Formula Injection in Chi with Mutual Tls

Formula Injection occurs when user-controlled data is inserted into executable formulas, often in spreadsheet exports or report generation endpoints. In the Chi web framework for Go, this typically arises when dynamic route parameters or query values are passed into templates or response streams that are later interpreted as formulas by client applications (for example, an exported CSV or XLSX with a cell expression like =SUM(A:A)). When Mutual TLS (mTLS) is enforced, the server authenticates the client via client certificates, but this does not inherently validate the content of requests. An authenticated client may still submit malicious payloads if input validation is missing.

With Mutual Tls, the TLS handshake guarantees the identity of the client, which can create a false sense of security. Developers may assume that mTLS-bound endpoints are safe from injection because only trusted clients can connect. However, if the endpoint accepts user-controlled data (e.g., a query parameter used to generate a formula) and reflects it into a response that is processed as a formula by downstream systems, the authenticated channel does not mitigate the injection risk. For instance, an endpoint /export protected by mTLS might accept a parameter formula and embed it into an Excel-compatible response without sanitization. An attacker with a valid certificate could supply =cmd|' /C calc'!A0 or a simple arithmetic injection like =1+1 to manipulate behavior or extract data via formula evaluation in the client application.

Chi does not provide built-in sanitization for dynamic values used in formula contexts. The framework encourages developers to construct responses manually or via templates, placing the burden on the application to ensure values are escaped or validated. When generating content that will be interpreted by spreadsheet software, you must treat all user-influenced data as untrusted, even under Mutual Tls. Attack patterns include embedding executable expressions in CSV cells, injecting worksheet functions, or leveraging encoding tricks to bypass naive filters. Because mTLS focuses on transport-layer identity and does not inspect payload semantics, the combination with Formula Injection in Chi highlights the need for output encoding, strict allowlists for formula fields, and context-aware escaping based on the client consumption format (e.g., CSV, XLSX, or JSON).

Remediation centers on strict input validation and context-aware escaping rather than relying on Mutual Tls for data safety. In Chi, use middleware to inspect and sanitize incoming query or body parameters before they are used in formula generation. For CSV exports, ensure values are properly escaped and avoid inserting user data directly into expressions. For binary formats like XLSX, use a library that handles formula escaping or generate formulas server-side without reflecting user input.

Below are concrete code examples demonstrating secure handling in Chi with Mutual Tls configured. The examples assume you have TLS certificates set up for both server and client verification. The first example shows a Chi router that validates a formula query parameter against an allowlist before inclusion in a CSV response.

import (
    "net/http"
    "regexp"
    "github.com/go-chi/chi/v5"
    "github.com/go-chi/render"
)

// allowlist of safe functions; extend based on your use case
var safeFormulas = map[string]bool{
    "SUM": true, "AVERAGE": true, "COUNT": true,
}

func isSafeFormula(input string) bool {
    // Allow only alphanumeric function names and basic operators, no external references
    matched, _ := regexp.MatchString(`^[A-Z]+\([A-Z0-9:,]+\)$`, input)
    if !matched {
        return false
    }
    // Extract function name
    // naive extraction for example; improve as needed
    for fn := range safeFormulas {
        if len(input) >= len(fn) && input[:len(fn)] == fn {
            return true
        }
    }
    return false
}

func exportHandler(w http.ResponseWriter, r *http.Request) {
    formula := r.URL.Query().Get("formula")
    if formula == "" {
        render.Respond(w, r, render.M{"error": "formula parameter required"})
        return
    }
    if !isSafeFormula(formula) {
        render.Respond(w, r, render.M{"error": "invalid formula"})
        return
    }
    // Safe to use in CSV; still escape quotes and special chars
    safeValue := regexp.MustCompile(`"`).ReplaceAllString(formula, `""`)
    csvContent := "Result
" + safeValue + "
"
    w.Header().Set("Content-Type", "text/csv")
    w.Write([]byte(csvContent))
}

func main() {
    r := chi.NewRouter()
    // Mutual Tls middleware would be applied here, ensuring client certs are verified
    // Example placeholder: r.Use(mtlsMiddleware)
    r.Get("/export", exportHandler)
    http.ListenAndServeTLS(":443", "server.crt", "server.key", r)
}

The second example demonstrates generating an XLSX file using a library like tealeg/xlsx, where user input is used only as cell values, not as formula expressions, preventing injection.

import (
    "github.com/go-chi/chi/v5"
    "github.com/tealeg/xlsx/v3"
    "net/http"
)

func reportHandler(w http.ResponseWriter, r *http.Request) {
    userInput := r.URL.Query().Get("label")
    // Validate label length and characters
    if userInput == "" || len(userInput) > 100 {
        render.Respond(w, r, render.M{"error": "invalid label"})
        return
    }
    file := xlsx.NewFile()
    sheet, _ := file.AddSheet("Report")
    row := sheet.AddRow()
    cell := row.AddCell()
    cell.SetString(userInput) // Safe: sets plain text, not a formula
    w.Header().Set("Content-Type", "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet")
    file.Write(w)
}

func main() {
    r := chi.NewRouter()
    // Mutual Tls enforced upstream or via middleware
    r.Get("/report", reportHandler)
    http.ListenAndServeTLS(":443", "server.crt", "server.key", r)
}

These approaches ensure that even with Mutual Tls securing the channel, formula injection risks are mitigated through validation, allowlisting, and safe output generation. Always treat client-influenced data as potentially malicious, regardless of transport-layer authentication.