Format String in Echo Go with Bearer Tokens

A format string vulnerability occurs when user-controlled input is passed directly to formatting functions like fmt.Printf without a format specifier. In the Echo Go framework, this risk becomes more significant when Bearer Tokens are handled through route parameters, headers, or query strings. If a developer logs or processes the token value using an unsafe format function, an attacker can supply format specifiers such as %s, %x, or %n to read from or write to memory. Because Bearer Tokens often contain sensitive information, a format string bug can expose token contents or lead to more severe memory disclosure.

Consider an endpoint that extracts a token from a header and logs it using a format string controlled by the developer:

auth := req.Request.Header.Get("Authorization")
// Unsafe: user input used directly as format string
fmt.Printf(auth)

If the header contains Authorization: Bearer abc123, an attacker can replace the value with Bearer %s %s %s to cause the program to read from the stack. In a real API, the token might be passed as a query parameter and then incorrectly formatted:

token := c.QueryParam("token")
// Unsafe: token used as format string
fmt.Printf(token)

When the request includes ?token=Bearer+%x+%x+%x, the application may leak memory contents, potentially revealing parts of the Bearer Token or other sensitive runtime data. Even when the token is used correctly as a string, concatenating it into a format string without explicit formatting creates an indirect vulnerability path. For example:

token := c.QueryParam("token")
logMessage := "Token received: " + token
fmt.Printf(logMessage)

If token contains format specifiers, the concatenated message still becomes a format string attack vector. Echo Go applications must treat Bearer Tokens as opaque strings and avoid any direct use in formatting functions to prevent memory disclosure or unintended behavior.

To prevent format string vulnerabilities when working with Bearer Tokens in Echo Go, always use explicit format strings and pass user input as arguments. This ensures that token values are treated strictly as data and never as format directives.

Safe logging with explicit format:

auth := req.Request.Header.Get("Authorization")
// Safe: format string is static, token is an argument
fmt.Printf("Authorization header: %s\n", auth)

When constructing messages that include a Bearer Token, use fmt.Sprintf with a static format string or structured logging where available:

token := c.QueryParam("token")
// Safe: token is passed as an argument, not used as a format string
message := fmt.Sprintf("Processing token: %s", token)
fmt.Println(message)

For validation or extraction, prefer functions that do not involve formatting at all. For example, validate the token structure without printing it directly:

token := c.QueryParam("token")
if len(token) > 7 && token[:7] == "Bearer " {
    actualToken := token[7:]
    // Use actualToken for authentication logic
    _ = actualToken
}

In middleware, avoid concatenating untrusted input into log messages that might later be formatted unsafely. Instead, use structured fields if your logging library supports them, or ensure that any dynamic parts are clearly separated from static format text.

These practices ensure that Bearer Tokens are handled safely and that format string weaknesses cannot be triggered through user-controlled values in Echo Go routes.