Heap Overflow in Echo Go with Basic Auth

A heap overflow in an Echo Go service that uses HTTP Basic Authentication occurs when user-controlled data from the authentication header is copied into a fixed-size buffer on the heap without proper length checks. In Go, this typically manifests as writing beyond the bounds of a slice or byte array that was allocated on the heap, for example by using copy(dst, src) where src is derived from the Authorization header and dst has a smaller capacity than src. Because Basic Auth credentials are base64-encoded and sent on every request, an attacker can repeatedly supply long credential strings to probe and exploit the boundary conditions.

The combination is significant because the authentication phase runs before application logic, so a heap overflow in credential handling can occur unauthenticated during the initial probe. This maps to findings in the Authentication and BOLA/IDOR checks of a middleBrick scan, which test the unauthenticated attack surface. If the overflow is reachable through path parameters or query inputs that are processed after authentication, the issue may also align with BFLA/Privilege Escalation and Property Authorization checks. An attacker can leverage such a condition to cause instability or, depending on memory layout, influence control flow, which would be reflected in the Data Exposure and Unsafe Consumption categories.

Consider an endpoint that parses the Basic Auth header and uses the credentials to populate a request context. A vulnerable pattern in Echo Go might look like this, where a fixed-size destination buffer is used without validating source length:

dst := make([]byte, 16)
src := []byte(authHeaderValue) // authHeaderValue from "Authorization: Basic ..."
copy(dst, src) // potential heap overflow if src > 16 bytes

In this example, src can be arbitrarily large, and copy will write beyond the 16-byte heap-allocated slice, corrupting adjacent memory. middleBrick’s Input Validation and LLM/AI Security checks do not directly test for memory safety issues like heap overflows, but they do identify malformed or oversized inputs that can be used in such probes. The scanner’s Inventory Management and Unsafe Consumption checks help surface endpoints where large or untrusted payloads are accepted, which is relevant when credentials or derived values are processed in unsafe ways.

To detect this class of issue during a scan, middleBrick sends requests with unusually long Basic Auth credentials as part of its unauthenticated attack surface testing. If the service exhibits crashes, panics, or inconsistent behavior, findings are surfaced with severity and remediation guidance. Developers should treat authentication header parsing as untrusted input and apply strict length validation and safe copying patterns to prevent heap corruption.

Remediation focuses on avoiding fixed-size heap buffers when handling Basic Auth credentials and validating input lengths before any copying. In Go, prefer using slices with dynamic bounds checks or hashing the credential into a fixed-size representation (e.g., a hash digest) rather than copying raw data.

Safe pattern using length validation:

const maxBasicCredLen = 1024 // reasonable upper bound for credentials
parts := strings.SplitN(string(authHeader), " ", 2)
if len(parts) != 2 || parts[0] != "Basic" {
    // return 400 Bad Request
    return
}
cred, err := base64.StdEncoding.DecodeString(parts[1])
if err != nil || len(cred) > maxBasicCredLen {
    // return 401 Unauthorized
    return
}
// Use cred as a byte slice for comparison or hashing; avoid unbounded copies

Instead of copying raw credentials into a small buffer, compute a fixed-size hash if you need to store or compare a secret:

import "crypto/sha256"
hash := sha256.Sum256(cred) // fixed-size 32-byte hash, safe to store

If you must compare credentials, use a constant-time comparison to avoid timing leaks and ensure you do not write beyond any fixed destination:

if subtle.ConstantTimeCompare(cred, expectedHash) != 1 {
    // return 401 Unauthorized
}

Echo Go handler example that avoids heap overflow by not copying unbounded data:

func safeBasicAuth(next echo.HandlerFunc) echo.HandlerFunc {
    return func(c echo.Context) error {
        auth := c.Request().Header.Get("Authorization")
        if !strings.HasPrefix(auth, "Basic ") {
            return echo.NewHTTPError(http.StatusUnauthorized, "missing basic auth")
        }
        token := strings.TrimPrefix(auth, "Basic ")
        decoded, err := base64.StdEncoding.DecodeString(token)
        if err != nil || len(decoded) == 0 {
            return echo.NewHTTPError(http.StatusUnauthorized, "invalid basic auth")
        }
        // Validate length to prevent abuse
        if len(decoded) > 1024 {
            return echo.NewHTTPError(http.StatusRequestEntityTooLarge, "credentials too large")
        }
        // Use decoded credentials safely, e.g., compare hash
        // ...
        return next(c)
    }
}

With these changes, the handler no longer copies raw user-controlled data into fixed-size heap buffers, mitigating the heap overflow risk. middleBrick’s CLI tool can be used to verify remediation by rescanning the endpoint: middlebrick scan <url>. The GitHub Action can enforce a score threshold to prevent regressions, and the MCP Server allows scanning directly from IDEs when modifying authentication code.