diff --git a/src/SharpCompress/Common/Tar/Headers/TarHeader.cs b/src/SharpCompress/Common/Tar/Headers/TarHeader.cs
index 27817f96..559a27bc 100644
--- a/src/SharpCompress/Common/Tar/Headers/TarHeader.cs
+++ b/src/SharpCompress/Common/Tar/Headers/TarHeader.cs
@@ -37,6 +37,7 @@ internal sealed partial class TarHeader
     internal IArchiveEncoding ArchiveEncoding { get; }
 
     internal const int BLOCK_SIZE = 512;
+    internal const long MAX_ENTRY_SIZE = long.MaxValue - (BLOCK_SIZE - 1);
 
     // Maximum size for long name/link headers to prevent memory exhaustion attacks
     // This is generous enough for most real-world scenarios (32KB)
@@ -631,23 +632,36 @@ internal sealed partial class TarHeader
 
     private static EntryType ReadEntryType(byte[] buffer) => (EntryType)buffer[156];
 
-    private long ReadSize(byte[] buffer)
+    private static long ReadSize(byte[] buffer)
     {
         var size =
-            (buffer[124] & 0x80) == 0x80 // if size in binary
-                ? BinaryPrimitives.ReadInt64BigEndian(buffer.AsSpan(0x80))
+            (buffer[124] & 0x80) != 0
+                ? ReadBase256Size(buffer)
                 : ReadAsciiInt64Base8(buffer, 124, 11);
 
-        // A negative payload size can rewind seekable archives to the same header.
+        // Negative sizes can rewind seekable archives; oversized sizes overflow padding.
         // Validate here so ordinary entries and metadata use the same sync/async guard.
-        if (size < 0)
+        if (size is < 0 or > MAX_ENTRY_SIZE)
         {
-            throw new InvalidFormatException($"Tar entry size {size} cannot be negative.");
+            throw new InvalidFormatException($"Invalid TAR entry size: {size}.");
         }
 
         return size;
     }
 
+    private static long ReadBase256Size(byte[] buffer)
+    {
+        // Only the last eight bytes fit in a long. Do not silently truncate the
+        // remaining value bits, including the sign bits of GNU negative sizes.
+        var leadingBits = (buffer[124] & 0x7F) | buffer[125] | buffer[126] | buffer[127];
+        if (leadingBits != 0)
+        {
+            throw new InvalidFormatException("TAR entry size does not fit in 64 bits.");
+        }
+
+        return BinaryPrimitives.ReadInt64BigEndian(buffer.AsSpan(128, 8));
+    }
+
     private static byte[] ReadBlock(BinaryReader reader)
     {
         var buffer = reader.ReadBytes(BLOCK_SIZE);
diff --git a/src/SharpCompress/Common/Tar/TarHeaderFactory.cs b/src/SharpCompress/Common/Tar/TarHeaderFactory.cs
index 1628b95a..e72b23c1 100644
--- a/src/SharpCompress/Common/Tar/TarHeaderFactory.cs
+++ b/src/SharpCompress/Common/Tar/TarHeaderFactory.cs
@@ -59,11 +59,12 @@ internal static partial class TarHeaderFactory
 
     private static long PadTo512(long size)
     {
-        var zeros = (int)(size % 512);
-        if (zeros == 0)
+        // PAX size overrides bypass ReadSize, so validate the effective size too.
+        if (size is < 0 or > TarHeader.MAX_ENTRY_SIZE)
         {
-            return size;
+            throw new InvalidFormatException($"Invalid TAR entry size: {size}.");
         }
-        return checked(512 - zeros + size);
+
+        return (size + 511) & ~511L;
     }
 }
diff --git a/tests/SharpCompress.Test/Tar/TarSizeValidationTests.cs b/tests/SharpCompress.Test/Tar/TarSizeValidationTests.cs
index 93510361..a8e8c6f6 100644
--- a/tests/SharpCompress.Test/Tar/TarSizeValidationTests.cs
+++ b/tests/SharpCompress.Test/Tar/TarSizeValidationTests.cs
@@ -21,11 +21,13 @@ public class TarSizeValidationTests
     [InlineData(-512L, false)]
     [InlineData(-1024L, false)]
     [InlineData(long.MinValue, false)]
+    [InlineData(long.MaxValue, false)]
+    [InlineData(TarHeader.MAX_ENTRY_SIZE + 1, false)]
     [InlineData(-1L, true)]
     [InlineData(-512L, true)]
     [InlineData(-1024L, true)]
     [InlineData(long.MinValue, true)]
-    public async Task Tar_Header_Rejects_Negative_Size(long size, bool negativeMarker)
+    public async Task Tar_Header_Rejects_Invalid_Size(long size, bool negativeMarker)
     {
         using var stream = new MemoryStream(CreateHeader(size, negativeMarker));
         using var reader = new BinaryReader(stream, Encoding.UTF8, leaveOpen: true);
@@ -41,6 +43,69 @@ public class TarSizeValidationTests
         );
     }
 
