Log Injection in Buffalo with Bearer Tokens

Log injection occurs when untrusted input is written directly into log files without proper sanitization, enabling an attacker to forge log entries, obscure real events, or inject malicious content such as newlines or structured log delimiters. In the Buffalo web framework, this risk is amplified when Bearer token authentication is used and token values or related request metadata are logged without validation or escaping. Because Bearer tokens often appear in headers (e.g., Authorization: Bearer ), developers may inadvertently log entire headers or selected header values for debugging or audit purposes. If an attacker can supply a crafted token containing newline characters, control characters, or log-like syntax (e.g., JSON-like fields or key-value pairs), they can manipulate the log structure, inject fake entries, or trigger log forging that complicates incident response and monitoring.

In Buffalo, common patterns that create exposure include using the standard library log.Printf or similar calls with request headers directly, or including the raw token in structured logs without normalization. For example, logging the Authorization header as-is can allow newline characters in the token to inject additional lines, making it appear as separate log events. Additionally, if token values contain sensitive information and are logged in plaintext, this contributes to data exposure, which is one of the 12 security checks performed by middleBrick. Attackers may also probe for verbose error messages or debug handlers that echo tokens, enabling output injection or information leakage through logs. Because log injection can be chained with other weaknesses—such as missing input validation or insufficient rate limiting—it is important to treat log entries as untrusted output and to validate and sanitize any data derived from requests before it is recorded.

middleBrick identifies log injection risks during scans by analyzing how request data, including headers and tokens, is handled and whether outputs are escaped or structured safely. It checks for missing normalization of newlines and control characters, and whether structured logging formats (such as JSON) are properly encoded. In environments where Bearer tokens are used for authentication, scans verify that tokens are not logged in raw form and that log entries cannot be manipulated via attacker-controlled input. This is particularly relevant when OpenAPI specifications are available, as middleBrick cross-references spec definitions with runtime behavior to highlight discrepancies between documented authentication and actual logging practices.

To remediate log injection risks when using Bearer tokens in Buffalo, sanitize and avoid logging raw token values, and ensure any logged request metadata is normalized and structured safely. Below are concrete code examples demonstrating secure practices in Go with the Buffalo framework.

1. Avoid logging raw Bearer tokens

Do not log the Authorization header directly. Instead, redact or omit sensitive authentication values before logging.

// Unsafe: logging the raw Authorization header
// logger.Printf("Request headers: %v", r.Header)

// Safe: redact the Authorization header before logging
const bearerPrefix = "Bearer "
auth := r.Header.Get("Authorization")
redactedAuth := "[redacted]"
if len(auth) > len(bearerPrefix) && strings.HasPrefix(auth, bearerPrefix) {
    redactedAuth = "[redacted]"
}
logger.Printf("Request authenticated: %v", redactedAuth)

2. Normalize and escape log data

If you must include token-derived or request metadata in logs, remove or escape newline and control characters to prevent log forging. Use structured logging with proper escaping for JSON or key-value formats.

import (
    "strings"
    "unicode/utf8"
)

func sanitizeForLog(input string) string {
    // Remove carriage return and newline to prevent log injection
    sanitized := strings.ReplaceAll(input, "\r", "")
    sanitized = strings.ReplaceAll(sanitized, "\n", "")
    // Ensure valid UTF-8 to avoid encoding issues in logs
    if !utf8.ValidString(sanitized) {
        return "[binary-data-removed]"
    }
    return sanitized
}

// Example usage when logging non-sensitive metadata alongside tokens
userID := sanitizeForLog(req.FormValue("user_id"))
tokenHeader := sanitizeForLog(r.Header.Get("Authorization"))
logger.Printf("user_id=%q auth_header=%q", userID, tokenHeader)

3. Use structured logging with explicit field handling

Prefer structured logging libraries that enforce field types and escaping. When using such a library, never pass raw header values directly; instead, explicitly redact or hash them.

import (
    "go.uber.org/zap"
)

func logRequest(logger *zap.Logger, r *http.Request) {
    auth := r.Header.Get("Authorization")
    var tokenField string
    if auth != "" && strings.HasPrefix(auth, "Bearer ") {
        tokenField = "[redacted]"
    } else {
        tokenField = auth // still not ideal; prefer [redacted] universally
    }
    logger.Info("request received",
        zap.String("method", r.Method),
        zap.String("path", r.URL.Path),
        zap.String("authorization", tokenField),
    )
}

4. Leverage middleware for consistent redaction

Implement middleware that sanitizes headers before requests proceed to application logic, ensuring logs generated by downstream handlers are safe.

func RedactAuthMiddleware(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        if auth := r.Header.Get("Authorization"); auth != "" {
            // Optionally validate format without storing raw value
            if strings.HasPrefix(auth, "Bearer ") {
                // Do not modify the header required by authentication logic,
                // but ensure logs downstream use redacted values.
                ctx := context.WithValue(r.Context(), "auth_redacted", true)
                r = r.WithContext(ctx)
            }
        }
        next.ServeHTTP(w, r)
    })
}

By combining these practices—redacting tokens before logging, normalizing input to remove injection characters, and using structured logging—you reduce the risk of log injection and related data exposure when Bearer tokens are used in Buffalo applications. Security checks such as those provided by middleBrick can help verify that these mitigations are consistently applied across endpoints.