diff --git a/PolyShim.Tests/NetCore21/RandomNumberGeneratorTests.cs b/PolyShim.Tests/NetCore21/RandomNumberGeneratorTests.cs
new file mode 100644
index 00000000..f3c79aea
--- /dev/null
+++ b/PolyShim.Tests/NetCore21/RandomNumberGeneratorTests.cs
@@ -0,0 +1,36 @@
+using System;
+using System.Security.Cryptography;
+using FluentAssertions;
+using Xunit;
+
+namespace PolyShim.Tests.NetCore21;
+
+public class RandomNumberGeneratorTests
+{
+    [Fact]
+    public void Fill_Span_Test()
+    {
+        // Arrange
+        var span = new Span<byte>(new byte[16]);
+
+        // Act
+        RandomNumberGenerator.Fill(span);
+
+        // Assert
+        // Verify that not all bytes are zero (cryptographic RNG should produce non-zero data).
+        // Note: Theoretically, a crypto RNG could produce all zeros (probability: 2^-128),
+        // but this is astronomically unlikely in practice.
+        span.ToArray().Should().Contain(b => b != 0);
+    }
+
+    [Fact]
+    public void Fill_EmptySpan_Test()
+    {
+        // Arrange
+        var span = new Span<byte>();
+
+        // Act & Assert
+        // Should not throw and should return immediately
+        RandomNumberGenerator.Fill(span);
+    }
+}
diff --git a/PolyShim/List-Signatures.ps1 b/PolyShim/List-Signatures.ps1
index d3edd323..5388d991 100644
--- a/PolyShim/List-Signatures.ps1
+++ b/PolyShim/List-Signatures.ps1
@@ -371,7 +371,7 @@ foreach ($file in $codeFiles) {
     }
 
     # Extract type declarations (must be internal)
-    $typePattern = 'internal\s+(?:(?:readonly|partial|static|sealed|ref)\s+)*(class|enum|struct|interface|record)\s+(\w+(?:<[^>]+>)?)'
+    $typePattern = 'internal\s+(?:(?:readonly|partial|static|sealed|abstract|ref)\s+)*(class|enum|struct|interface|record)\s+(\w+(?:<[^>]+>)?)'
     $typeMatches = [regex]::Matches($content, $typePattern)
 
     foreach ($match in $typeMatches) {
diff --git a/PolyShim/NetCore10/RandomNumberGenerator.cs b/PolyShim/NetCore10/RandomNumberGenerator.cs
new file mode 100644
index 00000000..a788faa5
--- /dev/null
+++ b/PolyShim/NetCore10/RandomNumberGenerator.cs
@@ -0,0 +1,38 @@
+#if NETSTANDARD && !NETSTANDARD1_3_OR_GREATER
+#nullable enable
+// ReSharper disable RedundantUsingDirective
+// ReSharper disable CheckNamespace
+// ReSharper disable InconsistentNaming
+// ReSharper disable PartialTypeWithSinglePart
+
+using System;
+using System.Diagnostics.CodeAnalysis;
+
+namespace System.Security.Cryptography;
+
+// Polyfill for RandomNumberGenerator class on .NET Standard < 1.3
+// Note: This uses Random instead of cryptographically secure RNG as System.Security.Cryptography
+// types are not available on these older platforms.
+[ExcludeFromCodeCoverage]
+internal abstract class RandomNumberGenerator : IDisposable
+{
+    public static RandomNumberGenerator Create() => new RandomWrapper();
+
+    public abstract void GetBytes(byte[] data);
+
+    protected virtual void Dispose(bool disposing) { }
+
+    public void Dispose()
+    {
+        Dispose(true);
+        GC.SuppressFinalize(this);
+    }
+
+    private sealed class RandomWrapper : RandomNumberGenerator
+    {
+        private readonly Random _random = new();
+
+        public override void GetBytes(byte[] data) => _random.NextBytes(data);
+    }
+}
+#endif
diff --git a/PolyShim/NetCore21/RandomNumberGenerator.cs b/PolyShim/NetCore21/RandomNumberGenerator.cs
new file mode 100644
index 00000000..bbce1b77
--- /dev/null
+++ b/PolyShim/NetCore21/RandomNumberGenerator.cs
@@ -0,0 +1,39 @@
+#if (NETCOREAPP && !NETCOREAPP2_1_OR_GREATER) || (NETFRAMEWORK) || (NETSTANDARD && !NETSTANDARD2_1_OR_GREATER)
+#nullable enable
+// ReSharper disable RedundantUsingDirective
+// ReSharper disable CheckNamespace
+// ReSharper disable InconsistentNaming
+// ReSharper disable PartialTypeWithSinglePart
+
+using System;
+using System.Diagnostics.CodeAnalysis;
+using System.Security.Cryptography;
+
+[ExcludeFromCodeCoverage]
+internal static class MemberPolyfills_NetCore21_RandomNumberGenerator
+{
+    extension(RandomNumberGenerator)
+    {
+        // https://learn.microsoft.com/dotnet/api/system.security.cryptography.randomnumbergenerator.fill
+        public static void Fill(Span<byte> data)
+        {
+            if (data.Length == 0)
+                return;
+
+            var rng = RandomNumberGenerator.Create();
+            try
+            {
+                var buffer = new byte[data.Length];
+                rng.GetBytes(buffer);
+                buffer.CopyTo(data);
+            }
+            finally
+            {
+                // Explicit cast needed for .NET Framework 3.5 where RandomNumberGenerator
+                // doesn't properly expose IDisposable for using statements.
+                ((IDisposable)rng).Dispose();
+            }
+        }
+    }
+}
+#endif
diff --git a/PolyShim/Signatures.md b/PolyShim/Signatures.md
index b08d43b5..4eee34af 100644
--- a/PolyShim/Signatures.md
+++ b/PolyShim/Signatures.md
@@ -1,8 +1,8 @@
 # Signatures
 
