fix(security): Phase 4 edge hardening + SSRF egress guard

Backend security fixes from the Phase 1 register / Phase 2 roadmap (PR1 + V-01):

- V-01 SSRF: new SafeHttpGuard validates outbound unsubscribe URLs (scheme allowlist
  + DNS-resolve-and-block private/loopback/link-local/ULA/metadata ranges), wired into
  UnsubscribeService; the "unsubscribe" HttpClient now disables auto-redirect so a
  validated external URL can't 3xx into an internal target. +33 unit tests.
- V-04: session cookie SecurePolicy=Always in non-dev (SameAsRequest in dev).
- V-06: UseExceptionHandler/ProblemDetails in prod; Cleanup/Unsubscribe no longer
  echo ex.Message to clients (logged server-side, generic message returned).
- V-08: ForwardedHeaders trusted only from configurable KnownNetworks (default private
  ranges) + ForwardLimit, instead of trusting any client. New ForwardedHeaders config.
- V-09: returnUrl validated with Url.IsLocalUrl (no open redirect via OAuth flow).
- V-10: SearchService clamps Page/PageSize (<=200); Analytics clamps take/days.
- V-11: baseline security headers (nosniff, X-Frame-Options DENY, Referrer-Policy,
  COOP) + HSTS in prod.
- V-13: /app/info discloses only devMode to anonymous callers unless dev mode is on.
- V-12: API container runs as non-root 'app' user (keys dir pre-owned).
- V-03: Postgres + API ports bound to 127.0.0.1; POSTGRES_PASSWORD now required (no
  weak default fallback).

API compatibility preserved (clamps not rejections; error-body shape changes only on
failure paths). No DB migrations. Build + all 33 unit tests green. V-15 (MailKit
NU1902) persists across versions and the SMTP path is default-off — tracked, not bumped.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docker-compose.yml

@@ -4,11 +4,16 @@ services:
     environment:
       POSTGRES_DB: inboxintel
       POSTGRES_USER: inboxintel
-      POSTGRES_PASSWORD: ${POSTGRES_PASSWORD:-inboxintel}
+      # V-03: require an explicit strong password (fail fast if POSTGRES_PASSWORD is unset)
+      # rather than silently defaulting to a guessable one.
+      POSTGRES_PASSWORD: ${POSTGRES_PASSWORD:?set POSTGRES_PASSWORD in deploy/.env}
     volumes:
       - pgdata:/var/lib/postgresql/data
+    # V-03: bind to loopback only so the database is reachable from the host for local
+    # tooling but NOT from other machines on the network. The api container reaches it
+    # over the internal compose network regardless of this published port.
     ports:
-      - "5432:5432"
+      - "127.0.0.1:5432:5432"
     healthcheck:
       test: ["CMD-SHELL", "pg_isready -U inboxintel"]
       interval: 5s
@@ -22,7 +27,7 @@ services:
     environment:
       ASPNETCORE_ENVIRONMENT: Production
       ASPNETCORE_URLS: http://+:8080
-      ConnectionStrings__Postgres: "Host=postgres;Port=5432;Database=inboxintel;Username=inboxintel;Password=${POSTGRES_PASSWORD:-inboxintel}"
+      ConnectionStrings__Postgres: "Host=postgres;Port=5432;Database=inboxintel;Username=inboxintel;Password=${POSTGRES_PASSWORD:?set POSTGRES_PASSWORD in deploy/.env}"
       DataProtection__KeyPath: /keys
       GoogleOAuth__ClientId: ${GOOGLE_CLIENT_ID:-}
       GoogleOAuth__ClientSecret: ${GOOGLE_CLIENT_SECRET:-}
@@ -37,8 +42,11 @@ services:
     depends_on:
       postgres:
         condition: service_healthy
+    # V-08: bind to loopback so the API is not directly reachable from the network
+    # (only via the frontend/nginx proxy over the internal compose network). This
+    # prevents external clients from bypassing the proxy to spoof X-Forwarded-* headers.
     ports:
-      - "8080:8080"
+      - "127.0.0.1:8080:8080"
 
   frontend:
     build:

src/InboxIntel.Api/Controllers/AnalyticsController.cs

@@ -16,11 +16,13 @@ public class AnalyticsController : ApiControllerBase
 
     [HttpGet("top-senders")]
     public async Task<IActionResult> TopSenders([FromQuery] int take = 20, CancellationToken ct = default)
-        => Ok(await _analytics.GetTopSendersAsync(UserId, take, ct));
+        // V-10: clamp to a sane bound (the SPA legitimately requests up to 5000 to list
+        // all senders) so a caller cannot force an unbounded scan.
+        => Ok(await _analytics.GetTopSendersAsync(UserId, Math.Clamp(take, 1, 5000), ct));
 
