Crlf Injection in Echo Go with Bearer Tokens

Crlf Injection occurs when user-controlled input containing carriage return (CR, \r) and line feed (\n) characters is reflected into HTTP headers without sanitization. In Echo Go, this often happens when API keys or tokens are handled as request parameters or headers and then written into downstream headers or logs. When Bearer Tokens are used for authentication, developers sometimes pass the token via a query parameter or a custom header and then re-use it in outgoing requests or responses. If the token value contains \r\n sequences and is placed into a header construction routine that does not validate or sanitize line breaks, an attacker can inject additional headers.

For example, an Echo Go handler that reads a token from a header and forwards it to a backend service might concatenate the token directly into an Authorization header without validation. A malicious token like Bearer abc123\r\nX-Injected: malicious would cause the server to send two separate headers: Authorization: Bearer abc123 and X-Injected: malicious. This can lead to request smuggling, cache poisoning, or bypass of authorization checks if the injected header alters routing or authentication logic. Even when tokens are extracted from standard Authorization headers, misuse of reflection in logging, debugging endpoints, or HTTP client wrappers can expose the same issue. The risk is compounded when the application does not enforce strict input validation on header values and treats Bearer Tokens as opaque strings while still performing string-based header assembly.

Because Echo Go is a popular framework for building HTTP APIs, improper handling of Bearer Tokens in combination with unchecked user input creates a clear path for Crlf Injection. The framework does not automatically sanitize header values, so developers must ensure that any dynamic content used in header construction is validated and encoded. Security scans that combine specification analysis with runtime testing can detect these injection points by checking whether reflected tokens in responses contain line breaks and whether they influence header parsing. Addressing this requires both input validation and careful design of how tokens are passed between services.

Remediation focuses on preventing CR and LF characters from being interpreted as header delimiters. When working with Bearer Tokens in Echo Go, avoid simple string concatenation for header construction and use the standard library’s type-safe header utilities. Always validate token input and reject or sanitize control characters before they reach any header-setting logic.

Example of vulnerable code

// Vulnerable: direct concatenation allows injection
func ProxyHandler(c echo.Context) error {
    token := c.Request().Header.Get("Authorization")
    // token might be "Bearer abc123\r\nX-Injected: malicious"
    req, _ := http.NewRequest("GET", "http://backend/api", nil)
    req.Header.Set("Authorization", token) // Unsafe if token contains \r\n
    resp, _ := http.DefaultClient.Do(req)
    return c.StringProxy(resp)
}

Secure remediation with validation

import (
    "net/http"
    "strings"
    "unicode"
)

// sanitizeLineBreaks rejects or removes CR/LF characters
func sanitizeLineBreaks(s string) string {
    return strings.Map(func(r rune) rune {
        if r == '\r' || r == '\n' {
            return -1
        }
        return r
    }, s)
}

// isValidBearerToken ensures the token follows expected format and contains no control chars
func isValidBearerToken(token string) bool {
    if !strings.HasPrefix(token, "Bearer ") {
        return false
    }
    payload := strings.TrimPrefix(token, "Bearer ")
    if payload == "" {
        return false
    }
    for _, r := range payload {
        if unicode.IsControl(r) {
            return false
        }
    }
    return true
}

func SecureProxyHandler(c echo.Context) error {
    auth := c.Request().Header.Get("Authorization")
    if !isValidBearerToken(auth) {
        return echo.NewHTTPError(http.StatusBadRequest, "invalid authorization header")
    }
    sanitized := sanitizeLineBreaks(auth)
    req, _ := http.NewRequest("GET", "http://backend/api", nil)
    req.Header.Set("Authorization", sanitized)
    resp, _ := http.DefaultClient.Do(req)
    return c.StringProxy(resp)
}

In this secure pattern, isValidBearerToken ensures the Authorization header starts with Bearer and contains no control characters, including CR and LF. The sanitizeLineBreaks function removes any remaining line break characters as a defensive measure. By using Header.Set only after validation, Echo Go applications avoid unintended header line splitting. These practices align with the broader security checks available through automated scans that verify whether tokens and other header values are safely handled.

For teams using the CLI, running middlebrick scan <url> can surface these issues by testing how the API reflects token-like values in headers and responses. Organizations on the Pro plan can enable continuous monitoring to detect regressions in header handling automatically. The GitHub Action can enforce that any API merged into main maintains these secure patterns, while the Web Dashboard helps track improvements over time.