Header Injection in Echo Go with Api Keys

Header injection in the Echo Go framework occurs when user-controlled input is placed directly into HTTP response headers without validation or sanitization. When API keys are handled in Echo Go routes, developers sometimes copy key values from request headers into other response headers for logging, forwarding, or custom tracing. If those values are reflected without encoding or validation, an attacker can inject additional header lines, enabling header manipulation or response splitting.

For example, an API key supplied in Authorization: ApiKey <token> might be read and written into a custom header like X-API-Key. If the application does not sanitize newline characters (\n, \r) or control characters, an attacker can provide a key such as abc\r\nX-Injected: malicious. Echo then reflects this into the response, causing the injected line to be interpreted as a separate header by downstream clients or proxies. This can lead to HTTP response splitting, cache poisoning, or the leakage of sensitive headers to unintended parties.

In Echo Go, routes that bind API keys from headers and then set them verbatim into outbound or logging headers expand the attack surface. The framework does not automatically sanitize header values; it is the developer’s responsibility to ensure that any user-derived data placed into headers is safe. Without explicit validation, an API key becomes a vector for header injection, particularly when combined with features like custom middleware that forwards or logs headers. The risk is compounded when the API key is used both for authentication and for reflection, because the same untrusted value influences both security logic and output formatting.

Real-world attack patterns mirror classic CVE-style scenarios involving response splitting and header smuggling. For instance, an injected Set-Cookie header could hijack sessions, while injected Location or Refresh headers may enable client-side redirection attacks. Because Echo Go applications often integrate with external services that replay headers, an injected value can propagate beyond the immediate response, amplifying the impact. This is why treating API keys as opaque credentials and never reflecting them without strict sanitization is essential.

To remediate header injection when handling API keys in Echo Go, treat API key values as untrusted input and ensure they are never directly reflected into headers. Use strict allowlists, avoid reflection, and apply safe encoding when headers must be propagated.

Problematic pattern to avoid:

func handler(c echo.Context) error {
    apiKey := c.Request().Header.Get("X-API-Key")
    // Dangerous: directly reflecting user-controlled API key into a header
    c.Response().Header().Set("X-Client-API-Key", apiKey)
    return c.String(http.StatusOK, "ok")
}

An attacker can supply X-API-Key: abc\r\nX-Injected: value and cause two headers to be created.

Safe remediation with allowlist validation:

import (
    "net/http"
    "regexp"
    "github.com/labstack/echo/v4"
)

var apiKeyRegex = regexp.MustCompile(`^[A-Za-z0-9\-_]+\.[A-Za-z0-9\-_]+\.?[A-Za-z0-9\-_]*$`)

func safeHandler(c echo.Context) error {
    apiKey := c.Request().Header.Get("Authorization")
    if apiKey == "" || !apiKeyRegex.MatchString(apiKey) {
        return echo.NewHTTPError(http.StatusBadRequest, "invalid api key")
    }
    // Do NOT reflect apiKey into other headers; use it only for auth checks
    isValid, err := validateKey(c.Request().Context(), apiKey)
    if err != nil || !isValid {
        return echo.NewHTTPError(http.StatusUnauthorized, "invalid key")
    }
    return c.String(http.StatusOK, "authenticated")
}

func validateKey(ctx context.Context, key string) (bool, error) {
    // Validate against a secure store or introspection endpoint
    return true, nil
}

If you must propagate a key for internal auditing, encode or hash it:

import (
    "crypto/sha256"
    "encoding/hex"
    "github.com/labstack/echo/v4"
)

func auditHandler(c echo.Context) error {
    apiKey := c.Request().Header.Get("Authorization")
    if apiKey == "" {
        return echo.NewHTTPError(http.StatusBadRequest, "missing key")
    }
    // Create a safe fingerprint instead of reflecting the raw key
    fingerprint := sha256.Sum256([]byte(apiKey))
    c.Response().Header().Set("X-API-Key-Fingerprint", hex.EncodeToString(fingerprint[:]))
    return c.Next()
}

Additional measures include disabling header copying in middleware, enforcing strict CORS policies, and using Echo’s middleware to reject requests with unexpected line-ending characters in header values. These steps reduce the risk of injection while preserving the utility of API key authentication.