Formula Injection in Echo Go with Basic Auth

Formula Injection occurs when an attacker can control a value that is interpreted as a formula by a downstream spreadsheet processor or report generator. In Echo Go applications that use HTTP Basic Authentication, this risk arises when credentials or derived values are reflected into exported data or administrative reports without proper sanitization.

Consider an Echo Go handler that authenticates via Basic Auth and then generates a CSV or Excel file from request parameters. If the username or a decoded credential is written directly into a cell, and that file is later opened in a spreadsheet application, a formula such as =cmd|' /C calc'!A0 can be executed in the context of the user opening the file. The combination of Basic Auth (where the username/password may be reused as identifiers) and dynamic data export creates a scenario where attacker-controlled data becomes code.

For example, an Echo Go service might decode the Authorization header, use the username as a report filter, and stream a CSV back with a filename derived from the user identity. If the username is attacker=cmd|' /C calc'!A0 (URL-encoded or otherwise injected at the application layer), the exported CSV may begin with that value in a cell. When a user opens the file in Excel or Google Sheets, the formula is evaluated, leading to unintended command execution or data exfiltration.

This pattern is particularly dangerous when the Basic Auth credentials are not treated as secrets in the data plane. Even though Basic Auth transmits credentials in base64 (not encryption), the decoded username may be repurposed as a data field. If that field reaches a spreadsheet or document generation pipeline without output encoding or validation, the attack chain from authentication to execution is complete.

In an automated scan, middleBrick tests unauthenticated attack surfaces and can detect indicators that exported data reflects user-controlled input. While the scanner does not execute payloads, it highlights places where authentication-derived data flows into export functionality without sanitization, pointing to the need for strict output encoding and contextual escaping.

Remediation focuses on ensuring that any data derived from authentication—especially the Basic Auth username—never reaches downstream consumers in a form that can be interpreted as executable formulas. Apply the following patterns in your Echo Go handlers.

1. Avoid using Basic Auth credentials in exported data

Do not write decoded username or password values into CSV, Excel, JSON, or HTML responses. If you must associate a user with data, use a sanitized, non-sensitive identifier (e.g., a numeric user ID) and validate it independently.

2. Encode output based on context

When generating CSVs destined for spreadsheet tools, apply strict escaping. For formulas that could start with =, +, -, or @, prepend a single quote ' or encode the value in a way that prevents evaluation.

// Example: safe CSV generation in Echo Go
import (
    "encoding/csv"
    "net/http"
    "github.com/labstack/echo/v4"
)

func exportHandler(c echo.Context) error {
    userID := c.Get("user_id").(int)          // trusted internal ID
    records := [][]string{
        {"Report", "Value"},
        {"User_" + itoa(userID), "100"},      // safe: no formula characters
    }
    w := c.Response().Writer
    csvWriter := csv.NewWriter(w)
    if err := csvWriter.WriteAll(records); err != nil {
        return echo.NewHTTPError(http.StatusInternalServerError, "failed to write CSV")
    }
    csvWriter.Flush(); return nil
}

func itoa(i int) string { return strconv.Itoa(i) }

3. Sanitize if credentials must be logged or displayed

If you must log or show authentication context, hash or truncate the username and disallow characters that could initiate formulas. Never reflect raw Authorization header content into downloadable files.

4. Validate and constrain Basic Auth usernames at the boundary

Reject usernames containing formula-indicative characters such as =, +, -, @, or control characters. Use a strict allowlist when possible.

// Example: validate Basic Auth username in Echo Go
func validateUsername(username string) bool {
    // Allow only alphanumerics and a safe subset
    matched, _ := regexp.MatchString(`^[A-Za-z0-9_.-]{1,64}$`, username)
    return matched
}

func authMiddleware(next echo.HandlerFunc) echo.HandlerFunc {
    return func(c echo.Context) error {
        user, pass, ok := c.Request().BasicAuth()
        if !ok || !validateUsername(user) {
            return echo.NewHTTPError(http.StatusUnauthorized, "invalid credentials")
        }
        // proceed with authentication logic
        c.Set("username", user)
        return next(c)
    }
}

5. Separate authentication from data export

Ensure that export endpoints do not rely on Basic Auth-derived fields for cell values. If you must include user context, use a separate, sanitized lookup that does not re-use raw credential material.