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From "Sebb (JIRA)" <>
Subject [jira] Resolved: (NET-345) Telnet client: not properly handling IAC bytes within subnegotiation messages
Date Thu, 03 Mar 2011 01:42:37 GMT


Sebb resolved NET-345.

    Resolution: Fixed

Thanks, applied patches with a few minor changes.
E.g. output is buffered, so simpler to double the IACs on the fly rather than using another

> Telnet client: not properly handling IAC bytes within subnegotiation messages
> -----------------------------------------------------------------------------
>                 Key: NET-345
>                 URL:
>             Project: Commons Net
>          Issue Type: Bug
>          Components: Telnet
>    Affects Versions: 2.0
>            Reporter: Archie Cobbs
>         Attachments: patch3.txt, patch4.txt
> Subnegotiation messages in telnet are sent using the sequence {{IAC SB ... IAC SE}}.
> Although it's not clearly spelled out in [RFC 854|],
any {{IAC}} ({{0xff}}) bytes inside these messages must be escaped by doubling. Other clients
do this and this is the only behavior that makes sense.
> The commons-net telnet client is failing both to escape and to unescape {{IAC}} bytes
within subnegotiation messages. Moreover, if it does receive a valid {{IAC IAC}} sequence
within a subnegotiation message, it will incorrectly jump back to "data" input mode, discarding
the message and introducing its remainder as garbage in the data stream.
> In addition, the code fails to check for an overflow of the subnegotiation buffer, which
would cause an {{ArrayIndexOutOfBounds}} exception if a malicious peer triggered this condition.
> Finally, a {{IAC SE}} sequence appearing by itself should probably be discarded, rather
than passing as a command to the handler.
> I'm attaching a patch to fix these issues.

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