SecureFileUpload.Core 1.0.0-preview.3

SecureFileUpload.Core

Defense-in-depth file upload pipeline for ASP.NET Core 10+ — AES-256-GCM envelope encryption, Argon2id key derivation, deep content validation, and pluggable virus scanning.

SecureFileUpload.Core is a battle-tested file-upload pipeline lifted from a production ASP.NET Core document-intake workflow, de-branded, hardened, and shipped as a single NuGet package. Every layer is implemented in code you can read; every limitation is named in KNOWN-GAPS.md; every security claim traces to a specific line in src/ per the audit in SECURITY-ANALYSIS.md.

"So whether you eat or drink or whatever you do, do it all for the glory of God." — 1 Corinthians 10:31

What's New in 1.0.0-preview.2

This release is a security-and-platform modernization. No new layers, no behavior changes — the upload pipeline is byte-for-byte the same. The crypto floor and the runtime target both moved up.

• Argon2id for KEK derivation. The master Key Encryption Key is now derived via Argon2id (RFC 9106, OWASP 2024+ recommendation) with memory-hard defaults — m=64 MiB, t=3, p=4. Memory-hardness raises the cost-per-guess on GPUs and ASICs by orders of magnitude over the prior PBKDF2-SHA256 derivation.
• Backward-compatible online upgrade. Files wrapped under prior PBKDF2 KEKs (600 000 and 210 000 iterations) still decrypt via FileUpload:KeyDerivation:LegacyKekFallback=true (default). No file on disk is bricked by the upgrade. New writes always use the Argon2id-derived KEK.
• Configurable KDF. Argon2id is the default; KeyDerivation:Algorithm = "Pbkdf2" is available for FIPS-restricted environments. All Argon2id parameters and the PBKDF2 iteration count are tunable from appsettings.json.
• .NET 10. Target framework consolidated on net10.0 only. Pin 1.0.x-preview.0 if you need a net8.0-only build.
• Packaging. Deterministic build, Source Link, .snupkg symbols, and README.md / LICENSE / SECURITY-ANALYSIS.md / KNOWN-GAPS.md bundled inside the package itself.

The crypto posture, parameters, and honest residual risks are documented in Implementation & Crypto Posture below and in SECURITY-ANALYSIS.md.

Why this exists

File upload is one of the most consistently mishandled surfaces in web development. Most tutorials show you how to receive a file. Very few defend against:

• Polyglot files (a valid JPEG that is also a working PHP shell)
• Double-extension attacks (photo.pdf.exe)
• MIME spoofing and magic-byte forgery
• Path traversal via filename manipulation
• PDF JavaScript injection (including inside FlateDecode-compressed object streams)
• ZIP-bomb / pixel-flood attacks via image decoding
• Log poisoning via crafted filenames
• Disk exhaustion via batched uploads
• Direct web-serving of attacker-controlled bytes

This package addresses every item on that list in code, then names the gaps it does not close. The red-team review in SECURITY-ANALYSIS.md traces each claim to its source line.

The 8-layer pipeline

Every uploaded file passes through eight serial layers. Failure at any content-decision layer rejects the file. The pipeline is fail-closed on content; the single fail-open seam is virus-scanner availability (Layer 7), and that is documented, counted, and never silently relabelled as clean — see KNOWN-GAPS.md §Gap 9.

