Build an authenticated document upload that treats browser file metadata as untrusted.

• Enforce request and application size limits.
• Do not trust extension or content type alone.
• Store outside the web root.
• Never use the original name as a storage path.

I reject impossible metadata early, use %PDF- only to reject obvious mismatches, and generate an opaque storage key. The file stays outside the web root in quarantine until deeper PDF validation and malware scanning succeed. A signature match alone does not prove the document is structurally valid or safe.

[Authorize]
[HttpPost("documents")]
[RequestSizeLimit(10_500_000)]
public async Task<ActionResult<DocumentResponse>> Upload(
    IFormFile file, CancellationToken ct)
{
    const long maxBytes = 10_000_000;
    if (file.Length is <= 0 or > maxBytes)
        return ValidationProblem(new() {
            ["file"] = ["Choose a non-empty PDF up to 10 MB."]
        });

    var storageKey = $"quarantine/{Guid.NewGuid():N}";
    var retained = false;
    try
    {
        await using var input = file.OpenReadStream();
        await storage.WriteAsync(storageKey, input, maxBytes, ct);

        var signature = await storage.ReadPrefixAsync(storageKey, 5, ct);
        if (!signature.AsSpan().SequenceEqual("%PDF-"u8))
            return ValidationProblem(new() {
                ["file"] = ["The file does not look like a PDF."]
            });

        var validation = await pdfValidator.ValidateAsync(storageKey, ct);
        var scan = validation.IsStructurallyValid
            ? await scanner.ScanAsync(storageKey, ct)
            : ScanResult.Rejected;
        if (!validation.IsStructurallyValid || !scan.IsSafe)
            return ValidationProblem(new() {
                ["file"] = ["The file could not be accepted."]
            });

        var document = Document.Create(User.RequireUserId(), storageKey,
            FileNames.ForDisplay(file.FileName), file.Length, "application/pdf");
        db.Documents.Add(document);
        await db.SaveChangesAsync(ct);
        retained = true;

        return CreatedAtAction(nameof(GetDocument), new { id = document.Id },
            DocumentResponse.From(document));
    }
    finally
    {
        if (!retained)
            await storage.DeleteIfExistsAsync(storageKey, CancellationToken.None);
    }
}

Signature sniffing is constant-size and only rejects obvious mismatches. Structural validation and malware scanning inspect the stored object and may be expensive, so larger or slower workflows should remain quarantined and complete asynchronously. Size limits, cleanup, generated keys, and download authorization are still required even after a scanner reports safe.

• The body exceeds the server limit.
• The stream ends before five bytes.
• Scanning fails or times out.
• Storage succeeds but database save fails.

• Upload a renamed executable.
• Assert generated keys never contain the original name.
• Verify rejected quarantine objects are removed.
• Cancel during streaming.

• Trusting ContentType or the original extension.
• Treating %PDF- as proof that a file is a valid and safe PDF.
• Saving under the original filename or inside wwwroot.
• Publishing the quarantine object before structural validation and scanning.
• Reading the whole file into one byte array.