Crlf Injection in Aspnet with Firestore

Crlf Injection occurs when an attacker can inject carriage return (CR, \r) and line feed (\n) characters into an HTTP header or a log entry, causing header splitting or log forging. In an Aspnet application that uses Cloud Firestore as a backend, this typically arises when user-controlled input is reflected into Firestore document fields or metadata and later rendered in contexts where CRLF sequences are interpreted by downstream systems.

Consider an Aspnet endpoint that writes a custom HTTP header value into Firestore before echoing it back in a response. If the input is not validated, an attacker can supply a value such as injected\r\nX-Content-Type-Options: nosniff. When the application writes this string to Firestore and later reads it to set a header, the embedded CRLF splits the header, injecting an additional X-Content-Type-Options: nosniff header. This can bypass security headers or enable response splitting if the application subsequently streams the data to a browser.

Additionally, Firestore documents often contain fields used to construct log lines or error messages. If a field like comment includes CRLF sequences and those documents are exported or streamed to a log aggregator, forged log entries can appear as legitimate events, undermining log integrity and aiding in post-exploitation activities. MiddleBrick’s checks for Log Forgery and Header Injection align with this attack surface, flagging endpoints where user input reaches headers or structured logs without canonicalization.

Firestore’s data model encourages nested objects and maps, which can inadvertently propagate CRLF-containing values across multiple fields and collections. When combined with Aspnet’s dynamic model binding, an attacker may supply payloads in nested properties that later surface in HTTP responses or server-side logs. Because Firestore does not enforce header-safe string constraints, the onus is on the application to validate and sanitize inputs before persistence and before any secondary use such as templating or export.

To detect this combination, a scan with middleBrick targets endpoints that accept input, write it to Firestore, and later retrieve it for header or log usage. The tool’s checks for Input Validation and Header/Log Forgery are run in parallel, and findings include concrete steps to canonicalize inputs and contextual-output-encode according to the OWASP API Top 10 and related standards.

Remediation focuses on ensuring that any user-controlled data destined for Firestore or later reflection in headers/logs is canonicalized and validated. Below are concrete examples for Aspnet using the official Google Cloud Firestore SDK for .NET.

First, define a helper that rejects or sanitizes CRLF characters at the boundary where input enters your domain model. This prevents tainted data from being written to Firestore and later reused in a header or log context.

using System;
using System.Text.RegularExpressions;

public static class ValidationHelper
{
    // Reject CR and LF in strings that will be stored in Firestore and later used in headers/logs
    private static readonly Regex CrlfRegex = new Regex(@"[\r\n]", RegexOptions.Compiled);

    public static string SanitizeForFirestore(string input)
    {
        if (string.IsNullOrEmpty(input))
        {
            return input;
        }
        if (CrlfRegex.IsMatch(input))
        {
            throw new ArgumentException("Input must not contain CR or LF characters.");
        }
        return input;
    }
}

Second, apply this validation in your Aspnet controller before creating or updating a Firestore document.

using Google.Cloud.Firestore;
using Microsoft.AspNetCore.Mvc;

[ApiController]
[Route("api/comments")]
public class CommentsController : ControllerBase
{
    private readonly FirestoreDb _firestore;

    public CommentsController(FirestoreDb firestore)
    {
        _firestore = firestore;
    }

    [HttpPost]
    public async Task PostComment([FromBody] CommentRequest request)
    {
        var sanitizedText = ValidationHelper.SanitizeForFirestore(request.Text);

        var docRef = _firestore.Collection("comments").Document();
        var comment = new Comment
        {
            Id = docRef.Id,
            Text = sanitizedText,
            CreatedAt = Timestamp.GetCurrentTimestamp()
        };
        await docRef.SetAsync(comment);

        return CreatedAtAction(nameof(GetComment), new { id = docRef.Id }, comment);
    }
}

public class CommentRequest
{
    public string Text { get; set; }
}

public class Comment
{
    public string Id { get; set; }
    public string Text { get; set; }
    public Timestamp CreatedAt { get; set; }
}

Third, if you must store free-form text that may later be rendered in logs, apply output encoding appropriate to the consuming context. For example, when emitting log lines, replace CRLF with a safe placeholder or use structured logging that avoids concatenation of raw fields into a single line.

using Microsoft.Extensions.Logging;

public class LogService
{
    private readonly ILogger _logger;

    public LogService(ILogger logger)
    {
        _logger = logger;
    }

    public void WriteCommentLog(string commentText)
    {
        // Replace CRLF to prevent log forging; alternatively use structured logging
        var safeText = commentText.Replace("\r", "\\r").Replace("\n", "\\n");
        _logger.LogInformation("Comment submitted: {Text}", safeText);
    }
}

Finally, for responses that set headers derived from Firestore data, always validate and avoid direct concatenation. Use Aspnet’s built-in header utilities which do not interpret CRLF.

using Microsoft.AspNetCore.Http;

public void Configure(IApplicationBuilder app)
{
    app.Use(async (context, next) =>
    {
        // Example: safe header value from Firestore
        var safeValue = "nosniff";
        context.Response.Headers.Append("X-Content-Type-Options", safeValue);
        await next();
    });
}