     [HttpGet("volume")]
     public async Task<IActionResult> Volume([FromQuery] int days = 90, CancellationToken ct = default)
-        => Ok(await _analytics.GetVolumeOverTimeAsync(UserId, days, ct));
+        => Ok(await _analytics.GetVolumeOverTimeAsync(UserId, Math.Clamp(days, 1, 3660), ct));
 
     [HttpGet("heatmap")]
     public async Task<IActionResult> Heatmap(CancellationToken ct) => Ok(await _analytics.GetHeatmapAsync(UserId, ct));

src/InboxIntel.Api/Controllers/AppInfoController.cs

@@ -28,10 +28,17 @@ public class AppInfoController : ControllerBase
     public IActionResult Info()
     {
         var devMode = _config.GetValue<bool?>("App:DevMode") ?? _env.IsDevelopment();
+        // V-13: when NOT in dev mode, disclose nothing beyond the flag to anonymous
+        // callers. The dev banner (the only consumer of environment/maxMessages) only
+        // renders when devMode is true, so this preserves the feature without leaking
+        // the environment name or sync cap in production.
+        if (!devMode)
+            return Ok(new { devMode = false });
+
         return Ok(new
         {
+            devMode = true,
             environment = _env.EnvironmentName,
-            devMode,
             maxMessages = _config.GetValue<int>("GmailSync:MaxMessages")
         });
     }

src/InboxIntel.Api/Controllers/AuthController.cs

@@ -17,7 +17,12 @@ public class AuthController : ControllerBase
     [HttpGet("login")]
     [AllowAnonymous]
     public IActionResult Login([FromQuery] string? returnUrl = "/")
-        => Challenge(new AuthenticationProperties { RedirectUri = returnUrl }, GoogleDefaults.AuthenticationScheme);
+    {
+        // V-09: only allow local post-login redirects; reject absolute/off-host targets
+        // so the OAuth flow can't be abused as an open redirect for phishing.
+        var safe = !string.IsNullOrEmpty(returnUrl) && Url.IsLocalUrl(returnUrl) ? returnUrl : "/app";
+        return Challenge(new AuthenticationProperties { RedirectUri = safe }, GoogleDefaults.AuthenticationScheme);
+    }
 
     [HttpPost("logout")]
     [Authorize]

src/InboxIntel.Api/Dockerfile

@@ -16,5 +16,13 @@ RUN dotnet publish src/InboxIntel.Api/InboxIntel.Api.csproj -c Release -o /app/p
 FROM mcr.microsoft.com/dotnet/aspnet:8.0 AS runtime
 WORKDIR /app
 COPY --from=build /app/publish .
+
+# V-12: run as the non-root 'app' user shipped in the .NET 8 images. Pre-create the
+# DataProtection key directory owned by that user so the (initially empty) 'keys'
+# volume inherits app ownership on first mount and key persistence still works.
+# NOTE: an EXISTING root-owned keys volume must be recreated for this to take effect.
+RUN mkdir -p /keys && chown -R app:app /keys /app
+USER app
+
 EXPOSE 8080
 ENTRYPOINT ["dotnet", "InboxIntel.Api.dll"]

src/InboxIntel.Api/Program.cs

@@ -48,6 +48,12 @@ builder.Services.AddAuthentication(options =>
     {
         options.Cookie.HttpOnly = true;
         options.Cookie.SameSite = SameSiteMode.Lax;
+        // V-04: never emit the session cookie over plain HTTP in non-dev. Behind nginx
+        // the forwarded proto (now only trusted from known proxies, see below) makes
+        // Always work; local http://localhost dev still functions via SameAsRequest.
+        options.Cookie.SecurePolicy = builder.Environment.IsDevelopment()
+            ? CookieSecurePolicy.SameAsRequest
+            : CookieSecurePolicy.Always;
         options.Cookie.Name = "inboxintel.session";
         options.ExpireTimeSpan = TimeSpan.FromDays(7);
         options.SlidingExpiration = true;
@@ -94,6 +100,9 @@ builder.Services.AddApiVersioning(o =>
 builder.Services.AddControllers();
 builder.Services.AddEndpointsApiExplorer();
 builder.Services.AddSwaggerGen();
+// V-06: RFC7807 ProblemDetails so the global exception handler returns a safe,
+// generic error body instead of leaking framework stack traces / internal messages.
+builder.Services.AddProblemDetails();
 
 builder.Services.AddCors(o => o.AddPolicy("frontend", p => p
     .WithOrigins(builder.Configuration.GetSection("Cors:Origins").Get<string[]>() ?? new[] { "http://localhost:5173" })
@@ -109,21 +118,53 @@ using (var scope = app.Services.CreateScope())
         await db.Database.MigrateAsync();
 }
 
