Crlf Injection in Buffalo with Bearer Tokens

Crlf Injection occurs when an attacker can inject carriage return (CR, \r) and line feed (\n) characters into HTTP headers, causing header injection or header splitting. In the Buffalo web framework for Go, this risk is amplified when Bearer Tokens are handled improperly in request construction or logging. If application code embeds user-controlled data—such as a token, username, or header value—directly into an HTTP header without strict validation or sanitization, an attacker can inject additional headers by including \r\n sequences.

Consider a scenario where a service builds an Authorization header from a user-supplied value or incorporates request parameters into log lines that include status lines. For example, if a handler reads a query parameter like token and uses it to form a header or log entry, an input such as abc123%0D%0AX-Injected:%20malicious (where %0D%0A decodes to \r\n) can introduce new headers or fake status lines. When Buffalo generates HTTP responses or forwards requests, these injected sequences can cause the server to append separate headers (e.g., X-Injected: malicious), potentially leading to response splitting, cache poisoning, or sensitive header manipulation.

The interaction with Bearer Tokens is particularly subtle. A developer might capture an incoming Bearer Token from an Authorization header and then echo it into logs or use it to construct downstream requests without adequate sanitization. Because the token often appears in status lines or log entries that resemble headers, CRLF characters in the token or in parameters can trick the server or intermediary proxies into misparsing the message. This can expose internal routes, bypass intended authorization boundaries, or manipulate logging formats in ways that obscure real issues. Even when tokens are validated, the framework does not automatically neutralize CR or LF characters; developers must explicitly reject or encode them.

Because Buffalo does not automatically sanitize header values, any user-controlled input that ends up in a header—including Bearer Tokens, query parameters, or form fields—must be treated as potentially hostile. Attack patterns like response splitting (CVE-2004-0907) or log injection become feasible when CRLF injection is possible. Tools such as middleBrick can detect these classes of issues by analyzing the unauthenticated attack surface and flagging header manipulation risks alongside findings from its 12 security checks, including Input Validation and Data Exposure. This helps teams understand where sanitization or strict allow-listing is required.

To prevent CRLF Injection when working with Bearer Tokens in Buffalo, validate and sanitize all inputs that may reach headers or log lines. The safest approach is to reject tokens containing CR or LF characters at the boundary, rather than attempting to escape them. Below are concrete patterns for Buffalo handlers that demonstrate secure handling of Bearer Tokens.

First, use a strict allow-list regular expression for token characters and explicitly reject control characters. For incoming requests, extract the Authorization header, verify it matches the Bearer scheme, and ensure the token does not contain \r or \n.

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

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

func SecureTokenHandler(c buffalo.Context) error {
    auth := c.Request().Header.Get("Authorization")
    if auth == "" {
        return c.Error(http.StatusUnauthorized, errors.New("authorization header required"))
    }
    parts := strings.Split(auth, " ")
    if len(parts) != 2 || strings.ToLower(parts[0]) != "bearer" {
        return c.Error(http.StatusBadRequest, errors.New("invalid authorization format"))
    }
    token := parts[1]
    if !bearerTokenRegex.MatchString(token) {
        return c.Error(http.StatusBadRequest, errors.New("invalid token characters"))
    }
    if strings.ContainsAny(token, "\r\n") {
        return c.Error(http.StatusBadRequest, errors.New("token contains invalid characters"))
    }
    // Use token safely for downstream calls or logging
    c.Logger().Info("authorized request", "user_id", lookupUserID(token))
    return nil
}

This pattern ensures that any CR or LF characters are caught before they can influence headers or log entries. It also avoids formatting the Authorization header via concatenation that could inadvertently introduce line breaks.

Second, when constructing outbound HTTP requests that include Bearer Tokens, avoid building raw header strings from concatenation with untrusted data. Instead, use the standard library’s header map, which handles proper formatting and prevents injection.

import (
    "net/http"
)

func CallProtectedAPI(token string) (*http.Response, error) {
    req, err := http.NewRequest(http.MethodGet, "https://api.example.com/resource", nil)
    if err != nil {
        return nil, err
    }
    req.Header.Set("Authorization", "Bearer "+token)
    // The Header.Set method safely replaces existing headers and does not interpret CR/LF as line breaks
    client := &http.Client{}
    return client.Do(req)
}

Finally, enforce logging hygiene by filtering or encoding any user-influenced values that may be written to logs. If tokens must appear in logs, replace them with a redacted placeholder or hash to prevent injection via log lines that resemble status lines.