Header Injection in Buffalo with Bearer Tokens

Header Injection in the Buffalo framework when Bearer Tokens are used for authentication can occur when untrusted input is improperly reflected into HTTP headers that influence authorization. For example, if a developer builds a route handler that takes a query or header value and directly assigns it to an Authorization header, an attacker can supply a malicious value such as a newline character sequence (\r\n) to inject additional headers like X-Original-URL or to split the header and inject a crafted Bearer token.

Consider a Buffalo app that forwards requests to an upstream service and constructs the Authorization header from user-controlled data:

// WARNING: vulnerable pattern
func forwardHandler(c buffalo.Context) error {
    token := c.Param("token")
    req, _ := http.NewRequest("GET", "https://api.example.com/resource", nil)
    req.Header.Set("Authorization", "Bearer " + token)
    client := &http.Client{}
    resp, _ := client.Do(req)
    return c.Render(200, r.HTML("forward.html"))
}

If token contains newline sequences, an attacker can inject extra headers:

token = "abc123\r\nX-Forwarded-For: 10.0.0.1"
// Resulting Authorization header lines may be split, causing the server to
// interpret injected text as separate headers or parameters.

In a Buffalo app, this can bypass intended Bearer Token validation if the injected header alters routing, logging, or is forwarded to backend services that trust internal headers. Additionally, if the app parses the Authorization header in a custom middleware that splits on commas or semicolons, an injected token can be interpreted as a second, attacker-controlled credential. This becomes especially risky when combined with other findings such as BOLA/IDOR or insecure direct object references, because a manipulated Authorization header could be used to escalate access to other users’ resources.

Another common pattern in Buffalo apps is to read the token from a header and pass it to an OAuth2 client or API client without strict validation:

// Another risky pattern
func apiProxy(c buffalo.Context) error {
    auth := c.Request().Header.Get("Authorization")
    // Expecting "Bearer eyJ..." but user can control the whole string
    parts := strings.Split(auth, " ")
    if len(parts) != 2 || parts[0] != "Bearer" {
        c.Response().WriteHeader(http.StatusUnauthorized)
        return c.Error(401, errors.New("invalid auth"))
    }
    token := parts[1]
    // Forward token without verifying format or audience
    client := &http.Client{}
    req, _ := http.NewRequest("GET", "https://upstream.example/data", nil)
    req.Header.Set("Authorization", auth)
    resp, _ := client.Do(req)
    return c.Render(200, r.JSON(resp.Header))
}

An attacker can supply a malformed Authorization value that passes the simple prefix check but injects additional semantics, such as using a second Bearer token or altering the header chain. Because Buffalo encourages convention-based routing and middleware stacks, such injected values may propagate to downstream handlers or logging facilities, increasing the attack surface. Proper validation of header syntax, disallowing newlines, and strictly formatting the Authorization header mitigate these risks.

Remediation focuses on strict parsing, rejecting unexpected control characters, and avoiding concatenation of user input into the Authorization header. Always treat the incoming token as an opaque value and do not reassemble the Authorization header by string concatenation. Prefer configuring Buffalo middleware to add the token from a trusted source or after validation.

Safe approach: validate the incoming Bearer token format and set the header explicitly without user-controlled injection points:

// Secure pattern: validate and use a pre-configured token or a verified source
func secureProxy(c buffalo.Context) error {
    // Ideally this token comes from session store, JWT verification, or server-side config
    token := verifyTokenFromSession(c) // implement your own secure token retrieval
    req, _ := http.NewRequest("GET", "https://api.example.com/resource", nil)
    req.Header.Set("Authorization", "Bearer " + token)
    // Ensure no newline characters are present
    if strings.ContainsAny(token, "\r\n") {
        c.Response().WriteHeader(http.StatusBadRequest)
        return c.Error(400, errors.New("invalid token"))
    }
    client := &http.Client{}
    resp, err := client.Do(req)
    if err != nil {
        appError(c, err)
        return err
    }
    defer resp.Body.Close()
    return c.Render(200, r.JSON(resp.Header))
}

If you must forward an Authorization header, normalize it by re-encoding or using a server-side token store rather than echoing user input. For JWT-based Bearer tokens, validate the token signature and claims in Buffalo middleware before allowing the request to proceed:

// JWT validation middleware example
func JWTAuth(next buffalo.Handler) buffalo.Handler {
    return func(c buffalo.Context) error {
        auth := c.Request().Header.Get("Authorization")
        const bearerPrefix = "Bearer "
        if !strings.HasPrefix(auth, bearerPrefix) {
            c.Response().WriteHeader(http.StatusUnauthorized)
            return c.Error(401, errors.New("missing bearer token"))
        }
        raw := strings.TrimPrefix(auth, bearerPrefix)
        if strings.ContainsAny(raw, "\r\n") {
            c.Response().WriteHeader(http.StatusBadRequest)
            return c.Error(400, errors.New("token contains invalid characters"))
        }
        // Verify token using jwt.Parse with your key and claims validation
        _, err := jwt.Parse(raw, func(token *jwt.Token) (interface{}, error) {
            if _, ok := token.Method.(*jwt.SigningMethodHMAC); !ok {
                return nil, fmt.Errorf("unexpected signing method")
            }
            return []byte(os.Getenv("JWT_SECRET")), nil
        })
        if err != nil {
            c.Response().WriteHeader(http.StatusUnauthorized)
            return c.Error(401, errors.New("invalid token"))
        }
        return next(c)
    }
}

For the CLI, you can scan your Buffalo endpoints with the middlebrick command to detect header-related issues during development:

$ middlebrick scan https://api.yourapp.com/openapi.json

Additionally, the GitHub Action can enforce security thresholds in CI/CD so that builds fail if a scan exposes header injection or other high-severity findings. This complements secure coding practices by catching regressions before deployment.

Finally, avoid logging raw Authorization headers and ensure that any error messages do not reflect user-supplied header values. These measures reduce both the likelihood and the impact of Header Injection in Buffalo applications using Bearer Tokens.