glibc iconv crash with ISO-2022-JP-3

[Tavis Ormandy <taviso () gmail com>]

Hello list, I suddenly got interested in mutt attack surface after
CVE-2021-3181, and some testing found a crash via charset conversion
glibc. It's just an abort(), I don't think there's any further impact.

I believe this would crash anything that does character conversion with
iconv. Mail clients do automatic charset conversion when they see a
Subject like:

    Subject: =?ISO-2022-JP-3?B?.....

or a MIME header like this:

    Content-Type: text/plain; charset=ISO-2022-JP-3

The impact is just that you can't open your mail client, because it
crashes as soon as it sees the subject.

Upstream bug: https://sourceware.org/bugzilla/show_bug.cgi?id=27256
Patch: https://sourceware.org/pipermail/libc-alpha/2021-January/122058.html

Thanks, Tavis.

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