Formula Injection in Gorilla Mux with Jwt Tokens

Formula Injection occurs when user-controlled data is interpreted as a formula (e.g., in spreadsheet exports or templating contexts), leading to unintended execution or data manipulation. When combined with Gorilla Mux routing and JWT-based authentication, the risk arises not from the router itself but from how authenticated route-level data and claims are handled downstream.

Gorilla Mux is a popular HTTP request router for Go that supports variable path patterns (e.g., /users/{id}) and route-specific middleware. In applications using JWT tokens for authentication, middleware typically validates the token, extracts claims (such as user ID or roles), and injects them into the request context. If any of these values—especially claims or parameters derived from token payloads—are later used in contexts that evaluate expressions (such as generating dynamic URLs, constructing queries, or exporting data to spreadsheets), Formula Injection can occur.

Consider a scenario where an API endpoint uses Gorilla Mux to route /export/{format} and the {format} path variable is influenced by a JWT claim (e.g., a tenant identifier). If the application builds a spreadsheet formula using that claim without sanitization, an attacker who can influence the JWT payload (via a compromised issuer or weak validation) could inject expressions like =cmd|' /C calc'!A0 in a cell export, triggering malicious behavior in the client application.

Another vector involves dynamic query construction. Suppose a handler uses claims from a validated JWT to filter data (e.g., userID := vars["userID"] from the token) and builds an Excel export with formulas referencing that ID. If userID is not strictly validated and is reflected into a formula context, an attacker may supply a value such as "123"!A1 to pull data from unintended cells, effectively performing data exfiltration through formula injection.

The interaction between Gorilla Mux routing and JWT authentication becomes critical when route variables or context values derived from tokens are passed into templating engines or export functions without proper encoding. Because Gorilla Mux provides direct access to matched variables via vars, developers must ensure these values are sanitized before any downstream use in formula-capable formats. JWT tokens themselves are not vulnerable to Formula Injection, but the way their claims and associated route parameters are consumed can expose applications if input validation and output encoding are neglected.

Real-world patterns include using JWT-authenticated endpoints to generate CSVs with formulas referencing user-specific identifiers. If an attacker can manipulate either the token claims (through a misconfigured identity provider) or the route parameters, they may inject formulas that execute upon opening the file. This highlights the need to treat data from both routing and authentication layers as potentially hostile when used in formulaic contexts.

Remediation focuses on strict validation of route variables and claims, and safe handling of data used in formula-capable outputs. Below are concrete patterns for a Gorilla Mux-based service using JWT authentication.

First, ensure JWT validation is strict and claims are not assumed safe. Use a well-maintained library and verify signatures and expected claims.

import (
    "github.com/dgrijalva/jwt-go"
    "net/http"
)

func ValidateToken(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        tokenString := extractToken(r)
        token, err := jwt.Parse(tokenString, func(token *jwt.Token) (interface{}, error) {
            if _, ok := token.Method.(*jwt.SigningMethodHMAC); !ok {
                return nil, http.ErrAbortHandler
            }
            return []byte("your-secret"), nil
        })
        if err != nil || !token.Valid {
            http.Error(w, "Unauthorized", http.StatusUnauthorized)
            return
        }
        claims, ok := token.Claims.(jwt.MapClaims)
        if !ok {
            http.Error(w, "Invalid claims", http.StatusUnauthorized)
            return
        }
        // Ensure expected claims are present and safe
        userID, ok := claims["user_id"]
        if !ok {
            http.Error(w, "Missing user_id", http.StatusBadRequest)
            return
        }
        ctx := context.WithValue(r.Context(), "userID", userID)
        next.ServeHTTP(w, r.WithContext(ctx))
    })
}

func extractToken(r *http.Request) string {
    // Implement bearer token extraction
    return ""
}

Second, sanitize and validate route variables before using them in any context that may interpret formulas. Explicitly type and constrain values.

import (
    "github.com/gorilla/mux"
    "strconv"
    "unicode"
)

func exportHandler(w http.ResponseWriter, r *http.Request) {
    vars := mux.Vars(r)
    format := vars["format"]
    // Validate against allowed values only
    if format != "csv" && format != "xlsx" {
        http.Error(w, "invalid format", http.StatusBadRequest)
        return
    }
    userID := r.Context().Value("userID")
    // Ensure userID is a safe integer or string without formula characters
    // Example: if userID is string, reject characters like '=', '+', '-', '@'
    if userIDStr, ok := userID.(string); ok {
        for _, ch := range userIDStr {
            if ch == '=' || ch == '+' || ch == '-' || ch == '@' {
                http.Error(w, "invalid user identifier", http.StatusBadRequest)
                return
            }
        }
    }
    // Proceed to generate safe output
}

func safeGenerateFormula(base string, id string) string {
    // Encode or quote values used in formulas
    return "\"" + id + "\"!A1"
}

Third, when generating spreadsheet exports, avoid reflecting untrusted values into formula syntax. Use libraries that write values as literals, not formulas. For example, using tealeg/xlsx, write strings as values rather than formulas.

import (
    "github.com/tealeg/xlsx"
)

func writeCell(sheet *xlsx.Sheet, rowIndex int, col string, value string) {
    row := sheet.Row(rowIndex)
    cell := row.AddCell()
    cell.SetString(value)
    // Do not use cell.SetFormula with untrusted input
}

By combining strict JWT validation, explicit allowlisting of route variables, and safe output generation, you mitigate Formula Injection risks in Gorilla Mux applications that rely on JWT tokens.