Integer Overflow in Fiber with Basic Auth

Integer overflow in a Fiber-based API that uses HTTP Basic Authentication can arise when user-controlled numeric values (e.g., quantity, batch size, or buffer length) are parsed from request parameters, headers, or JSON bodies without proper validation. In Go, when an arithmetic operation exceeds the maximum value of the integer type, it wraps around, producing a small or negative number that can bypass size checks, trigger negative-length allocations, or lead to out-of-bounds memory access patterns.

Combining Basic Authentication with integer overflow is notable because authentication happens before request routing. If the Authorization header is parsed to extract credentials and then used to derive or index into numeric values (for example, computing a per-user rate-limit bucket or a buffer size), an attacker can supply a crafted numeric parameter that overflows after type conversion. This can weaken enforcement of authentication boundaries, expose private data, or cause the server to serve incorrect content.

Consider a Fiber route that accepts an amount query parameter to allocate a buffer for the authenticated user’s request. If amount is parsed as an unsigned integer and multiplied by a per-user factor derived from Basic Auth credentials (e.g., a tenant ID or tier index), an overflow can produce a very small buffer. The server may then copy more data than the buffer can hold, leading to data exposure or instability. Because Basic Authentication sends credentials on each request, the overflow may be repeatable and easier to trigger in authenticated sessions.

Example scenario: an authenticated request includes ?amount=4294967295 (max 32-bit unsigned) and the handler computes size := amount * 4. On a 32-bit system or if the result is stored in a 32-bit unsigned integer, the multiplication wraps to a small number (e.g., 60), causing a small allocation that leads to a buffer overflow when copying request body data. This pattern is detectable through runtime analysis that correlates authentication events with numeric input validation failures.

To detect this class of issues, scans should test authenticated endpoints with large, boundary, and negative-like values for numeric inputs, and verify that size or length checks remain consistent after arithmetic. MiddleBrick’s checks for Input Validation, Property Authorization, and Unsafe Consumption are particularly relevant because they examine how numeric controls interact with authenticated contexts and parsed specifications.

OpenAPI/Swagger definitions can help by documenting expected ranges and types for parameters used in authenticated flows. MiddleBrick resolves full $ref graphs and cross-references spec definitions with runtime findings, highlighting mismatches where the spec declares a smaller range than the implementation allows. This is important because Basic Authentication does not change parameter semantics, but it does create a persistent context in which those parameters are evaluated.

Remediation focuses on validating and sanitizing numeric inputs before using them in arithmetic, and ensuring that authentication-derived data does not directly control memory or size calculations. In Fiber, always parse numeric values with explicit type checks, range checks, and overflow-safe operations before using them in buffer sizes, loops, or index calculations.

Below is a concrete, secure example of a Fiber handler that uses Basic Authentication and safely handles a numeric query parameter. The code decodes credentials using strings.Split, validates the username and password, parses the amount query parameter with strconv.ParseUint, and applies explicit overflow checks before using the value.

package main

import (
    "net/http"
    "strconv"
    "strings"

    "github.com/gofiber/fiber/v2"
)

// authenticateBasic is a simple Basic Auth validator.
func authenticateBasic(c *fiber.Ctx) error {
    auth := c.Get("Authorization")
    if len(auth) < 6 || !strings.HasPrefix(auth, "Basic ") {
        return c.Status(401).SendString("Unauthorized")
    }
    payload, err := base64.StdEncoding.DecodeString(auth[6:])
    if err != nil {
        return c.Status(401).SendString("Unauthorized")
    }
    parts := strings.SplitN(string(payload), ":", 2)
    if len(parts) != 2 || parts[0] != "user" || parts[1] != "pass" {
        return c.Status(401).SendString("Forbidden")
    }
    return c.Next()
}

func safeHandler(c *fiber.Ctx) error {
    auth := c.Get("Authorization")
    // Basic Auth already validated by middleware; extract tenant factor if needed.
    const maxAllowed = 1024
    amountStr := c.Query("amount")
    if amountStr == "" {
        return c.Status(400).SendString("amount is required")
    }
    amount, err := strconv.ParseUint(amountStr, 10, 32)
    if err != nil {
        return c.Status(400).SendString("invalid amount")
    }
    if amount == 0 || amount > maxAllowed {
        return c.Status(400).SendString("amount out of range")
    }
    // Safe arithmetic: amount is uint32, known bounds prevent overflow.
    bufferSize := amount * 4
    if bufferSize/4 != amount { // overflow check
        return c.Status(400).SendString("amount too large")
    }
    buf := make([]byte, bufferSize)
    // Use buf for processing authenticated request data...
    return c.JSON(fiber.Map{"buffer_size": bufferSize})
}

func main() {
    app := fiber.New()
    app.Use(authenticateBasic)
    app.Get("/process", safeHandler)
    app.Listen(":3000")
}

Key points in this remediation:

• Parse with strconv.ParseUint specifying bit size (e.g., 32) to enforce range before arithmetic.
• Check multiplication for overflow by verifying that reversing the operation returns the original value (e.g., bufferSize/4 == amount).
• Cap values with a server-side constant (maxAllowed) to prevent resource exhaustion regardless of authentication context.
• Keep authentication checks separate from numeric validation so that credentials do not flow into size calculations.

For deployments, combine this with MiddleBrick’s CLI to scan endpoints from the terminal (middlebrick scan <url>) and with the GitHub Action to fail builds if risk scores drop below your chosen threshold. The Pro plan’s continuous monitoring can alert you when authenticated numeric inputs drift out of safe ranges across API versions.