┌──────────────────────────────────────────────────────────────────────────┐
│                         INCOMING FILE UPLOAD                              │
└──────────────────────────────┬───────────────────────────────────────────┘
                               │
       ┌───────────────────────▼────────────────────────┐
       │  Layer 1   File size (per-file + batch total)   │
       │            Minimum size per format               │
       └───────────────────────┬────────────────────────┘
       ┌───────────────────────▼────────────────────────┐
       │  Layer 2   Extension allowlist                   │
       │            .jpg .jpeg .png .webp .pdf            │
       └───────────────────────┬────────────────────────┘
       ┌───────────────────────▼────────────────────────┐
       │  Layer 3   MIME ↔ extension cross-validation     │
       │            Browser MIME must match extension     │
       └───────────────────────┬────────────────────────┘
       ┌───────────────────────▼────────────────────────┐
       │  Layer 4   Magic-byte signature check            │
       │            JPEG / PNG / WebP fourCC / PDF        │
       └───────────────────────┬────────────────────────┘
       ┌───────────────────────▼────────────────────────┐
       │  Layer 5   Filename inspection                   │
       │            Double-extension, Unicode bidi,       │
       │            path traversal, Windows reserved      │
       └───────────────────────┬────────────────────────┘
       ┌───────────────────────▼────────────────────────┐
       │  Layer 6   Deep content validation               │
       │            JPEG/PNG/WebP structural walkers,     │
       │            PDF byte-pattern scan,                │
       │            FlateDecode-compressed PDF stream     │
       │            decompression and re-scan             │
       └───────────────────────┬────────────────────────┘
       ┌───────────────────────▼────────────────────────┐
       │  Layer 7   Virus scan (pluggable)                │
       │            Windows Defender OR ClamAV / clamd    │
       │            Detection fail-closed,                │
       │            Availability fail-open (tracked)      │
       └───────────────────────┬────────────────────────┘
       ┌───────────────────────▼────────────────────────┐
       │  Layer 8   Encrypted storage                     │
       │            AES-256-GCM envelope (v2):            │
       │              per-file random DEK                 │
       │              wrapped under Argon2id-derived KEK  │
       │            Image recompression strips polyglot   │
       │            tails before encryption.              │
       │            Randomized filename, outside wwwroot. │
       │            Path traversal re-checked before      │
       │            write via PathHelper.IsPathUnderBase. │
       └──────────────────────────────────────────────────┘

Install

dotnet add package SecureFileUpload.Core

Requires .NET 10+ with ASP.NET Core. The package references Microsoft.AspNetCore.App as a framework reference, so nothing extra ships inside it — your runtime's existing ASP.NET Core does the heavy lifting.

If you need a net8.0 build, pin to a 1.0.x-preview.0 version of this package — Argon2id and net10.0 arrived in preview.2.

Quick start

1. Register the services

// Program.cs
using SecureFileUpload.Services;

builder.Services.AddSecureFileUpload();

// Match this to FileUpload:MaxTotalUploadBytes in appsettings.json.
builder.Services.Configure<FormOptions>(options =>
{
    options.MultipartBodyLengthLimit = 53_477_376; // 51 MB
});

AddSecureFileUpload() registers FileContentValidator, the platform-appropriate IVirusScanService (Windows Defender on Windows, ClamAV elsewhere), and IFileUploadService in one call. The scanner backend is picked at startup; switching it is one DI line, not a code change.

2. Receive an upload

[HttpPost]
[RequestSizeLimit(53_477_376)]
public async Task<IActionResult> Submit(MyInputModel model)
{
    if (!ModelState.IsValid)
        return View(model);

    var result = await _fileUploadService.UploadFilesAsync(
        Request.Form.Files, model.LastName, "intake");

    if (!result.Success)
    {
        // result.Errors are user-safe; result.WorkflowOutcome is
        // AllSaved | PartialSaved | AllRejected | NoFiles.
        foreach (var msg in result.Errors)
            ModelState.AddModelError(string.Empty, msg);
        return View(model);
    }

    return RedirectToAction(nameof(Submitted), new { id = result.SubmissionFolder });
}

3. Serve a file safely

// Wire the reference controller behind an authenticated, MFA-gated route.
// SecureFileDownloadController forces Content-Disposition: attachment,
// a strict CSP, X-Frame-Options: DENY, COOP/COEP/CORP, and re-checks
// path traversal at read time — the file never renders inline.
services.AddControllers().AddApplicationPart(typeof(SecureFileDownloadController).Assembly);

A complete appsettings.json reference is in Configuration below.

Implementation & Crypto Posture

This section names primitives, parameters, and residual risks. It is the single source of truth for the cryptographic posture; the marketing tagline is at the top of this README.

Honest limitations

• The KDF salt is in source. It is identical across deployments of this library version. The protection model assumes the secret lives in a real secrets manager (Key Vault, AWS Secrets Manager, env var injected by the platform) — never appsettings.json committed to a repo.
• Argon2id is not FIPS-validated. Compliance-bound deployments must select Pbkdf2 explicitly.
• The KEK lives in process memory. A memory-disclosure or core-dump capability on the host bypasses the KDF entirely. Mitigations are deployment-level (least privilege, ASLR, sealed VMs, confidential compute).
• No HSM / KMS integration in v1. A KMS-backed KEK is tracked in docs/hardening-roadmap.md as the right next step for high-assurance deployments.
• Argon2id parameters are CPU/RAM-bound. On a constrained container, startup derivation may take longer than the ~250–500 ms target. The library logs KDF_ARGON2ID_DERIVED | ElapsedMs=... so this is measurable.