+    [Theory]
+    [InlineData(TarHeader.MAX_ENTRY_SIZE - 1)]
+    [InlineData(TarHeader.MAX_ENTRY_SIZE)]
+    public async Task Tar_Header_Accepts_Maximum_Paddable_Size(long size)
+    {
+        using var stream = new MemoryStream(CreateHeader(size));
+        using var reader = new BinaryReader(stream, Encoding.UTF8, leaveOpen: true);
+        var header = new TarHeader(new ArchiveEncoding());
+
+        Assert.True(header.Read(reader));
+        Assert.Equal(size, header.Size);
+
+        stream.Position = 0;
+        using var asyncStream = new AsyncOnlyStream(stream, disposeStream: false);
+        using var asyncReader = new AsyncBinaryReader(asyncStream, leaveOpen: true);
+        Assert.True(await header.ReadAsync(asyncReader, cancellationToken: CancellationToken.None));
+        Assert.Equal(size, header.Size);
+    }
+
+    [Theory]
+    [InlineData(124, false)]
+    [InlineData(125, false)]
+    [InlineData(126, false)]
+    [InlineData(127, false)]
+    [InlineData(124, true)]
+    [InlineData(125, true)]
+    [InlineData(126, true)]
+    [InlineData(127, true)]
+    public async Task Tar_ArchiveFactory_Rejects_Binary_Size_With_Ignored_High_Bits(
+        int offset,
+        bool useAsync
+    )
+    {
+        var header = CreateHeader(0);
+        // Keep the binary marker, but encode a size larger than a long can hold.
+        header[offset] |= 0x01;
+        WriteChecksum(header);
+        using var stream = new MemoryStream(header);
+        if (useAsync)
+        {
+            using var asyncStream = new AsyncOnlyStream(stream, disposeStream: false);
+            await Assert.ThrowsAsync<ArchiveOperationException>(async () =>
+            {
+                await using var archive = await ArchiveFactory.OpenAsyncArchive(
+                    asyncStream,
+                    ReaderOptions.ForExternalStream,
+                    CancellationToken.None
+                );
+            });
+        }
+        else
+        {
+            Assert.Throws<ArchiveOperationException>(() =>
+            {
+                using var archive = ArchiveFactory.OpenArchive(
+                    stream,
+                    ReaderOptions.ForExternalStream
+                );
+            });
+        }
+        Assert.True(stream.CanRead);
+    }
+
     [Theory]
     [InlineData(false)]
     [InlineData(true)]
@@ -262,10 +327,15 @@ public class TarSizeValidationTests
         BinaryPrimitives.WriteInt64BigEndian(header.AsSpan(128, 8), size);
         header[156] = (byte)'0';
         Encoding.ASCII.GetBytes("ustar\0" + "00").CopyTo(header, 257);
+        WriteChecksum(header);
+        return header;
+    }
+
+    private static void WriteChecksum(byte[] header)
+    {
         var checksum = TarHeader.RecalculateChecksum(header);
         Encoding
             .ASCII.GetBytes(Convert.ToString(checksum, 8).PadLeft(6, '0') + "\0 ")
             .CopyTo(header, 148);
-        return header;
     }
 }