diff --git a/src/api/SIGCM2.Application/Audit/AuditOptions.cs b/src/api/SIGCM2.Application/Audit/AuditOptions.cs
index de19e3d..6e63a68 100644
--- a/src/api/SIGCM2.Application/Audit/AuditOptions.cs
+++ b/src/api/SIGCM2.Application/Audit/AuditOptions.cs
@@ -1,6 +1,11 @@
 namespace SIGCM2.Application.Audit;
 
-/// Bound from appsettings section "Audit". Extensible via configuration.
+/// Bound from appsettings section "Audit".
+/// Extensibility model: ADDITIVE via `IConfiguration` array binding. Setting
+/// `Audit:SanitizedKeys:N` at indices beyond the defaults APPENDS custom keys;
+/// indices 0..10 OVERWRITE the defaults. To fully replace, use a `PostConfigure`
+/// in DI. This mirrors the standard .NET array-binding quirk intentionally —
+/// the default keys are security-critical and should not be silently dropped.
 public sealed class AuditOptions
 {
     public const string SectionName = "Audit";
diff --git a/src/api/SIGCM2.Infrastructure/Audit/JsonSanitizer.cs b/src/api/SIGCM2.Infrastructure/Audit/JsonSanitizer.cs
new file mode 100644
index 0000000..c1b1f1b
--- /dev/null
+++ b/src/api/SIGCM2.Infrastructure/Audit/JsonSanitizer.cs
@@ -0,0 +1,63 @@
+using System.Text.Json;
+using System.Text.Json.Nodes;
+
+namespace SIGCM2.Infrastructure.Audit;
+
+/// UDT-010 (#REQ-AUD-5): serializes a metadata object to JSON and strips keys in the blacklist
+/// at every nesting level. Case-insensitive match. Null input → null output (never throws).
+/// Produces valid JSON consumable by AuditEvent.Metadata (ISJSON=1 constraint).
+public static class JsonSanitizer
+{
+    /// Serializes <paramref name="obj"/> to JSON and removes any property whose key matches
+    /// (case-insensitively) an entry in <paramref name="blacklist"/>. Recursive into nested
+    /// objects and arrays. Returns null if the input is null.
+    public static string? Sanitize(object? obj, IReadOnlyCollection<string> blacklist)
+    {
+        if (obj is null) return null;
+
+        var node = JsonSerializer.SerializeToNode(obj);
+        if (node is null) return null;
+
+        if (blacklist.Count > 0)
+        {
+            var blacklistLower = blacklist
+                .Select(k => k.ToLowerInvariant())
+                .ToHashSet();
+            Strip(node, blacklistLower);
+        }
+
+        return node.ToJsonString();
+    }
+
+    private static void Strip(JsonNode node, HashSet<string> blacklistLower)
+    {
+        switch (node)
+        {
+            case JsonObject obj:
+                var keysToRemove = obj
+                    .Where(kvp => blacklistLower.Contains(kvp.Key.ToLowerInvariant()))
+                    .Select(kvp => kvp.Key)
+                    .ToList();
+
+                foreach (var key in keysToRemove)
+                    obj.Remove(key);
+
+                foreach (var kvp in obj.ToList())
+                {
+                    if (kvp.Value is not null)
+                        Strip(kvp.Value, blacklistLower);
+                }
+                break;
+
+            case JsonArray arr:
+                foreach (var item in arr)
+                {
+                    if (item is not null)
+                        Strip(item, blacklistLower);
+                }
+                break;
+
+            // JsonValue: scalar, nothing to strip
+        }
+    }
+}
diff --git a/src/api/SIGCM2.Infrastructure/DependencyInjection.cs b/src/api/SIGCM2.Infrastructure/DependencyInjection.cs
index baa417f..1efd8a9 100644
--- a/src/api/SIGCM2.Infrastructure/DependencyInjection.cs
+++ b/src/api/SIGCM2.Infrastructure/DependencyInjection.cs
@@ -8,6 +8,7 @@ using Microsoft.IdentityModel.Tokens;
 using SIGCM2.Application.Abstractions;
 using SIGCM2.Application.Abstractions.Persistence;
 using SIGCM2.Application.Abstractions.Security;
+using SIGCM2.Application.Audit;
 using SIGCM2.Application.Auth;
 using SIGCM2.Infrastructure.Http;
 using SIGCM2.Infrastructure.Messaging;
@@ -68,6 +69,9 @@ public static class DependencyInjection
         services.AddHttpContextAccessor();
         services.AddScoped<IClientContext, ClientContext>();
 
+        // UDT-010: Audit options (SanitizedKeys blacklist) — overridable via appsettings "Audit".