-- **Total:** 340
-- **Types:** 74
-- **Members:** 266
+- **Total:** 343
+- **Types:** 76
+- **Members:** 267
 
 ___
 
@@ -236,6 +236,8 @@ ___
 - `OSPlatform`
   - [**[struct]**](https://learn.microsoft.com/dotnet/api/system.runtime.interopservices.osplatform) <sup><sub>.NET Core 1.0</sub></sup>
   - [`OSPlatform FreeBSD`](https://learn.microsoft.com/dotnet/api/system.runtime.interopservices.osplatform.freebsd) <sup><sub>.NET Core 3.0</sub></sup>
+- `OSPlatformAttribute`
+  - [**[class]**](https://learn.microsoft.com/dotnet/api/system.runtime.versioning.osplatformattribute) <sup><sub>.NET 5.0</sub></sup>
 - `OverloadResolutionPriorityAttribute`
   - [**[class]**](https://learn.microsoft.com/dotnet/api/system.runtime.compilerservices.overloadresolutionpriorityattribute) <sup><sub>.NET 9.0</sub></sup>
 - `Parallel`
@@ -275,6 +277,9 @@ ___
   - [`T[] GetItems<T>(T[], int)`](https://learn.microsoft.com/dotnet/api/system.random.getitems#system-random-getitems-1(-0()-system-int32)) <sup><sub>.NET 8.0</sub></sup>
   - [`void NextBytes(Span<byte>)`](https://learn.microsoft.com/dotnet/api/system.random.nextbytes#system-random-nextbytes(system-span((system-byte)))) <sup><sub>.NET Core 2.1</sub></sup>
   - [`void Shuffle<T>(T[])`](https://learn.microsoft.com/dotnet/api/system.random.shuffle#system-random-shuffle-1(-0())) <sup><sub>.NET 8.0</sub></sup>
+- `RandomNumberGenerator`
+  - **[class]** <sup><sub>.NET Core 1.0</sub></sup>
+  - [`void Fill(Span<byte>)`](https://learn.microsoft.com/dotnet/api/system.security.cryptography.randomnumbergenerator.fill) <sup><sub>.NET Core 2.1</sub></sup>
 - `Range`
   - [**[struct]**](https://learn.microsoft.com/dotnet/api/system.range) <sup><sub>.NET Core 3.0</sub></sup>
 - `ReadOnlyMemory<T>`