Skip to content

Fix Base64/Base64Url decoding with whitespace and small destinations #123258

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Closed
Copilot wants to merge 4 commits into from
Closed

Fix Base64/Base64Url decoding with whitespace and small destinations #123258

Show file tree
Hide file tree
Changes from all commits
Commits
Show all changes
4 commits
Select commit Hold shift + click to select a range
ea07134
Initial plan
Copilot Jan 16, 2026
7bc4549
Add Base64.DecodeFromUtf8 assertions to Convert tests
Copilot Jan 16, 2026
2d200c5
Fix Base64 whitespace handling in InvalidDataFallback
Copilot Jan 16, 2026
703acc3
Improve Base64 whitespace handling fallback logic
Copilot Jan 16, 2026
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
16 changes: 16 additions & 0 deletions ...raries/System.Private.CoreLib/src/System/Buffers/Text/Base64Helper/Base64DecoderHelper.cs
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -269,6 +269,7 @@ internal static unsafe OperationStatus DecodeFrom<TBase64Decoder, T>(TBase64Deco

static OperationStatus InvalidDataFallback(TBase64Decoder decoder, ReadOnlySpan<T> source, Span<byte> bytes, ref int bytesConsumed, ref int bytesWritten, bool isFinalBlock)
{
ReadOnlySpan<T> originalSource = source;
source = source.Slice(bytesConsumed);
bytes = bytes.Slice(bytesWritten);

Expand Down Expand Up @@ -297,6 +298,7 @@ static OperationStatus InvalidDataFallback(TBase64Decoder decoder, ReadOnlySpan<
}

// Skip over the starting whitespace and continue.
int whitespaceConsumed = localConsumed;
bytesConsumed += localConsumed;
source = source.Slice(localConsumed);

Expand All @@ -306,6 +308,20 @@ static OperationStatus InvalidDataFallback(TBase64Decoder decoder, ReadOnlySpan<
bytesWritten += localWritten;
if (status is not OperationStatus.InvalidData)
{
// If we got DestinationTooSmall and have remaining non-whitespace data,
// fall back to block-wise decoding which can handle small destinations better.
if (status == OperationStatus.DestinationTooSmall && !source.IsEmpty)
{
// Check if there's non-whitespace remaining
int nonWhitespaceIdx = decoder.IndexOfAnyExceptWhiteSpace(source.Slice(localConsumed));
if (nonWhitespaceIdx >= 0)
{
// Reset and use blockwise decoder from the start
bytesConsumed = 0;
bytesWritten = 0;
return decoder.DecodeWithWhiteSpaceBlockwiseWrapper(decoder, originalSource, bytes, ref bytesConsumed, ref bytesWritten, isFinalBlock);
}
}
break;
}

Expand Down
15 changes: 15 additions & 0 deletions src/libraries/System.Runtime/tests/System.Runtime.Extensions.Tests/System/Convert.cs
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -6,6 +6,8 @@
using System.Linq;
using System.Text;
using System.Collections.Generic;
using System.Buffers.Text;
using System.Buffers;

using Test.Cryptography;

Expand Down Expand Up @@ -297,6 +299,8 @@ public static void TryFromBase64String(string encoded, byte[] expected)
bool success = Convert.TryFromBase64String(encoded, actual, out int bytesWritten);
Assert.False(success);
Assert.Equal(0, bytesWritten);

Assert.Equal(OperationStatus.InvalidData, Base64.DecodeFromUtf8(Encoding.UTF8.GetBytes(encoded), actual, out _, out _));
}
else
{
Expand All @@ -307,6 +311,10 @@ public static void TryFromBase64String(string encoded, byte[] expected)
Assert.True(success);
Assert.Equal<byte>(expected, actual);
Assert.Equal(expected.Length, bytesWritten);

Assert.Equal(OperationStatus.Done, Base64.DecodeFromUtf8(Encoding.UTF8.GetBytes(encoded), actual, out int bytesConsumed, out bytesWritten));
Assert.Equal(encoded.Length, bytesConsumed);
Assert.Equal(expected.Length, bytesWritten);
}

// Buffer too short
Expand All @@ -316,6 +324,9 @@ public static void TryFromBase64String(string encoded, byte[] expected)
bool success = Convert.TryFromBase64String(encoded, actual, out int bytesWritten);
Assert.False(success);
Assert.Equal(0, bytesWritten);

Assert.Equal(OperationStatus.DestinationTooSmall, Base64.DecodeFromUtf8(Encoding.UTF8.GetBytes(encoded), actual, out _, out bytesWritten));
Assert.Equal(actual.Length, bytesWritten);
}

// Buffer larger than needed
Expand All @@ -327,6 +338,10 @@ public static void TryFromBase64String(string encoded, byte[] expected)
Assert.Equal(99, actual[expected.Length]);
Assert.Equal<byte>(expected, actual.Take(expected.Length));
Assert.Equal(expected.Length, bytesWritten);

Assert.Equal(OperationStatus.Done, Base64.DecodeFromUtf8(Encoding.UTF8.GetBytes(encoded), actual, out int bytesConsumed, out bytesWritten));
Assert.Equal(encoded.Length, bytesConsumed);
Assert.Equal(expected.Length, bytesWritten);
}
}
}
Expand Down
Loading