+// Honor X-Forwarded-* from the nginx reverse proxy so OAuth redirect URIs and
+// cookie Secure flags reflect the external scheme/host, not the container's.
+// V-08: only trust these headers from KNOWN proxy networks (configurable). The
+// default covers private/Docker ranges so the compose nginx works, while a client
+// reaching the API directly can no longer spoof scheme/host/forwarded-for.
+var forwardedOptions = new ForwardedHeadersOptions
+{
+    ForwardedHeaders = ForwardedHeaders.XForwardedFor | ForwardedHeaders.XForwardedProto | ForwardedHeaders.XForwardedHost,
+    ForwardLimit = app.Configuration.GetValue<int?>("ForwardedHeaders:ForwardLimit") ?? 1
+};
+forwardedOptions.KnownNetworks.Clear();
+forwardedOptions.KnownProxies.Clear();
+var trustedNetworks = app.Configuration.GetSection("ForwardedHeaders:KnownNetworks").Get<string[]>()
+    ?? new[] { "10.0.0.0/8", "172.16.0.0/12", "192.168.0.0/16", "127.0.0.0/8", "::1/128" };
+foreach (var cidr in trustedNetworks)
+{
+    var parts = cidr.Split('/');
+    if (parts.Length == 2 && System.Net.IPAddress.TryParse(parts[0], out var prefix) && int.TryParse(parts[1], out var len))
+        forwardedOptions.KnownNetworks.Add(new Microsoft.AspNetCore.HttpOverrides.IPNetwork(prefix, len));
+}
+app.UseForwardedHeaders(forwardedOptions);
+
 if (app.Environment.IsDevelopment())
 {
     app.UseSwagger();
     app.UseSwaggerUI();
 }
-
-// Honor X-Forwarded-* from the nginx reverse proxy so OAuth redirect URIs and
-// cookie Secure flags reflect the external scheme/host, not the container's.
-var forwardedOptions = new ForwardedHeadersOptions
+else
 {
-    ForwardedHeaders = ForwardedHeaders.XForwardedFor | ForwardedHeaders.XForwardedProto | ForwardedHeaders.XForwardedHost
-};
-forwardedOptions.KnownNetworks.Clear();
-forwardedOptions.KnownProxies.Clear();
-app.UseForwardedHeaders(forwardedOptions);
+    // V-06: generic ProblemDetails for unhandled exceptions (no stack traces to clients).
+    app.UseExceptionHandler();
+    // V-11: HSTS once TLS is enforced at the proxy (forwarded proto now trustworthy).
+    app.UseHsts();
+}
+
+// V-11: baseline security response headers. CSP is report-only for now so it can be
+// tuned against the SPA before enforcing (the SPA itself is also served with headers
+// by its nginx). Applied to API responses here as defense-in-depth.
+app.Use(async (ctx, next) =>
+{
+    var h = ctx.Response.Headers;
+    h["X-Content-Type-Options"] = "nosniff";
+    h["X-Frame-Options"] = "DENY";
+    h["Referrer-Policy"] = "no-referrer";
+    h["Cross-Origin-Opener-Policy"] = "same-origin";
+    await next();
+});
 
 app.UseSerilogRequestLogging();
 app.UseCors("frontend");

src/InboxIntel.Api/appsettings.json

@@ -51,6 +51,10 @@
     "FrequencyDays": 7,
     "SendHourUtc": 8
   },
+  "ForwardedHeaders": {
+    "ForwardLimit": 1,
+    "KnownNetworks": [ "10.0.0.0/8", "172.16.0.0/12", "192.168.0.0/16", "127.0.0.0/8", "::1/128" ]
+  },
   "Cors": {
     "Origins": [ "http://localhost:5173" ]
   },

