Log Injection in Echo Go with Hmac Signatures

Log injection occurs when untrusted data is written into log entries without proper sanitization, enabling log forging or log poisoning. In Echo Go, if a developer includes request metadata, headers, or user-controlled fields directly into log strings and then signs the resulting log line with Hmac Signatures, the signature does not protect the log’s semantic integrity—it only attests that the bytes were produced by a holder of the key. This creates a vulnerability because an attacker can inject newline characters, structured delimiters, or JSON-like fragments into the logged data, producing multiple log entries or appending malicious content that appears consistent with the signature scope. The signature remains valid even as the log message is manipulated, because the signature covers the final byte sequence rather than enforcing strict schema constraints on each log field.

Consider an Echo Go handler that logs a request ID and username and then signs the concatenated string with Hmac Signatures:

func handler(c echo.Context) error {
    user := c.QueryParam("user")
    requestID := c.Response().Header().Get("X-Request-ID")
    logLine := fmt.Sprintf("user=%s request_id=%s", user, requestID)
    mac := hmac.New(sha256.New, []byte(os.Getenv("SECRET")))
    mac.Write([]byte(logLine))
    signature := hex.EncodeToString(mac.Sum(nil))
    log.Printf("%s sig=%s", logLine, signature)
    return c.String(http.StatusOK, "ok")
}

If the user parameter contains a newline (e.g., alice\nrole=admin), the log line can split into multiple logical entries while the signature still covers the entire output. The signature does not prevent this injection; it merely confirms the bytes were generated by the service. This can be leveraged for log forging, where an attacker’s injected line appears as a legitimate, signed log entry. The same risk exists if structured logging formats (e.g., key=value or JSON) are used without escaping or validation, because the signature does not enforce structure—it only signs whatever string is produced.

Additionally, if log lines are later parsed by automated systems that rely on format assumptions (e.g., expecting a single, newline-terminated entry), injected newlines or control characters can cause mis-parsing, leading to missed events or false positives. Because the Hmac Signatures in this context only verify integrity of the whole signed string and not the trustworthiness or safety of individual data elements, log injection remains a reporting and parsing risk rather than an authentication risk. The scanner category Data Exposure does not flag this as a leak, but it is a security weakness in observability and auditability.

Remediation focuses on ensuring that log entries are well-structured and that injected delimiters cannot alter the logical format. Do not rely on signatures alone to enforce log correctness; instead, sanitize and serialize inputs before constructing the string to be signed and logged. Use a structured logging format with proper escaping, and sign only the canonical representation of the structured data if you need integrity.

First, validate and sanitize user inputs before they enter log strings. For string parameters that must appear in logs, remove or encode newline and carriage return characters:

func sanitizeUserInput(input string) string {
    return strings.ReplaceAll(strings.ReplaceAll(input, "\n", "\\n"), "\r", "\\r")
}

Second, construct log entries using a structured approach that prevents delimiter injection. Prefer key=value with fixed delimiters or JSON, and ensure field values are escaped. If you choose to keep a simple format, explicitly reject or replace control characters:

func buildLogLine(user, requestID string) string {
    user = sanitizeUserInput(user)
    // Reject or replace newlines to ensure one logical entry per line
    if strings.ContainsAny(user, "\n\r") {
        user = "invalid_user"
    }
    return fmt.Sprintf("user=%s request_id=%s", user, requestID)
}

Third, if you require integrity verification across log lines, sign the structured canonical form (e.g., JSON) rather than a freeform string. This makes it harder to inject fields without breaking the signature. Here is an example that builds a JSON log object, computes Hmac Signatures over its bytes, and appends the signature as a separate field:

import (
    "crypto/hmac"
    "crypto/sha256"
    "encoding/hex"
    "encoding/json"
    "net/http"
    "github.com/labstack/echo/v4"
)

type logRecord struct {
    User      string `json:"user"`
    RequestID string `json:"request_id"`
}

func handler(c echo.Context) error {
    user := c.QueryParam("user")
    requestID := c.Response().Header().Get("X-Request-ID")
    record := logRecord{User: user, RequestID: requestID}
    body, err := json.Marshal(record)
    if err != nil {
        return c.String(http.StatusInternalServerError, "encode error")
    }
    mac := hmac.New(sha256.New, []byte(os.Getenv("SECRET")))
    mac.Write(body)
    signature := hex.EncodeToString(mac.Sum(nil))
    logLine := fmt.Sprintf("%s sig=%s", string(body), signature)
    log.Println(logLine)
    return c.String(http.StatusOK, "ok")
}

With this approach, newline injection within user is contained by JSON string encoding, and the signature covers the canonical JSON bytes. Note that the signature does not prevent malformed input from being logged, but it ensures that the structured record cannot be altered without breaking the signature verification. Continue to apply input validation and output encoding at the boundaries, and avoid placing raw user-controlled data directly into log lines that are both signed and parsed by automated tools.