Log Injection in Aspnet with Hmac Signatures

Log injection in ASP.NET occurs when an attacker can control or influence data that is written to application or framework logs. Because logs are often aggregated, indexed, and reviewed by humans or automated systems, injected entries can forge audit trails, hide related malicious activity, or trigger automated alerting logic.

HMAC signatures are commonly used in ASP.NET to ensure integrity and authenticity of requests or payloads (e.g., webhook signatures, anti-forgery tokens, or API authentication). When HMAC verification is performed but the application logs details from the incoming request—such as headers, query strings, or body—without properly sanitizing or separating the signature from logged data, a log injection vulnerability can arise.

Consider an endpoint that validates an HMAC signature and then logs the request payload. If the payload or headers include newline characters or structured log delimiters, an attacker can append additional log lines by injecting crafted data. For example, a header value containing %0a or literal newline characters can cause the log entry to span multiple lines, making injected content appear as part of the application’s normal logging output. In structured logging formats like JSON, an attacker who can control a field value can break out of the current JSON object and append new JSON objects or text, corrupting the log stream and potentially obscuring the original request context tied to the HMAC verification step.

In ASP.NET, this often maps to the OWASP API Top 10 category API1:2023 – Broken Object Level Authorization and API5:2023 – Injection, because log injection can be chained with other weaknesses to evade detection or manipulate auditability. Real-world patterns include failing to validate or sanitize inputs before passing them to logging frameworks such as Serilog or Microsoft.Extensions.Logging, and including raw signature values or request paths directly in log messages without normalization or encoding.

An example of a vulnerable logging pattern in ASP.NET Core:

// Vulnerable: logs raw user-controlled data that may contain newline characters
var payload = await new StreamReader(Request.Body).ReadToEndAsync();
_logger.LogInformation($"Received payload: {payload}");

If payload contains newline characters, the log entry can be split, and additional lines can be injected. Even when HMAC validation passes, the log entry may misrepresent which data was covered by the signature, complicating forensic analysis.

middleBrick scans identify such log injection risks by correlating runtime input vectors with logging behavior and flagging unencoded user-controlled data written to logs, even when HMAC checks are present. This helps teams understand how seemingly trusted validation steps (like HMAC verification) can coexist with dangerous logging practices.

Remediation focuses on ensuring that user-controlled data never directly pollutes structured logs and that log entries remain atomic and well-formed. When HMAC signatures are used, treat the signature as sensitive metadata and avoid logging raw signature values or concatenated raw inputs.

1) Sanitize inputs before logging. If you must log payloads or headers, remove or escape newline characters and control characters, and enforce length limits. For structured logging, serialize objects explicitly rather than injecting raw strings.

2) Separate concerns in log entries. Log verification outcomes and metadata (e.g., request ID, anonymized identifiers) rather than raw user-controlled content. This keeps logs useful for auditing without enabling injection.

3) Use a consistent logging abstraction that enforces structured output. With Serilog, for example, use properties instead of message templates that concatenate raw input.

Secure logging example in ASP.NET Core:

// Secure: log structured metadata and avoid raw payload concatenation
var requestId = Activity.Current?.Id ?? Guid.NewGuid().ToString();
_logger.LogInformation("RequestProcessed {RequestId} {Path} {Method} HmacVerified {HmacVerified}",
    requestId,
    Request.Path,
    Request.Method,
    hmacVerified);
// If you must include payload content, sanitize it first
var safePayload = System.Text.RegularExpressions.Regex.Replace(payload, @"[\r\n]", string.Empty);
_logger.LogDebug("SanitizedPayloadLength: {Length}", safePayload.Length);

In this example, the log statement uses structured logging placeholders, ensuring that input values are serialized as distinct properties rather than interpolated into the message template. Newline characters are stripped before logging any potentially sensitive or user-controlled content, and the HMAC verification result is logged as a discrete boolean property rather than embedding the raw signature in the message.

4) If you rely on OpenAPI/Swagger specs, document expected header formats and validation outcomes. middleBrick’s OpenAPI/Swagger spec analysis can help identify mismatches between documented behavior and runtime logging practices, especially when $ref resolutions expose shared schemas that may encourage unsafe logging patterns.

5) For applications using the middleBrick CLI, you can integrate scans into your workflow: middlebrick scan <url>. For CI/CD, the GitHub Action can enforce a minimum security score before deployment, and the MCP Server enables scanning API endpoints directly from AI coding assistants, helping catch log injection and HMAC misuse early.