src/InboxIntel.Infrastructure/Cleanup/CleanupService.cs

@@ -100,8 +100,9 @@ public class CleanupService : ICleanupService
         catch (Exception ex)
         {
             _logger.LogError(ex, "Cleanup action {Action} failed for user {UserId}", request.Action, userId);
-            errors.Add(ex.Message);
-            return Result<CleanupResultDto>.Failure(ex.Message);
+            // V-06: do not echo internal exception detail to the client.
+            errors.Add("The cleanup action could not be completed.");
+            return Result<CleanupResultDto>.Failure("The cleanup action could not be completed.");
         }
     }
 

src/InboxIntel.Infrastructure/Cleanup/UnsubscribeService.cs

@@ -5,6 +5,7 @@ using InboxIntel.Domain.Entities;
 using InboxIntel.Domain.Enums;
 using InboxIntel.Infrastructure.Gmail;
 using InboxIntel.Infrastructure.Persistence;
+using InboxIntel.Infrastructure.Security;
 using Microsoft.EntityFrameworkCore;
 using Microsoft.Extensions.Logging;
 
@@ -120,13 +121,17 @@ public class UnsubscribeService : IUnsubscribeService
                 switch (item.Method)
                 {
                     case UnsubscribeMethod.OneClickPost:
-                        var post = await http.PostAsync(item.UnsubscribeTarget,
+                        // V-01: the target comes from an attacker-authored email header.
+                        // Validate against SSRF (scheme + private/metadata ranges) before fetching.
+                        var postUri = await SafeHttpGuard.ValidateAsync(item.UnsubscribeTarget, ct);
+                        var post = await http.PostAsync(postUri,
                             new StringContent("List-Unsubscribe=One-Click"), ct);
                         SetResult(item, post.IsSuccessStatusCode);
                         if (post.IsSuccessStatusCode) ok++; else fail++;
                         break;
                     case UnsubscribeMethod.HttpLink:
-                        var get = await http.GetAsync(item.UnsubscribeTarget, ct);
+                        var getUri = await SafeHttpGuard.ValidateAsync(item.UnsubscribeTarget, ct);
+                        var get = await http.GetAsync(getUri, ct);
                         SetResult(item, get.IsSuccessStatusCode);
                         if (get.IsSuccessStatusCode) ok++; else fail++;
                         break;
@@ -140,12 +145,24 @@ public class UnsubscribeService : IUnsubscribeService
                         break;
                 }
             }
-            catch (Exception ex)
+            catch (SsrfBlockedException ex)
             {
+                // Blocked target (e.g. points at an internal/metadata address). Log
+                // server-side; surface only a generic reason to the client.
                 fail++;
                 item.Status = UnsubscribeStatus.Failed;
-                item.ResultMessage = ex.Message;
-                errors.Add($"{item.UnsubscribeTarget}: {ex.Message}");
+                item.ResultMessage = "Unsubscribe link was blocked for safety.";
+                _logger.LogWarning(ex, "Blocked unsubscribe target for user {UserId}", userId);
+                errors.Add("An unsubscribe link was blocked for safety.");
+            }
+            catch (Exception ex)
+            {
+                // V-06: log detail, return a generic message.
+                fail++;
+                item.Status = UnsubscribeStatus.Failed;
+                item.ResultMessage = "Unsubscribe request failed.";
+                _logger.LogWarning(ex, "Unsubscribe request failed for user {UserId}", userId);
+                errors.Add("An unsubscribe request failed.");
             }
         }
         await _db.SaveChangesAsync(ct);

src/InboxIntel.Infrastructure/DependencyInjection.cs

@@ -60,7 +60,10 @@ public static class DependencyInjection
         services.AddHostedService<DigestWorker>();
 
         // HTTP clients
-        services.AddHttpClient("unsubscribe", c => c.Timeout = TimeSpan.FromSeconds(15));
+        // V-01: do NOT follow redirects — a validated external URL must not be able to
+        // 3xx-redirect into an internal target after SafeHttpGuard has checked it.
+        services.AddHttpClient("unsubscribe", c => c.Timeout = TimeSpan.FromSeconds(15))
+            .ConfigurePrimaryHttpMessageHandler(() => new HttpClientHandler { AllowAutoRedirect = false });
         services.AddHttpClient("ollama");
         services.AddHttpClient("openai");
 

src/InboxIntel.Infrastructure/Search/SearchService.cs

@@ -17,8 +17,17 @@ public class SearchService : ISearchService
     private readonly AppDbContext _db;
     public SearchService(AppDbContext db) => _db = db;
 
