Header Injection in Echo Go with Hmac Signatures

Header injection in the Echo Go framework occurs when untrusted input is placed into HTTP response headers without validation or sanitization. This can allow an attacker to inject additional headers, split the header block, or manipulate downstream caching and routing behavior. When Hmac Signatures are used to authenticate request integrity, header injection can weaken the security model if the signature is computed over an attacker-controlled header or if the signature verification logic is bypassed via injected headers.

Consider an API that uses Hmac Signatures to verify that requests originate from a trusted source. The server typically computes an HMAC over selected request components—such as method, path, selected headers, and body—and expects the client to send that signature in a header (e.g., X-API-Signature). If the server incorporates an attacker-controllable header into the signed payload, or if header injection allows an attacker to influence which headers are included in the verification, the integrity guarantee can be undermined. For example, an attacker might inject a header that is concatenated into the signature base string, causing the server to accept a modified request while still producing a valid Hmac Signature.

Echo Go applications that dynamically build headers for logging, correlation, or routing may inadvertently reflect attacker input into response headers. If the server uses a header value as part of the Hmac computation without strict allowlisting, an attacker can alter the signed context. Additionally, if signature verification logic is implemented as a middleware that inspects request headers and then sets or forwards headers without canonicalization, an attacker can exploit header injection to change effective routing or to bypass intended checks. Common techniques include newline characters in header values that cause header splitting, or duplicate headers that some frameworks process inconsistently, leading to ambiguous verification results.

In practice, this manifests when an API endpoint echoes a client-supplied header (e.g., X-Custom-Context) into a response header and that header is included in the Hmac payload. An attacker can inject crafted values that change the signature context or inject additional headers like X-Forwarded-For or X-Real-IP that the server might mistakenly trust. The vulnerability is compounded when the server does not enforce strict header normalization before computing or comparing Hmac Signatures, allowing an attacker to bypass integrity checks without needing to know the secret key.

To illustrate, a vulnerable Echo Go handler might read a header from the request, incorporate it into the Hmac payload, and then set a response header based on the same user input. An attacker can supply newline characters or duplicate headers to manipulate how the server parses and verifies the signature. This specific combination—Echo Go, Hmac Signatures, and header injection—creates a pathway where integrity controls can be evaded, enabling tampered requests to appear authentic.

Remediation centers on ensuring that only trusted, allowlisted headers are included in the Hmac computation and that user input is never directly reflected into response headers without strict validation. In Echo Go, implement a canonicalization step that extracts, normalizes, and selects headers before building the signature base string. Avoid including user-controlled headers in the signed payload unless they are explicitly expected and validated.

Below is a secure example of computing and verifying an Hmac Signature in Echo Go. The code defines a helper to build a canonical string from an allowlisted set of headers, uses Hmac-SHA256, and compares signatures in constant time to avoid timing attacks. It also demonstrates safe header handling to prevent injection.

// secure_hmac.go
package main

import (
    "crypto/hmac"
    "crypto/sha256"
    "encoding/hex"
    "net/http"
    "sort"
    "strings"

    "github.com/labstack/echo/v4"
)

// allowlisted headers that may influence the signature
var allowedHeaders = []string{"x-request-id", "x-correlation-id", "content-type"}

// canonicalizeAndSign builds a canonical string from selected headers and signs it
func canonicalizeAndSign(req *http.Request, secret string) string {
    var parts []string
    for _, h := range allowedHeaders {
        if values := req.Header[http.CanonicalHeaderKey(h)]; len(values) > 0 {
            // normalize: lower-case header name, trim spaces, join multiple values with comma
            normalizedValues := make([]string, len(values))
            for i, v := range values {
                normalizedValues[i] = strings.TrimSpace(v)
            }
            parts = append(parts, h+":"+strings.Join(normalizedValues, ","))
        }
    }
    // sort for deterministic order
    sort.Strings(parts)
    canonical := strings.Join(parts, "\n")
    mac := hmac.New(sha256.New, []byte(secret))
    mac.Write([]byte(canonical))
    return hex.EncodeToString(mac.Sum(nil))
}

// verifyHmacSignature verifies the request signature using the same canonical method
func verifyHmacSignature(req *http.Request, secret, receivedSig string) bool {
    expected := canonicalizeAndSign(req, secret)
    return hmac.Equal([]byte(expected), []byte(receivedSig))
}

// SignatureMiddleware is an Echo middleware that validates Hmac Signatures
func SignatureMiddleware(secret string) echo.MiddlewareFunc {
    return func(next echo.HandlerFunc) echo.HandlerFunc {
        return func(c echo.Context) error {
            req := c.Request()
            receivedSig := req.Header.Get("X-Api-Signature")
            if receivedSig == "" {
                return c.String(http.StatusBadRequest, "missing signature")
            }
            if !verifyHmacSignature(req, secret, receivedSig) {
                return c.String(http.StatusUnauthorized, "invalid signature")
            }
            return next(c)
        }
    }
}

// ExampleHandler demonstrates safe header usage: do not include user input directly
func ExampleHandler(c echo.Context) error {
    // Extract a trusted header value safely
    requestID := c.Request().Header.Get("X-Request-Id")
    if requestID == "" {
        requestID = "none"
    }
    // Set a response header with strict allowlist and no user injection
    c.Response().Header().Set("X-Request-Id", requestID)
    return c.JSON(http.StatusOK, map[string]string{"status": "ok"})
}

func main() {
    e := echo.New()
    secret := "your-256-bit-secret" // store securely, e.g., from env
    e.Use(SignatureMiddleware(secret))
    e.GET("/api/data", ExampleHandler)
    e.Logger.Fatal(e.Start(":8080"))
}

Key remediation points:

• Never include raw user-supplied headers in the Hmac payload; only use explicitly allowlisted headers.
• Normalize header names to canonical case and trim whitespace to avoid bypass via case variation or extra spaces.
• Handle multiple header values deterministically (e.g., join with commas) and sort header components to ensure canonical ordering.
• Use constant-time comparison (hmac.Equal) to prevent timing side-channels in signature verification.
• Do not reflect untrusted header values into response headers; if necessary, use a strict allowlist and avoid echoing user input.
• Reject requests with newline characters or other control characters in header values to mitigate header splitting and injection attempts.

By combining strict header allowlisting, canonicalization, and secure comparison, the integrity provided by Hmac Signatures remains robust even in the presence of injection attempts in the Echo Go framework.