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Fix ArgumentOutOfRangeException in XmlReader when parsing malformed UTF-8 sequences #118081

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Fix ArgumentOutOfRangeException in XmlReader when parsing malformed UTF-8 sequences #118081

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fc8f546
Initial plan
Copilot Jul 26, 2025
a81f2ac
Fix ArgumentOutOfRangeException in XmlTextReaderImpl.ReadData
Copilot Jul 26, 2025
680d0bc
Clearer reference to the issue in the code comment
jeffhandley Jul 26, 2025
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5 changes: 5 additions & 0 deletions src/libraries/System.Private.Xml/src/System/Xml/Core/XmlTextReaderImpl.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -3418,6 +3418,11 @@ private int ReadData()
{
_ps.bytesUsed = 0;
}
else if (bytesLeft < 0)
{
// This can happen when encoding switch causes bytePos to be calculated incorrectly for malformed data
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Would it be better to detect this situation earlier to avoid introducing invalid ParsingState in the first place?

It is hard to reason about what all else can misbehave due to invalid ParsingState.

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Yeah, I agree and that was my hesitation in this comment too. It was worth a shot seeing if Copilot could tackle this, but I'm going to close the PR and leave the issue open for further investigation later.

Throw(SR.Xml_InvalidCharInThisEncoding);
}
else
{
Debug.Assert(_ps.bytes != null);
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14 changes: 14 additions & 0 deletions src/libraries/System.Private.Xml/tests/XmlReader/Tests/ReaderEncodingTests.cs
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Expand Up @@ -122,5 +122,19 @@ public static void ReadWithIncompleteBytes()
XmlException ex = Assert.Throws<XmlException>(() => reader.Read());
Assert.Contains(_invalidCharMessageStart, ex.Message);
}

[Fact]
public static void ReadWithMalformedUtf8InXmlDeclaration()
{
// This test verifies the fix for issue dotnet/runtime#113061 where malformed UTF-8 sequences
// in XML declaration caused ArgumentOutOfRangeException instead of XmlException
string data = "<?xml version=\"1.0\xbf\"?>";
byte[] bytes = System.Text.Encoding.UTF8.GetBytes(data);
var reader = XmlReader.Create(new MemoryStream(bytes));

// Should throw XmlException, not ArgumentOutOfRangeException
XmlException ex = Assert.Throws<XmlException>(() => reader.Read());
Assert.Contains(_invalidCharInThisEncoding, ex.Message);
}
}
}
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