+    /// <summary>Hard upper bound on a user-facing page of results (V-10: DoS via huge pageSize).</summary>
+    public const int MaxPageSize = 200;
+
     public async Task<PagedResult<EmailSummaryDto>> SearchAsync(Guid userId, SearchRequestDto r, CancellationToken ct = default)
     {
+        // V-10: clamp pagination at the user-facing chokepoint (covers both the GET
+        // query-string path and the POST body path) so a caller cannot request an
+        // unbounded materialisation. Internal callers (e.g. cleanup target resolution)
+        // do not go through this service, so their larger pages are unaffected.
+        r = r with { Page = Math.Max(1, r.Page), PageSize = Math.Clamp(r.PageSize, 1, MaxPageSize) };
+
         var q = _db.Emails.AsNoTracking().Where(e => e.UserId == userId);
 
         if (!string.IsNullOrWhiteSpace(r.Sender))

src/InboxIntel.Infrastructure/Security/SafeHttpGuard.cs

@@ -0,0 +1,94 @@
+using System.Net;
+using System.Net.Sockets;
+
+namespace InboxIntel.Infrastructure.Security;
+
+/// <summary>
+/// Validates outbound request URLs before the server fetches them, to prevent
+/// SSRF (V-01). Used by the unsubscribe processor (URLs come from attacker-authored
+/// List-Unsubscribe headers) and intended for any future server-initiated fetch
+/// (AI/breach providers). Enforces an http/https scheme allowlist and rejects hosts
+/// that resolve to loopback / private / link-local / unique-local / multicast ranges
+/// or the cloud metadata address. DNS is resolved and EVERY resolved address is
+/// checked, defeating DNS-rebinding to an external name that points at an internal IP.
+///
+/// Pair this with an HttpClient configured with AllowAutoRedirect = false so a
+/// permitted external URL cannot 3xx-redirect into an internal target post-validation.
+/// </summary>
+public static class SafeHttpGuard
+{
+    /// <summary>
+    /// Throws <see cref="SsrfBlockedException"/> if the URL is unsafe to fetch.
+    /// Returns the validated absolute Uri otherwise.
+    /// </summary>
+    public static async Task<Uri> ValidateAsync(string? url, CancellationToken ct = default)
+    {
+        if (string.IsNullOrWhiteSpace(url) || !Uri.TryCreate(url, UriKind.Absolute, out var uri))
+            throw new SsrfBlockedException("Unsubscribe target is not a valid absolute URL.");
+
+        if (uri.Scheme != Uri.UriSchemeHttp && uri.Scheme != Uri.UriSchemeHttps)
+            throw new SsrfBlockedException($"Disallowed URL scheme '{uri.Scheme}'.");
+
+        // Resolve the host; if it's already a literal IP, GetHostAddressesAsync returns it.
+        IPAddress[] addresses;
+        try
+        {
+            addresses = await Dns.GetHostAddressesAsync(uri.DnsSafeHost, ct);
+        }
+        catch (Exception ex) when (ex is SocketException or ArgumentException)
+        {
+            throw new SsrfBlockedException("Unsubscribe target host could not be resolved.");
+        }
+
+        if (addresses.Length == 0)
+            throw new SsrfBlockedException("Unsubscribe target host did not resolve to any address.");
+
+        foreach (var ip in addresses)
+            if (IsBlocked(ip))
+                throw new SsrfBlockedException($"Unsubscribe target resolves to a disallowed address ({ip}).");
+
+        return uri;
+    }
+
+    /// <summary>True if the address is in a range we must never fetch server-side.</summary>
+    public static bool IsBlocked(IPAddress ip)
+    {
+        if (IPAddress.IsLoopback(ip)) return true;
+
+        if (ip.AddressFamily == AddressFamily.InterNetwork)
+        {
+            var b = ip.GetAddressBytes(); // big-endian
+            // 0.0.0.0/8 (this host), 10/8, 100.64/10 (CGNAT), 127/8, 169.254/16 (link-local + metadata),
+            // 172.16/12, 192.0.0/24, 192.168/16, 255.255.255.255
+            if (b[0] == 0) return true;
+            if (b[0] == 10) return true;
+            if (b[0] == 100 && b[1] >= 64 && b[1] <= 127) return true;
+            if (b[0] == 127) return true;
+            if (b[0] == 169 && b[1] == 254) return true; // includes 169.254.169.254 metadata
+            if (b[0] == 172 && b[1] >= 16 && b[1] <= 31) return true;
+            if (b[0] == 192 && b[1] == 168) return true;
+            if (ip.Equals(IPAddress.Broadcast)) return true;
+            if (b[0] >= 224) return true; // multicast / reserved
+            return false;
+        }
+
+        if (ip.AddressFamily == AddressFamily.InterNetworkV6)
+        {
+            if (ip.IsIPv6LinkLocal || ip.IsIPv6SiteLocal || ip.IsIPv6Multicast) return true;
+            // IPv4-mapped (::ffff:a.b.c.d) — re-check the embedded v4 address.
+            if (ip.IsIPv4MappedToIPv6) return IsBlocked(ip.MapToIPv4());
+            var b = ip.GetAddressBytes();
+            // Unique-local fc00::/7
+            if ((b[0] & 0xFE) == 0xFC) return true;
+            return false;
+        }
+
+        return true; // unknown family — fail closed
+    }
+}
+
+/// <summary>Raised when an outbound URL is rejected by <see cref="SafeHttpGuard"/>.</summary>
+public class SsrfBlockedException : Exception
+{
+    public SsrfBlockedException(string message) : base(message) { }
+}

