Format String in Fiber with Api Keys

A format string vulnerability in a Fiber application that handles API keys occurs when user-controlled input is passed directly to a logging or formatting function without proper sanitization. In Go, functions like fmt.Printf interpret format verbs such as %s, %x, or %p. If an attacker can influence the format string, they can read from or write to memory, potentially leaking sensitive data like API keys stored in variables.

Consider a scenario where a Fiber route receives an API key via a query parameter and logs it using a user-controlled format string:

app.Get("/log", func(c *fiber.Ctx) error {
    format := c.Query("format", "key: %s")
    apiKey := c.Get("X-API-Key")
    fmt.Printf(format, apiKey)
    return c.SendString("logged")
})

If an attacker sends a request like ?format=%x:%x:%x:%x, the output may reveal adjacent memory contents, including the API key or other sensitive runtime data. Even if the API key is passed as an argument, a malformed format string can cause the program to read beyond the provided arguments on the stack or heap, leading to information disclosure.

This becomes especially critical when API keys are logged for debugging and the format string is not strictly controlled. An attacker can chain a format string read with other techniques to infer memory layout or, in more complex cases, attempt to modify variables in memory. Because Fiber routes often handle authentication and secrets, any inadvertent exposure of API keys through format strings can lead to unauthorized access and lateral movement within a system.

The risk is not limited to direct logging. If user input influences formatting in middleware or helper utilities that process API keys, the same class of vulnerability persists. For example, constructing log messages by concatenating user input without using format verbs safely can still result in unintended interpretation if the logging backend applies additional formatting.

To assess this risk, scans check whether format functions receive dynamic format strings and whether sensitive values like API keys are part of the formatting operation. Findings highlight the need to treat format strings as untrusted and to isolate sensitive data from any user-influenced formatting logic.

Remediation centers on ensuring that format strings are static and that sensitive values such as API keys are never interpolated using user-controlled format verbs. The safest approach is to avoid dynamic format strings entirely and to log API keys using explicit, safe methods.

1. Use static format strings with explicit verb placement:

app.Get("/log", func(c *fiber.Ctx) error {
    apiKey := c.Get("X-API-Key")
    fmt.Printf("apiKey: %s", apiKey)
    return c.SendString("logged")
})

By fixing the format string, fmt.Printf will only interpret the provided arguments as data, preventing memory disclosure through format verbs supplied by the client.

2. Prefer structured logging that does not rely on format strings:

type LogEntry struct {
    Event string "json:"event""
    APIKey string "apiKey"
}

app.Get("/log", func(c *fiber.Ctx) error {
    apiKey := c.Get("X-API-Key")
    entry := LogEntry{Event: "api_key_accessed", APIKey: apiKey}
    data, _ := json.Marshal(entry)
    fmt.Println(string(data))
    return c.SendString("logged")
})

Structured logging serializes data explicitly and avoids format string parsing on the server side, which eliminates the risk of format string attacks while improving log interoperability.

3. If dynamic formatting is required for legacy reasons, validate and restrict the format string:

allowedFormats := map[string]bool{"key: %s": true, "prefix=%s": true}

app.Get("/log", func(c *fiber.Ctx) error {
    format := c.Query("format", "key: %s")
    if !allowedFormats[format] {
        return c.Status(fiber.StatusBadRequest).SendString("invalid format")
    }
    apiKey := c.Get("X-API-Key")
    fmt.Printf(format, apiKey)
    return c.SendString("logged")
})

Whitelisting allowed format strings ensures that an attacker cannot inject arbitrary verbs or sequences that could lead to memory disclosure. This approach is less robust than using static formats and should be avoided when simpler solutions suffice.

4. Avoid logging API keys altogether in production environments:

app.Get("/secure", func(c *fiber.Ctx) error {
    apiKey := c.Get("X-API-Key")
    if apiKey == "" {
        return c.Status(fiber.StatusUnauthorized).SendString("missing key")
    }
    // Use apiKey for authentication without logging it
    return c.SendString("ok")
})

The most secure remediation is to minimize the exposure surface by not logging sensitive values such as API keys. Validate their presence for authentication purposes without persisting them in logs where a format string vulnerability could expose them.

These fixes align with secure coding practices for handling secrets and ensure that format string behaviors cannot be influenced by untrusted input, thereby protecting API keys in Fiber applications.