For the full code-traced security review, see SECURITY-ANALYSIS.md. For things this pipeline does not protect against, see KNOWN-GAPS.md.

Configuration

{
  "FileUpload": {
    "StorageRoot": "../uploads",
    "MaxFileSizeBytes": 10485760,
    "MaxFileCount": 5,
    "MaxTotalUploadBytes": 52428800,
    "MinStorageFreeBytes": 536870912,
    "MinTempFreeBytes": 536870912,
    "LowDiskWarningBytes": 2147483648,
    "RecompressImages": true,
    "JpegRecompressQuality": 95,
    "EncryptionEnabled": false,
    "EncryptionSecret": "CHANGE_THIS_TO_A_REAL_SECRET_MINIMUM_32_CHARS",
    "KeyDerivation": {
      "Algorithm": "Argon2id",
      "Argon2id": {
        "MemoryKiB": 65536,
        "Iterations": 3,
        "Parallelism": 4
      },
      "Pbkdf2": {
        "Iterations": 600000
      },
      "LegacyKekFallback": true
    }
  },
  "FileContent": {
    "InspectCompressedPdfStreams": true,
    "MaxCompressedStreamsToInspect": 64,
    "MaxDecompressedStreamBytes": 16777216,
    "RejectEncryptedPdfs": true,
    "RejectInteractivePdfs": false,
    "MaxImageWidth": 10000,
    "MaxImageHeight": 10000,
    "MaxImagePixels": 40000000
  },
  "VirusScan": {
    "Enabled": false,
    "WindowsDefender": {
      "MpCmdRunPath": "C:\\Program Files\\Windows Defender\\MpCmdRun.exe",
      "TempScanPath": "C:\\Temp\\VirusScan",
      "TimeoutSeconds": 30
    },
    "ClamAv": {
      "Host": "localhost",
      "Port": 3310,
      "TimeoutSeconds": 30,
      "MaxStreamBytes": 26214400
    }
  }
}

Dependencies

Declared by the NuGet package — no manual installation required:

• ASP.NET Core 10+ shared framework (via FrameworkReference)
• SixLabors.ImageSharp 3.1.x — image identification and polyglot-tail recompression
• Konscious.Security.Cryptography.Argon2 1.3.x — Argon2id KEK derivation

Required at runtime if VirusScan:Enabled=true (not NuGet packages):

• Windows Defender (MpCmdRun.exe) — Windows only, used by WindowsDefenderScanService
• ClamAV (clamd listening on TCP) — Linux / macOS / containers, used by ClamAvScanService

The scanner is selected automatically by AddSecureFileUpload() based on OperatingSystem.IsWindows().

Source layout

Docs

• SECURITY-ANALYSIS.md — full code-traced security review, with each claim pointing at source lines
• KNOWN-GAPS.md — honest limitations and what this does NOT protect against
• docs/threat-model.md — what attack each layer defeats
• docs/hardening-roadmap.md — recommended next steps toward the strongest realistic posture
• tests/attack-vectors.md — per-layer attack test cases
• tests/Fuzz/ — SharpFuzz + AFL++ harness for the deep content validator

Release process

Publishing is handled by GitHub Actions in .github/workflows/nuget-publish.yml.

• Push to main runs build, pack, and a fuzz-harness build — no publish.
• Push a v* tag to publish to NuGet.org. The workflow derives the package version from the tag (v1.0.0-preview.3 → 1.0.0-preview.3).
• --skip-duplicate is used, so re-running on an existing version is non-destructive.

git tag v1.0.0-preview.3
git push origin v1.0.0-preview.3

The <Version> in src/SecureFileUpload.Core.csproj is the source of truth for local packs and CI artifacts; tag builds override it for the published NuGet version.

Contributing

Issues and PRs welcome — especially:

• Unit-test coverage for the per-layer validation paths
• Additional format validators (GIF, BMP deep content validation)
• Async / queued virus-scan worker for higher-volume deployments
• KMS / HSM-backed KEK provider

If you're filing a security report, please open a private advisory on GitHub rather than a public issue.

License

MIT. Use freely. Attribution appreciated, not required.

"So whether you eat or drink or whatever you do, do it all for the glory of God." — 1 Corinthians 10:31