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fr3akb0y #1
Member since Sep 2017 · 17 posts
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Subject: Unicode Tags throwing an exception of type 'agsXMPP.Xml.Xpnet.InvalidTokenException'
Hello alex,

It has been found that some of the users purposefully sending presence updates/iq packets with Unicode tags to disconnect the session,

Later debugging i got to know that exception is throwing from tokenizeContent function of agsxmpp.Xml.Xpnet.Encoding.cs

Could you please help me to solve this issue?

<presence from="user@Host" to="user2@Host"> <???? xmlns='????'>????</????></presence>

I know this should be restricted from the Server itself, But i need the solution to fix the exception or disconnection without skipping the presence update


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Alex #2
Member since Feb 2003 · 4375 posts · Location: Germany
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The XPNet tokenizer (parser) does validations for allowed characters.
When you want to allow certain invalid characters then you have to relax the parser and patch it yourself to skip those validations.

Alex
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fr3akb0y #3
Member since Sep 2017 · 17 posts
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Thank you so much for the reply, Could you please give me a clue about where should i start as those codes are really confusing ?



On function TOK scanLt()

case BT_LEAD3:
                if (end - off < 3)
                    throw new PartialCharException(off);
                if (byteType3(buf, off) != BT_NMSTRT)
                    throw new InvalidTokenException(off);
                off += 3;
                break;

This is where the exception is throwing
This post was edited on 2020-07-31, 12:34 by fr3akb0y.
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Alex #4
Member since Feb 2003 · 4375 posts · Location: Germany
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Write some unit test cases, then you can debug the code and verify your expected results.

Alex
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fr3akb0y #5
Member since Sep 2017 · 17 posts
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I have commented all exception throwing codes that i got while debugging and is working normally now, will it make any other issues?
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Alex #6
Member since Feb 2003 · 4375 posts · Location: Germany
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The exceptions are there for a reason.
You cannot just remove all of them.

Alex
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fr3akb0y #7
Member since Sep 2017 · 17 posts
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Unfortunately i had no other choice, Thank you for your replies
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Alex #8
Member since Feb 2003 · 4375 posts · Location: Germany
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In reply to post #5
Quote by fr3akb0y:
I have commented all exception throwing codes that i got while debugging and is working normally now, will it make any other issues?

yes this will cause issues. There are exceptions which are required for the code to work correctly with streaming XML.

Alex
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Alex #9
Member since Feb 2003 · 4375 posts · Location: Germany
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Look at the 2 latest commits from here:
https://github.com/matrix-xmpp/matrix-vnext/commits/master

Alex
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fr3akb0y #10
Member since Sep 2017 · 17 posts
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This is what i exactly did, And i noticed in those commits you pasted have the exactly same Unicode string those i got from attackers
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Alex #11
Member since Feb 2003 · 4375 posts · Location: Germany
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The XML snippet was sent to me by another user of the library. This is why I put them as is in the unit tests.
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