Log Injection in Buffalo with Hmac Signatures

Log injection occurs when untrusted input is written directly into application logs without validation, encoding, or separation, enabling attackers to forge log entries, break log formats, or inject additional control characters. In the Buffalo web framework, a common pattern is to compute an HMAC over selected request parameters or headers and include elements of that data in log lines. When the data used in the HMAC computation (e.g., a user-controlled header or query parameter) is also concatenated into the log message as a raw string, the log entry becomes malleable. For example, if you include the raw value of an X-Request-ID header directly in a log line, an attacker can append newline characters or structured delimiters to alter the perceived sequence of events in the log. Because the HMAC is typically computed over the same untrusted value, an attacker cannot forge a valid HMAC without the secret key, but they can still manipulate how that value appears in logs, creating confusion during incident investigation. In Buffalo, this risk is heightened when logs are used for security monitoring or audit trails, because injected newlines or structured payloads (such as JSON fragments) can make a single malicious request look like multiple events or can obscure the true origin of a transaction. The presence of Hmac signatures does not prevent log injection; it only ensures integrity of the signed data. If the logged content includes the original untrusted input verbatim, the signature remains valid while the log record becomes untrustworthy. This specific combination — using Hmac signatures for integrity but logging raw input — creates a false sense of security, where defenders assume a signed log entry implies authenticity, when in fact the content may have been crafted by an attacker to evade detection or to trigger log injection vulnerabilities in downstream analysis tools.

To mitigate log injection when using Hmac signatures in Buffalo, ensure that any data written to logs is sanitized, and that log lines follow a strict, immutable structure. Never directly interpolate untrusted input into log messages. Instead, use structured logging with explicit fields and encode or omit dangerous characters. Below is a concrete example of a Buffalo middleware that computes an HMAC over a selected header and logs safely by separating the signed value from the log payload.

// middleware/hmac_logger.go
package middleware

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

	"github.com/gobuffalo/buffalo"
)

const logSeparator = " | "

func HmacLogger(secret string) buffalo.MiddlewareFunc {
	return func(next buffalo.Handler) buffalo.Handler {
		return buffalo.HandlerFunc(func(c buffalo.Context) error {
			// Example: sign a custom header to ensure integrity in logs
			source := c.Request().Header.Get("X-Request-ID")
			mac := hmac.New(sha256.New, []byte(secret))
			mac.Write([]byte(source))
			signature := hex.EncodeToString(mac.Sum(nil))

			// Safe structured logging: separate fields with a delimiter and avoid raw newlines in values
			cleanSource := replaceNewlinesAndPipes(source)
			c.Response().Header().Set("X-Request-ID-Sig", signature)
			// Use key=value pairs to keep logs parseable and avoid injection
			c.Logger().Infof("event=request id=%s sig=%s method=%s path=%s", cleanSource, signature, c.Request().Method, c.Request().URL.Path)

			return next(c)
		})
	}
}

func replaceNewlinesAndPipes(s string) string {
	// Replace characters that can break log line structure
	s = strings.ReplaceAll(s, "\n", "\\n")
	s = strings.ReplaceAll(s, "\r", "\\r")
	s = strings.ReplaceAll(s, "|", "\\|")
	return s
}

In this pattern, the HMAC is computed over the raw header value to provide integrity, but the logged representation is cleaned before concatenation. The log line uses a consistent delimiter ( | ) and key=value pairs, which reduces the risk of newline or structured injection breaking the log format. For remediation focused specifically on Hmac signatures, ensure that the signature is stored and logged as a separate field and not embedded inside a free-text message where injection could obscure it. Additionally, avoid including sensitive or user-controlled data in log messages even if it is Hmac-signed, because signed logs can still be used for injection if the parsing logic downstream trusts the signature context implicitly. The combination of signature integrity and sanitized logging ensures that attackers cannot manipulate the log stream’s structure while you retain the ability to verify that the logged request data has not been tampered with after signing.