Heap Overflow in Fiber with Basic Auth

A heap overflow in a Fiber-based service that also uses HTTP Basic Authentication can occur when user-controlled data from the Authorization header is copied into a heap-allocated buffer without proper length checks. Because Basic Auth credentials are transmitted in the request header as a base64-encoded string, a client can send an unusually long or malformed value. If the server decodes this value and stores it in a fixed-size or improperly bounded heap buffer, an oversized input can overflow the buffer, corrupting adjacent memory.

This combination is notable because the authentication data is parsed before request routing and middleware execution in Fiber. An attacker can target the parsing logic directly without needing to exploit business logic or authenticated endpoints. In a black-box scan, middleBrick tests unauthenticated attack surfaces; for Basic Auth–protected endpoints, it submits malformed credentials to observe whether the service crashes, returns inconsistent responses, or exhibits memory-related anomalies indicative of a heap overflow.

Heap overflows may lead to arbitrary code execution, denial of service, or information disclosure. They often map to CWE-122 (Heap-based Buffer Overflow) and can intersect with the OWASP API Security Top 10 categories such as Broken Object Level Authorization (BOLA) when the overflow affects identity or permission checks. Because the vulnerability resides in low-level request handling, it can bypass higher-level protections if input validation is limited to route or body parsing only.

During a scan, middleBrick runs checks in parallel, including Input Validation and Authentication. For Basic Auth, it crafts requests with oversized or malformed credentials to detect missing length validation and inspects responses for signs of instability. Findings include severity ratings and remediation guidance, emphasizing the need to treat header-derived data as untrusted input regardless of authentication mechanism.

To mitigate heap overflow risks with Basic Authentication in Fiber, ensure that decoded credentials are handled with bounded buffers and strict length validation. Avoid unsafe C-style string operations and prefer managed string types or explicit size checks. Always validate the length of the username and password before copying them into any fixed-size structure.

Example of vulnerable code that decodes Basic Auth and copies credentials into a fixed-size array:

// Vulnerable: no length validation on decoded credentials
app.Get("/protected", func(c *fiber.Ctx) error {
    auth := c.get("Authorization")
    if auth == "" {
        return c.SendStatus(fiber.StatusUnauthorized)
    }
    // Expects "Basic base64string"
    parts := strings.Split(auth, " ")
    if len(parts) != 2 || parts[0] != "Basic" {
        return c.SendStatus(fiber.StatusBadRequest)
    }
    decoded, err := base64.StdEncoding.DecodeString(parts[1])
    if err != nil {
        return c.SendStatus(fiber.StatusBadRequest)
    }
    var cred [64]byte
    copy(cred[:], decoded) // heap overflow if decoded longer than 64
    // use cred...
    return c.SendString("ok")
})

Remediation using safe length checks and bounded copying:

// Secure: validate length before copying
app.Get("/protected", func(c *fiber.Ctx) error {
    auth := c.get("Authorization")
    if auth == "" {
        return c.SendStatus(fiber.StatusUnauthorized)
    }
    parts := strings.Split(auth, " ")
    if len(parts) != 2 || parts[0] != "Basic" {
        return c.SendStatus(fiber.StatusBadRequest)
    }
    decoded, err := base64.StdEncoding.DecodeString(parts[1])
    if err != nil {
        return c.SendStatus(fiber.StatusBadRequest)
    }
    const maxCredLen = 64
    if len(decoded) > maxCredLen {
        return c.SendStatus(fiber.StatusRequestEntityTooLarge)
    }
    var cred [maxCredLen]byte
    copy(cred[:], decoded) // safe: length guaranteed <= maxCredLen
    // use cred...
    return c.SendString("ok")
})

For production-grade security, combine these checks with broader practices such as using the middleBrick CLI to scan from terminal (middlebrick scan <url>) or integrating the GitHub Action to fail builds if risk scores degrade. The Dashboard can help track scores over time, while the Pro plan adds continuous monitoring and CI/CD pipeline gates to catch regressions early.