Formula Injection in Echo Go with Bearer Tokens

Formula Injection occurs when untrusted input is used to construct formulas that are later evaluated by a spreadsheet processor or similar engine. In the Echo Go web framework, this can arise when an API endpoint accepts user-controlled values and embeds them into spreadsheet-like outputs (for example, CSV or Excel files) without proper sanitization. When Bearer Tokens are involved—typically passed via the Authorization header to authenticate requests—the risk pattern becomes more nuanced: the token itself is not the formula, but data derived from or influenced by the token or its metadata may be injected into a formula context.

Consider an endpoint that generates a downloadable report containing a cost calculation. The handler might read an access token from the Authorization header, extract a tenant or plan identifier, and use that value in a computed cell. If the extracted value is numeric but user-influenced (e.g., plan tier mapped to a multiplier), and it is concatenated into a formula string such as =A1*VALUE, an attacker who can influence the token’s associated metadata might inject expressions like 1+1 or external references like [other.xlsx]Sheet!A1. This is Formula Injection: the token context enables the data path, but the injection happens when the value is placed into a formula without validation or escaping.

Echo Go does not inherently evaluate formulas; the vulnerability occurs downstream in the consumer (e.g., Excel, LibreOffice) that parses the generated file. Attack patterns include using crafted tokens or token-derived fields to smuggle executable expressions, which can lead to arbitrary code execution when the file is opened. This aligns with the broader OWASP API Top 10 category of Injection, where trust boundaries are mismanaged. Because the scan checks Input Validation and Property Authorization in parallel, findings will highlight cases where token-derived data reaches output generation without sanitization or type-constraining conversions.

Real-world examples include using a tenant ID extracted from a token to build a pricing cell, or including a session-derived sequence number in a SUM formula. If the token format contains structured claims (e.g., JSON Web Token payload segments) and those claims are reflected into CSV formulas without escaping leading equals signs or external references, the attack surface expands. The scanner’s Input Validation checks look for missing allowlists on data that feeds formula fields, while Property Authorization checks ensure token-scoped properties are not over-permissive in what they can influence.

Remediation focuses on two layers: strict validation/sanitization of any data derived from token claims before it enters a formula context, and safe output generation for spreadsheet-consumable formats. In Echo Go, this means intercepting token-derived values in handlers and applying allowlists or escaping before concatenation.

Example 1: Unsafe usage with token-derived numeric multiplier

// Unsafe: token-derived value used directly in formula
claims := getClaimsFromToken(c)            // returns map[string]interface{}
multiplier, _ := claims["tierMultiplier"].(float64)
cellFormula := fmt.Sprintf("=A1*%v", multiplier) // vulnerable if multiplier can be controlled
c.Response().Header().Set(echo.HeaderContentType, "text/csv")
fmt.Fprintf(c.Response(), "Result,Formula\n100,%s\n", cellFormula)

Example 2: Safe remediation with allowlist and escaping

// Safe: validate multiplier against an allowlist and escape leading equals
claims := getClaimsFromToken(c)
multiplier, ok := claims["tierMultiplier"].(float64)
if !ok || !isValidMultiplier(multiplier) {
    return echo.NewHTTPError(http.StatusBadRequest, "invalid tier")
}
// Ensure the value is numeric and not used to inject formulas
cellValue := 100 * multiplier
cellFormula := fmt.Sprintf("=A1*%v", cellValue) // safe: multiplier is numeric, not string from attacker
c.Response().Header().Set(echo.HeaderContentType, "text/csv")
fmt.Fprintf(c.Response(), "Result,Formula\n%d,%s\n", int(cellValue), cellFormula)

func isValidMultiplier(v float64) bool {
    allowed := map[float64]bool{1.0: true, 1.5: true, 2.0: true}
    return allowed[v]
}

Example 3: Safe CSV generation with escaped formulas

// If you must include a formula string from token-derived data, escape leading '='
formulaInput := getFormulaFragmentFromTokenClaims(c) // e.g., user-controlled or derived
escaped := strings.ReplaceAll(formulaInput, "=", "~~EQ~~") // simple transform; adjust per policy
// Later, consumer-side can revert if needed; but do not eval on server
cellFormula := fmt.Sprintf("=HYPERLINK(\"#\",\"~~EQ~~1+1\")")
c.Response().Header().Set(echo.HeaderContentType, "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet")
// Write using a library that escapes dangerous tokens; here we show the concept
fmt.Fprintf(c.Response(), "Calculation,Note\n100,%s\n", cellFormula)

Additional recommendations

• Use structured output libraries (e.g., tealeg/xlsx) instead of manual CSV concatenation when possible; they handle escaping internally.
• Apply Principle of Least Privilege to token scopes so that even if data is injected, the impact is limited.
• Combine with middleBrick scans (CLI: middlebrick scan <url> or GitHub Action for CI/CD) to detect when token-derived fields reach formula-like outputs.