Heap Overflow in Chi

Go’s memory safety eliminates most classic buffer overflows, but heap overflows can still arise when code steps outside the safety net — typically via unsafe pointers, cgo calls, or third‑party C libraries. In a Chi‑based HTTP service, the router itself does not allocate unsafe memory, but handlers often receive raw request data that is passed to C code for performance‑critical processing (e.g., custom parsers, crypto, or legacy protocol libraries). If the handler does not validate the size of the incoming data before handing it to a C function that expects a fixed‑size buffer, a malicious client can send an oversized payload that overruns the heap‑allocated buffer, corrupting adjacent heap objects.

Consider a Chi handler that uses cgo to invoke a C function process_input expecting a buffer of exactly 1024 bytes:

package main

import (
	"net/http"
	"unsafe"

	"github.com/go-chi/chi/v5"
	"yourmodule/cgohelper" // hypothetical cgo package
)

func vulnerableHandler(w http.ResponseWriter, r *http.Request) {
	// Read the entire body without any size limit
	body, _ := io.ReadAll(r.Body)
	// Convert Go slice to unsafe pointer
	ptr := unsafe.Pointer(&body[0])
	// Call C function that expects a fixed 1024‑byte buffer
	cgohelper.ProcessInput(ptr, C.size_t(len(body))) // len(body) may be >1024
	w.WriteHeader(http.StatusOK)
}

func main() {
	r := chi.NewRouter()
	r.Post("/process", vulnerableHandler)
	http.ListenAndServe(":8080", r)
}

If a client sends a POST with a body larger than 1024 bytes, the C function writes past the end of its heap‑allocated buffer, overwriting heap metadata or adjacent objects. The result can be a crash, silent data corruption, or, in the worst case, arbitrary code execution if the overflow overwrites function pointers or return addresses stored on the heap.

Similar patterns appear when Chi handlers extract URL path parameters or query values and pass them as []byte to C functions without length checks, or when custom middleware uses unsafe.Slice to create a slice from a raw pointer supplied by a C library.

Detecting a heap overflow in a running Chi service relies on observing abnormal behavior when the service processes unusually large inputs. Because middleBrick performs unauthenticated, black‑box scanning, it can trigger these conditions without source code or credentials.

During its Input Validation check, middleBrick sends a series of progressively larger payloads (e.g., 1 KB, 10 KB, 100 KB, 1 MB) to each discovered endpoint. If an endpoint begins to return HTTP 500 errors, empty responses, or the connection is reset after a certain size, middleBrick flags the endpoint as potentially vulnerable to memory‑corruption issues such as heap overflows. The scanner also monitors for delayed responses or spikes in response time that can indicate the process is handling a fault condition (e.g., a segmentation fault caught by the runtime and turned into a 500).

In addition, middleBrick’s Data Exposure and Rate Limiting checks can indirectly hint at heap problems: an endpoint that crashes under load may cause intermittent 502/504 errors from upstream proxies, which the scanner records as part of its overall risk score.

While middleBrick cannot pinpoint the exact line of code, the combination of:

• consistent failure only when payload size exceeds a certain threshold,
• no authentication or configuration required to reproduce the issue, and
• the endpoint being otherwise functional for normal‑sized requests
• points strongly to a heap overflow caused by unchecked size conversion before a C call or unsafe memory operation.

Developers can then reproduce the finding locally with tools like go test -race, GOTRACEBACK=crash, or dlv to confirm a segmentation fault or heap corruption.

The most reliable fix is to eliminate the path where unchecked data reaches unsafe or C code. In Chi handlers, always validate and limit the size of any data that will be handed off to lower‑level libraries.

Here is the same handler rewritten safely:

package main

import (
	"io"
	"net/http"

	"github.com/go-chi/chi/v5"
	"yourmodule/cgohelper"
)

const maxInputSize = 1024 // must match the C buffer size

func safeHandler(w http.ResponseWriter, r *http.Request) {
	// Limit the request body to maxInputSize bytes
	limitedReader := io.LimitReader(r.Body, maxInputSize)
	body, err := io.ReadAll(limitedReader)
	if err != nil {
		// If the body is larger than the limit, io.ReadAll returns ErrUnexpectedEOF
		http.Error(w, "request body too large", http.StatusBadRequest)
		return
	}
	// At this point len(body) <= maxInputSize
	ptr := unsafe.Pointer(&body[0])
	cgohelper.ProcessInput(ptr, C.size_t(len(body)))
	w.WriteHeader(http.StatusOK)
}

func main() {
	r := chi.NewRouter()
	r.Post("/process", safeHandler)
	http.ListenAndServe(":8080", r)
}

Key changes:

• io.LimitReader caps the amount of data read from r.Body. If the client sends more than maxInputSize, the handler returns a 400 error before any unsafe code runs.
• The length passed to the C function is now guaranteed to be within the buffer’s bounds.
• No unsafe.Pointer arithmetic is performed beyond the slice’s legitimate range.

If the C library truly needs a fixed‑size buffer, consider copying the data into a Go‑allocated [maxInputSize]byte array and passing a pointer to that array; this makes the size explicit and avoids reliance on the original slice’s length.

For cases where you cannot avoid cgo (e.g., interfacing with a legacy library), wrap the call in a helper that validates the input length and returns an error to the Chi handler if the check fails. Additionally, enable GODEBUG=cgocheck=2 in development to have the Go runtime validate pointer passing from Go to C.

Finally, add middleware to enforce a global request‑size limit across all routes:

func sizeLimitMiddleware(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		r.Body = http.MaxBytesReader(w, r.Body, 1<<20) // 1 MiB limit
		next.ServeHTTP(w, r)
	})
}

func main() {
	r := chi.NewRouter()
	r.Use(sizeLimitMiddleware)
	r.Post("/process", safeHandler)
	http.ListenAndServe(":8080", r)
}

By combining explicit size limits, safe data copying, and careful cgo boundaries, you eliminate the heap‑overflow risk while preserving the performance benefits of your native code.