+        services.Configure<AuditOptions>(configuration.GetSection(AuditOptions.SectionName));
+
         // Dispatcher
         services.AddScoped<IDispatcher, Dispatcher>();
 
diff --git a/tests/SIGCM2.Application.Tests/Infrastructure/Audit/AuditOptionsBindingTests.cs b/tests/SIGCM2.Application.Tests/Infrastructure/Audit/AuditOptionsBindingTests.cs
new file mode 100644
index 0000000..09e236a
--- /dev/null
+++ b/tests/SIGCM2.Application.Tests/Infrastructure/Audit/AuditOptionsBindingTests.cs
@@ -0,0 +1,71 @@
+using FluentAssertions;
+using Microsoft.Extensions.Configuration;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Options;
+using SIGCM2.Application.Audit;
+using SIGCM2.Infrastructure;
+using Xunit;
+
+namespace SIGCM2.Application.Tests.Infrastructure.Audit;
+
+/// UDT-010 Batch 3 — AuditOptions binding smoke tests.
+/// Validates that AddInfrastructure binds AuditOptions from the "Audit" config section
+/// and falls back to the POCO defaults when the section is absent.
+public sealed class AuditOptionsBindingTests
+{
+    private static IServiceProvider BuildProvider(IEnumerable<KeyValuePair<string, string?>>? overrides = null)
+    {
+        // Minimum required config for AddInfrastructure to succeed.
+        var inMemory = new Dictionary<string, string?>
+        {
+            ["ConnectionStrings:SqlServer"] = "Server=nowhere;Database=x;Integrated Security=true;",
+            ["Jwt:Issuer"] = "test",
+            ["Jwt:Audience"] = "test",
+            ["Jwt:AccessTokenMinutes"] = "60",
+            ["Jwt:RefreshTokenDays"] = "7",
+            ["Jwt:PrivateKeyPath"] = "unused-in-this-test.pem",
+            ["Jwt:PublicKeyPath"] = "unused-in-this-test.pem",
+        };
+
+        if (overrides is not null)
+            foreach (var kv in overrides)
+                inMemory[kv.Key] = kv.Value;
+
+        var config = new ConfigurationBuilder().AddInMemoryCollection(inMemory).Build();
+        var services = new ServiceCollection();
+        services.AddInfrastructure(config);
+        return services.BuildServiceProvider();
+    }
+
+    [Fact]
+    public void AuditOptions_WithoutConfigSection_UsesPocoDefaults()
+    {
+        using var sp = (ServiceProvider)BuildProvider();
+
+        var opts = sp.GetRequiredService<IOptions<AuditOptions>>().Value;
+
+        opts.SanitizedKeys.Should().Contain("password");
+        opts.SanitizedKeys.Should().Contain("refreshToken");
+        opts.SanitizedKeys.Should().Contain("apiKey");
+    }
+
+    [Fact]
+    public void AuditOptions_WithConfigSection_AddsToDefaults_PerIConfigurationArrayBinding()
+    {
+        // IConfiguration array binding is ADDITIVE, not REPLACE: config values at indices 0..N
+        // overwrite those indices but defaults beyond N are preserved. This is intended for
+        // AuditOptions — extensibility is additive (append, not replace).
