes it to a C function via cgo:

// #include <string.h>
import "C"
import (
    "encoding/base64"
    "net/http"
    "strings"
)

func authHandler(w http.ResponseWriter, r *http.Request) {
    auth := r.Header.Get("Authorization")
    if auth == "" {
        http.Error(w, "Unauthorized", http.StatusUnauthorized)
        return
    }
    parts := strings.SplitN(auth, " ", 2)
    if len(parts) != 2 || parts[0] != "Basic" {
        http.Error(w, "Unauthorized", http.StatusUnauthorized)
        return
    }
    decoded, err := base64.StdEncoding.DecodeString(parts[1])
    if err != nil {
        http.Error(w, "Unauthorized", http.StatusUnauthorized)
        return
    }
    // Assume format: username:password
    creds := strings.SplitN(string(decoded), ":", 2)
    if len(creds) != 2 {
        http.Error(w, "Unauthorized", http.StatusUnauthorized)
        return
    }
    password := creds[1]
    cPassword := C.CString(password) // copies Go string to C-managed heap
    defer C.free(unsafe.Pointer(cPassword))
    C.strcpy(C.CString(make([]byte, 32)), cPassword) // vulnerable: fixed 32-byte buffer
}

In this contrived example, the heap-allocated C string from password is copied into a fixed 32-byte buffer using strcpy, which does not perform bounds checking. If the decoded password exceeds 31 characters (plus null terminator), a heap overflow occurs. An attacker could send a long password in the Basic Auth header to overflow the buffer, potentially corrupting heap metadata or overwriting function pointers. While Go’s runtime provides some memory safety, interactions with C via cgo remove those protections, making overflows possible. This becomes a realistic concern when integrating with native libraries or when using assembly that manipulates memory directly.

middleBrick scans such endpoints during black-box testing of the unauthenticated attack surface. Even without credentials, the presence of Basic Auth parsing and cgo usage can be inferred from runtime behavior and flagged. The scanner’s checks for Input Validation and Unsafe Consumption would highlight the missing length validation on the Authorization header and the unsafe use of C memory operations, providing remediation guidance to avoid fixed-size buffers and to use bounded functions like strncpy or, better, avoid C string manipulation entirely.

Remediation focuses on avoiding unsafe memory operations and ensuring that decoded credentials are handled with length-aware functions. In Go, prefer using slices and the standard library instead of cgo for string manipulation. If interaction with C is necessary, always use bounded copying functions and validate input lengths before conversion.

Secure example using only Go, avoiding cgo and fixed buffers:

import (
    "encoding/base64"
    "net/http"
    "strings"
)

func secureAuthHandler(w http.ResponseWriter, r *http.Request) {
    auth := r.Header.Get("Authorization")
    if auth == "" {
        http.Error(w, "Unauthorized", http.StatusUnauthorized)
        return
    }
    parts := strings.SplitN(auth, " ", 2)
    if len(parts) != 2 || parts[0] != "Basic" {
        http.Error(w, "Unauthorized", http.StatusUnauthorized)
        return
    }
    decoded, err := base64.StdEncoding.DecodeString(parts[1])
    if err != nil {
        http.Error(w, "Unauthorized", http.StatusUnauthorized)
        return
    }
    // Validate expected format and length
    if !strings.Contains(string(decoded), ":") {
        http.Error(w, "Unauthorized", http.StatusUnauthorized)
        return
    }
    creds := strings.SplitN(string(decoded), ":", 2)
    if len(creds[1]) > 128 { // reasonable password length cap
        http.Error(w, "Request Entity Too Large", http.StatusRequestEntityTooLarge)
        return
    }
    password := creds[1]
    // Use password safely as a Go string; no C interaction
    _ = password // process authentication logic here
}

If cgo is unavoidable, use strncpy with explicit size and ensure the source length is checked:

// #include <string.h>
import "C"
import (
    "encoding/base64"
    "net/http"
    "strings"
    "unsafe"
)

func safeCpyAuthHandler(w http.ResponseWriter, r *http.Request) {
    auth := r.Header.Get("Authorization")
    decoded, _ := base64.StdEncoding.DecodeString(strings.SplitN(auth, " ", 2)[1])
    if len(decoded) > 127 {
        http.Error(w, "Request Entity Too Large", http.StatusRequestEntityTooLarge)
        return
    }
    cBuf := C.CString(string(make([]byte, 32))) // 32-byte fixed buffer on heap
    defer C.free(unsafe.Pointer(cBuf))
    pwd := C.CString(string(decoded))
    defer C.free(unsafe.Pointer(pwd))
    C.strncpy(cBuf, pwd, 31) // bounded copy
    cBuf[31] = '\0'      // enforce null termination
    // use cBuf safely within C bounds
}

Key remediation points: validate the length of the decoded password before any buffer assignment, avoid unbounded memory copy functions, and prefer Go-native handling over cgo. middleBrick’s scans can detect missing length checks and unsafe C usage, guiding developers toward safer credential handling.