Double Free in Buffalo with Api Keys

Double Free is a memory safety class of vulnerability where a program deallocates the same memory region twice. In the context of Buffalo, a Go web framework, this typically arises when application code or a plugin manages objects and their lifecycle inconsistently. When API keys are handled as first-class request-scoped values, the interaction between Buffalo's request lifecycle and key management can expose Double Free conditions under specific allocation and release patterns.

Consider a Buffalo application that parses and attaches API keys to the request context for authentication. If the key material is stored in a C-managed buffer (e.g., via cgo or a foreign library) and also referenced by Go structures, a Double Free can occur when both the C layer and the Go runtime attempt to free the same underlying memory. This is more likely when the API key is copied into a C string for validation or signing and the developer forgets to manage ownership correctly, leading to a premature free or a double release during request teardown.

Additionally, plugins or middleware that reuse buffers across requests can introduce the flaw. For example, a plugin that caches decoded API keys in a sync.Pool and returns pointers to reused memory may cause a Double Free if one request frees the memory while another request is still using it. The unauthenticated attack surface tested by middleBrick (which scans endpoints without credentials) can surface these issues when API key handling paths are triggered by unauthenticated requests, such as public endpoints that still parse an API key header for optional authorization.

Real-world patterns that align with this risk include unsafe CGo usage, incorrect finalizer implementations, or mishandling of byte slices that back C allocations. The presence of an API key in the request flow amplifies the impact because key processing is often security-sensitive, and a Double Free can lead to arbitrary code execution or denial of service.

Remediation focuses on ensuring clear ownership semantics and avoiding shared mutable memory across Go and C boundaries. Use Go-managed structures for API keys and avoid cgo unless strictly necessary. When cgo is required, ensure that memory is freed in a single, well-defined location and that references are invalidated after use.

Example 1: Safe API key handling without cgo

Keep API keys as Go strings or byte slices and avoid passing them to C. This eliminates cross-runtime deallocation risks.

// Safe: API key stays in Go memory.
package main

import (
    "net/http"
    "github.com/gobuffalo/buffalo"
)

func apiKeyMiddleware(next buffalo.Handler) buffalo.Handler {
    return func(c buffalo.Context) error {
        key := c.Request().Header.Get("X-API-Key")
        // Store as a plain string in context; no C allocation.
        c.Set("api_key", key)
        return next(c)
    }
}

func main() {
    app := buffalo.New(buffalo.Options{})
    app.Use(apiKeyMiddleware)
    app.GET("/public", func(c buffalo.Context) error {
        _ = c.Get("api_key").(string) // safe type assertion
        return c.Render(200, r.String("ok"))
    })
    app.Serve()
}

Example 2: Controlled cgo usage with explicit ownership

If you must use C for cryptographic operations, allocate and free memory in C with a single owner. Do not let Go and C both hold references to the same allocation.

//go:crypto C
#include <stdlib.h>

void process_key(const char* key, size_t len) {
    // C-side processing; memory is freed within C before returning.
    char* copy = malloc(len + 1);
    if (!copy) { /* handle error */ }
    memcpy((void*)copy, key, len);
    copy[len] = '\0';
    // Use copy...
    free(copy); // freed exactly once in C
}

//export ProcessKey
func ProcessKey(key *C.char, length C.size_t) {
    C.process_key(key, length)
}

// Go side: call the C function without retaining pointers.
package main

import (
    "C"
    "unsafe"
    "github.com/gobuffalo/buffalo"
)

func cgoMiddleware(next buffalo.Handler) buffalo.Handler {
    return func(c buffalo.Context) error {
        key := c.Request().Header.Get("X-API-Key")
        cKey := C.CString(key)
        defer C.free(unsafe.Pointer(cKey)) // ensure free only if you allocated with C.CString
        C.ProcessKey(cKey, C.size_t(len(key)))
        return next(c)
    }
}

func main() {
    app := buffalo.New(buffalo.Options{})
    app.Use(cgoMiddleware)
    app.GET("/secure", func(c buffalo.Context) error {
        // No key retained in Go after the middleware returns.
        return c.Render(200, r.String("processed"))
    })
    app.Serve()
}

Example 3: Avoiding sync.Pool reuse of pointers to C memory

Do not store pointers to C-allocated memory in pools that may be reused across requests without deep copies or proper lifetime management.

// Unsafe pattern to avoid: reusing C pointers via sync.Pool.
/*
var keyPool = sync.Pool{
    New: func() interface{} {
        return C.malloc(256)
    },
}
*/
// Instead, use Go-managed buffers.
var bufPool = sync.Pool{
    New: func() interface{} {
        return make([]byte, 256)
    },
}

func safeHandler(c buffalo.Context) error {
    buf := bufPool.Get().([]byte)
    defer bufPool.Put(buf)
    copy(buf, c.Request().Header.Get("X-API-Key"))
    // process buf as Go bytes
    return nil
}

General practices

• Prefer Go-managed strings and slices for API keys; avoid cgo unless necessary.
• If using C allocations, ensure a single owner and free in the same runtime context.
• Do not cache pointers to C memory in pools or global variables without strict lifetime control.
• Validate and sanitize API keys in Go land before any cross-runtime boundary.