+        using var sp = (ServiceProvider)BuildProvider(new[]
+        {
+            new KeyValuePair<string, string?>("Audit:SanitizedKeys:11", "customSecret"),
+            new KeyValuePair<string, string?>("Audit:SanitizedKeys:12", "internalToken"),
+        });
+
+        var opts = sp.GetRequiredService<IOptions<AuditOptions>>().Value;
+
+        // The 11 defaults remain + the 2 extras appear at the configured indices.
+        opts.SanitizedKeys.Should().Contain("password");      // default survived
+        opts.SanitizedKeys.Should().Contain("customSecret");  // appended via config
+        opts.SanitizedKeys.Should().Contain("internalToken"); // appended via config
+    }
+}
diff --git a/tests/SIGCM2.Application.Tests/Infrastructure/Audit/JsonSanitizerTests.cs b/tests/SIGCM2.Application.Tests/Infrastructure/Audit/JsonSanitizerTests.cs
new file mode 100644
index 0000000..5382b32
--- /dev/null
+++ b/tests/SIGCM2.Application.Tests/Infrastructure/Audit/JsonSanitizerTests.cs
@@ -0,0 +1,173 @@
+using FluentAssertions;
+using SIGCM2.Infrastructure.Audit;
+using Xunit;
+
+namespace SIGCM2.Application.Tests.Infrastructure.Audit;
+
+/// UDT-010 Batch 3 — JsonSanitizer unit tests (Strict TDD).
+/// Validates #REQ-AUD-5: metadata MUST be stripped of blacklisted keys before persisting.
+public sealed class JsonSanitizerTests
+{
+    private static readonly string[] DefaultBlacklist =
+    [
+        "password", "passwordHash", "token", "refreshToken",
+        "accessToken", "cvv", "card", "cardNumber",
+        "secret", "apiKey", "privateKey",
+    ];
+
+    [Fact]
+    public void Sanitize_NullInput_ReturnsNull()
+    {
+        var result = JsonSanitizer.Sanitize(null, DefaultBlacklist);
+        result.Should().BeNull();
+    }
+
+    [Fact]
+    public void Sanitize_EmptyBlacklist_PreservesAllKeys()
+    {
+        var result = JsonSanitizer.Sanitize(new { password = "x", username = "y" }, []);
+
+        result.Should().NotBeNull();
+        result.Should().Contain("\"password\"").And.Contain("\"username\"");
+    }
+
+    [Fact]
+    public void Sanitize_FlatObject_RemovesBlacklistedKeysAndKeepsOthers()
+    {
+        var result = JsonSanitizer.Sanitize(
+            new { password = "secret1", username = "juan", email = "j@x.com" },
+            DefaultBlacklist);
+
+        result.Should().NotBeNull();
+        result.Should().NotContain("\"password\"");
+        result.Should().NotContain("secret1");
+        result.Should().Contain("\"username\":\"juan\"");
+        result.Should().Contain("\"email\":\"j@x.com\"");
+    }
+
+    [Fact]
+    public void Sanitize_NestedObject_RemovesBlacklistedKeysAtEveryLevel()
+    {
+        var input = new
+        {
+            user = new { password = "x", email = "y@z.com" },
+            payload = new { nested = new { token = "abc", safe = "keep" } },
+        };
+
+        var result = JsonSanitizer.Sanitize(input, DefaultBlacklist);
+
+        result.Should().NotBeNull();
+        result.Should().NotContain("\"password\"").And.NotContain("\"token\"")
+            .And.NotContain("\"x\"").And.NotContain("abc");
+        result.Should().Contain("\"email\":\"y@z.com\"");
+        result.Should().Contain("\"safe\":\"keep\"");
+    }
+
+    [Fact]
+    public void Sanitize_ArrayOfObjects_StripsBlacklistedFromEachElement()
+    {
+        var input = new
+        {
+            items = new object[]
+            {
+                new { password = "a", name = "first" },
+                new { token = "b", name = "second" },
+                new { name = "third" },
+            },
+        };
+
+        var result = JsonSanitizer.Sanitize(input, DefaultBlacklist);
+
+        result.Should().NotBeNull();