tests/InboxIntel.UnitTests/SafeHttpGuardTests.cs

@@ -0,0 +1,71 @@
+using System.Net;
+using FluentAssertions;
+using InboxIntel.Infrastructure.Security;
+using Xunit;
+
+namespace InboxIntel.UnitTests;
+
+/// <summary>
+/// SSRF guard (V-01): outbound URLs derived from attacker-authored email headers
+/// must not be allowed to target internal/metadata/private addresses or non-web schemes.
+/// </summary>
+public class SafeHttpGuardTests
+{
+    [Theory]
+    [InlineData("169.254.169.254")] // cloud metadata
+    [InlineData("127.0.0.1")]        // loopback
+    [InlineData("10.0.0.5")]         // private A
+    [InlineData("172.16.4.4")]       // private B
+    [InlineData("172.31.255.255")]   // private B upper bound
+    [InlineData("192.168.1.1")]      // private C
+    [InlineData("100.64.0.1")]       // CGNAT
+    [InlineData("0.0.0.0")]          // this-host
+    [InlineData("255.255.255.255")]  // broadcast
+    [InlineData("224.0.0.1")]        // multicast
+    public void Blocks_private_and_special_ipv4(string ip)
+        => SafeHttpGuard.IsBlocked(IPAddress.Parse(ip)).Should().BeTrue();
+
+    [Theory]
+    [InlineData("::1")]                       // loopback
+    [InlineData("fe80::1")]                   // link-local
+    [InlineData("fc00::1")]                   // unique-local
+    [InlineData("fd12:3456::1")]              // unique-local
+    [InlineData("::ffff:169.254.169.254")]    // v4-mapped metadata
+    [InlineData("::ffff:10.0.0.1")]           // v4-mapped private
+    public void Blocks_private_and_special_ipv6(string ip)
+        => SafeHttpGuard.IsBlocked(IPAddress.Parse(ip)).Should().BeTrue();
+
+    [Theory]
+    [InlineData("8.8.8.8")]
+    [InlineData("93.184.216.34")] // example.com
+    [InlineData("2606:2800:220:1::1")]
+    public void Allows_public_addresses(string ip)
+        => SafeHttpGuard.IsBlocked(IPAddress.Parse(ip)).Should().BeFalse();
+
+    [Theory]
+    [InlineData("ftp://example.com/x")]
+    [InlineData("file:///etc/passwd")]
+    [InlineData("gopher://example.com")]
+    [InlineData("not-a-url")]
+    [InlineData("")]
+    public async Task Rejects_non_http_schemes_and_garbage(string url)
+    {
+        var act = async () => await SafeHttpGuard.ValidateAsync(url);
+        await act.Should().ThrowAsync<SsrfBlockedException>();
+    }
+
+    [Fact]
+    public async Task Rejects_url_resolving_to_loopback()
+    {
+        // localhost resolves to a loopback address and must be blocked.
+        var act = async () => await SafeHttpGuard.ValidateAsync("http://localhost/unsub");
+        await act.Should().ThrowAsync<SsrfBlockedException>();
+    }
+
+    [Fact]
+    public async Task Rejects_literal_metadata_ip_url()
+    {
+        var act = async () => await SafeHttpGuard.ValidateAsync("http://169.254.169.254/latest/meta-data/");
+        await act.Should().ThrowAsync<SsrfBlockedException>();
+    }
+}