Log Injection in Fiber with Api Keys

Log injection occurs when untrusted data is written directly into log files without sanitization, allowing an attacker to forge log entries, inject newlines, or hide follow-up activity. In a Fiber application that relies on API keys for authentication or rate-limiting, the combination of user-supplied key material and verbose logging creates a clear injection path. When an API key is accepted via header, query parameter, or form field and then echoed into logs, an attacker can embed control characters or structured log syntax to manipulate log appearance.

For example, if a Fiber middleware logs the received API key verbatim and the key contains a newline (%0A or %0D sequences URL-encoded by some clients) or structured characters such as ", {, or ], the resulting log line may appear as a valid entry when it is actually a crafted injection. In distributed tracing or audit contexts, this can cause misattribution of requests, mask malicious activity, or trigger log-based alerting rules in ways that obscure true anomalies. Because API keys often appear in authentication logs, an injected entry can simulate a valid key usage event, complicating incident response and forensic analysis.

In the context of the 12 security checks run by middleBrick, log injection intersects with Data Exposure and Input Validation categories. The scanner tests whether API key values reflected in application responses or logs are properly sanitized. It also examines whether authentication endpoints log keys in plaintext, which can lead to sensitive data exposure if log files are inadvertently shared or accessed. Because middleBrick performs black-box testing without credentials, it checks whether crafted API key inputs—containing newline, carriage return, or structured payloads—result in malformed or ambiguous log output without triggering validation errors.

Consider a route that authenticates via an API key header and logs the key for debugging:

app.Get("/resource", func(c *gin.Context) {
    apiKey := c.GetHeader("X-API-Key")
    log.Printf("API key used: %s", apiKey)
    // authentication logic
})

If an attacker sends X-API-Key: abc123%0A%20%20%22event%22:%20%22logout%22 (URL-decoded newline and JSON fragment), the log may produce two lines where the second appears as a structured injection, potentially misleading SIEM parsing. middleBrick’s checks for Input Validation and Data Exposure would flag such behavior, highlighting the need to sanitize and restrict what is recorded.

Remediation focuses on preventing raw API key values from being logged and ensuring that any logged representation is sanitized, normalized, and non-repudiable. Do not log API keys in full; instead log a hashed or truncated representation, and avoid including keys in error messages or debug output. Validate and normalize input before it reaches logging code to neutralize newline and control-character injection attempts.

Below are concrete Fiber patterns that address log injection risks tied to API keys. The first example shows a safe middleware approach that hashes the key before logging and rejects suspicious characters early:

import (
    "crypto/sha256"
    "encoding/hex"
    "log"
    "net/http"
    "strings"

    "github.com/gofiber/fiber/v2"
)

func apiKeyMiddleware(c *fiber.Ctx) error {
    apiKey := c.GetHeader("X-API-Key")
    if apiKey == "" {
        return c.Status(fiber.StatusUnauthorized).JSON(fiber.Map{"error": "missing api key"})
    }
    // Normalize: reject control characters and extra whitespace
    trimmed := strings.TrimSpace(apiKey)
    if trimmed != apiKey || strings.ContainsAny(trimmed, "\r\n") {
        return c.Status(fiber.StatusBadRequest).JSON(fiber.Map{"error": "invalid api key format"})
    }
    // Log a safe representation
    hashed := sha256.Sum256([]byte(trimmed))
    log.Printf("API key used: %s", hex.EncodeToString(hashed[:]))
    // Store trimmed key in context for downstream handlers if needed
    c.Locals("apiKey", trimmed)
    return c.Next()
}

This approach ensures that newline or carriage return characters are caught during validation, and only a deterministic hash is recorded. If you need to correlate logs without storing the raw key, consider a truncated hash or a deterministic redaction scheme that still preserves uniqueness without exposing the full value.

Another pattern uses structured logging with explicit field names and strict type safety, avoiding concatenation that could be abused:

import (
    "log"

    "github.com/gofiber/fiber/v2"
)

func safeHandler(c *fiber.Ctx) error {
    apiKey := c.GetHeader("X-API-Key")
    if apiKey == "" {
        return c.SendStatus(fiber.StatusUnauthorized)
    }
    // Structured log entry with controlled fields
    log.Printf(`{"event":"api_request","api_key_truncated":"%s"}`, apiKey[:8])
    // Proceed with authenticated logic
    return c.JSON(fiber.Map{"status": "ok"})
}

In this variant, only the first few characters are included, and the log line is emitted as a single, self-contained message to reduce the risk of line-split injection. middleBrick’s scans can validate that such logging behavior is consistent with secure patterns by checking whether reflected API key material is sanitized and whether inputs containing line breaks are rejected.

Finally, integrate these checks into your development workflow using the CLI and CI/CD tooling. With the middleBrick CLI you can run scans from the terminal:

middlebrick scan https://api.example.com

Incorporate the GitHub Action to add API security checks to your CI/CD pipeline and fail builds if risk scores drop below your chosen threshold. For interactive debugging, the MCP Server lets you scan APIs directly from your AI coding assistant within the editor.