+        result.Should().NotContain("\"password\"").And.NotContain("\"token\"");
+        result.Should().Contain("\"name\":\"first\"");
+        result.Should().Contain("\"name\":\"second\"");
+        result.Should().Contain("\"name\":\"third\"");
+    }
+
+    [Fact]
+    public void Sanitize_CaseInsensitiveMatching_StripsAllVariants()
+    {
+        var input = new Dictionary<string, object?>
+        {
+            ["Password"] = "a",
+            ["PASSWORD"] = "b",
+            ["pAsSwOrD"] = "c",
+            ["username"] = "keep",
+        };
+
+        var result = JsonSanitizer.Sanitize(input, new[] { "password" });
+
+        result.Should().NotBeNull();
+        result.Should().NotContain("\"a\"").And.NotContain("\"b\"").And.NotContain("\"c\"");
+        result.Should().Contain("\"username\":\"keep\"");
+    }
+
+    [Fact]
+    public void Sanitize_PreservesPrimitives_Numbers_Bools_Null()
+    {
+        var input = new { count = 42, flag = true, missing = (string?)null, password = "secret" };
+
+        var result = JsonSanitizer.Sanitize(input, DefaultBlacklist);
+
+        result.Should().NotBeNull();
+        result.Should().Contain("\"count\":42");
+        result.Should().Contain("\"flag\":true");
+        result.Should().Contain("\"missing\":null");
+        result.Should().NotContain("\"password\"");
+    }
+
+    [Fact]
+    public void Sanitize_ProducesValidJson()
+    {
+        var input = new { password = "x", user = new { token = "y", name = "z" } };
+        var result = JsonSanitizer.Sanitize(input, DefaultBlacklist);
+
+        result.Should().NotBeNull();
+        // Round-trip parse validates shape
+        var parsed = System.Text.Json.JsonDocument.Parse(result!);
+        parsed.RootElement.TryGetProperty("password", out _).Should().BeFalse();
+        parsed.RootElement.GetProperty("user").TryGetProperty("token", out _).Should().BeFalse();
+        parsed.RootElement.GetProperty("user").GetProperty("name").GetString().Should().Be("z");
+    }
+
+    [Fact]
+    public void Sanitize_AlreadySerializedJsonString_IsTreatedAsStringNotObject()
+    {
+        // Callers that want to sanitize an already-serialized JSON string must parse+re-sanitize
+        // themselves — the sanitizer treats a string argument as a JSON string primitive.
+        var result = JsonSanitizer.Sanitize("""{"password":"x"}""", DefaultBlacklist);
+
+        result.Should().NotBeNull();
+
+        // Round-trip parse proves the result is a JSON string primitive (not an object).
+        var parsed = System.Text.Json.JsonDocument.Parse(result!);
+        parsed.RootElement.ValueKind.Should().Be(System.Text.Json.JsonValueKind.String);
+        parsed.RootElement.GetString().Should().Be("""{"password":"x"}""");
+    }
+
+    [Fact]
+    public void Sanitize_AuditOptionsDefaultKeys_AreAllEffective()
+    {
+        // Defensive: AuditOptions.SanitizedKeys must match what tests expect.
+        var opts = new SIGCM2.Application.Audit.AuditOptions();
+
+        var input = new
+        {
+            password = "1", passwordHash = "2", token = "3", refreshToken = "4",
+            accessToken = "5", cvv = "6", card = "7", cardNumber = "8",
+            secret = "9", apiKey = "10", privateKey = "11",
+            keep = "yes",
+        };
+
+        var result = JsonSanitizer.Sanitize(input, opts.SanitizedKeys);
+
+        result.Should().NotBeNull();
+        foreach (var bad in new[] { "password", "passwordHash", "token", "refreshToken",
+            "accessToken", "cvv", "card", "cardNumber", "secret", "apiKey", "privateKey" })
+        {
+            result.Should().NotContain($"\"{bad}\"", because: $"'{bad}' is in default blacklist");
+        }
+        result.Should().Contain("\"keep\":\"yes\"");
+    }
+}