Crlf Injection in Fiber with Api Keys

Crlf Injection occurs when an attacker can inject a CRLF sequence (carriage return + line feed, represented as \r\n) into a header or status-line field. In the Fiber web framework for Go, this commonly arises when user-controlled input is placed into response headers without sanitization. When API keys are handled in headers—such as an X-API-Key request header used for authentication—the risk becomes more specific: an attacker who can control the API key value or a parameter that influences header construction may be able to inject additional headers or split the response.

Consider a scenario where an API key is passed in a header and the application builds a custom header reflecting the key value. If the input is not validated, a payload like validkey\r\nX-Injected: secret can cause the server to emit two separate logical lines in the header section. This can lead to response splitting, HTTP response smuggling, or the injection of arbitrary headers such as Set-Cookie or Location. Because Fiber allows developers to set headers programmatically (e.g., using ctx.Response().Header.Set()), failing to sanitize newline characters in values that eventually appear in headers can bypass intended access controls or enable cache poisoning and session fixation.

In the context of unauthenticated scanning, middleBrick tests for Crlf Injection by probing endpoints that accept or reflect user input in headers. It also checks whether API key handling flows expose header-splitting opportunities. Findings include severity, evidence of where the newline character enters the response path, and guidance to ensure that any user input used in headers is sanitized. This is particularly important when API keys are used as identifiers that are echoed back in headers or logs, as the combination of a flexible header API and insufficient input validation can unintentionally widen the attack surface.

Even when API keys are expected to be opaque strings, defensive handling is required: treat all input as untrusted, enforce a strict character set, and avoid concatenating raw user data into header values. middleBrick’s 12 security checks run in parallel and include specific tests for input validation as it relates to header and response integrity, helping to surface Crlf Injection risks early in the development lifecycle.

Remediation focuses on two areas: sanitizing any user-controlled data that reaches response headers, and validating API key formats before use. Below are concrete Fiber code examples that demonstrate insecure patterns and their secure counterparts.

Insecure example — reflecting an API key directly into a header:

// Insecure: raw user input used in header
app.Get("/resource", func(ctx *fiber.Ctx) error {
    apiKey := ctx.Query("api_key", "")
    // Dangerous: apiKey may contain \r\n
    ctx.Response().Header.Set("X-API-Key", apiKey)
    return ctx.SendString("ok")
})

This pattern allows newline characters in apiKey to split headers. An attacker sending ?api_key=abc\r\nX-Smuggled:true can cause the server to emit an additional header.

Secure remediation — validate and sanitize:

// Secure: validate API key format and sanitize header values
import "strings"

func isValidAPIKey(key string) bool {
    // Accept only alphanumeric and limited safe characters
    for _, r := range key {
        if !(r >= 'a' && r <= 'z' || r >= 'A' && r <= 'Z' || r >= '0' && r <= '9' || r == '-' || r == '_') {
            return false
        }
    }
    return len(key) >= 16 && len(key) <= 64
}

app.Get("/resource", func(ctx *fiber.Ctx) error {
    apiKey := ctx.Query("api_key", "")
    if !isValidAPIKey(apiKey) {
        return ctx.Status(fiber.StatusBadRequest).SendString("invalid api_key")
    }
    // Sanitize: reject newlines explicitly (defense in depth)
    safeKey := strings.ReplaceAll(apiKey, "\r", "")
    safeKey = strings.ReplaceAll(safeKey, "\n", "")
    ctx.Response().Header.Set("X-API-Key", safeKey)
    return ctx.SendString("ok")
})

Additional recommendations:

• Do not reflect raw API keys in response headers unless strictly necessary; consider returning a normalized token or an error if the key is not recognized rather than echoing it back.
• Use Fiber’s built-in mechanisms to set headers and avoid string concatenation that may introduce newlines. For example, prefer ctx.Response().Header.Set() with a pre-validated value.
• Apply strict allowlists for API key characters and length. Reject any input containing carriage returns, line feeds, or other control characters before using it in headers, cookies, or URLs.
• If you must log API keys, ensure they are truncated or masked and that logs are protected, but note that logging unsanitized values can still aid injection attacks if logs are later reflected.

By combining format validation with explicit sanitization, you reduce the risk of Crlf Injection in Fiber applications that rely on API keys in headers. middleBrick’s scans can help verify that such controls are effective by checking for improper header handling and input validation gaps.