Hello, everyone.
Last week, we received notice of a chat exploit to crash other users' Steam Clients through sending a chat message to a person on your friendslist. We did not take much of it, due to the user-that-was-reporting-it's demeanor.
As of two days ago, we received proof of this exploit through modmail. It involves sending one of two (or more we are not sure of) strings of text through chat.
I'm going to borrow words from /u/DirtDiglett, another one of our mods for this part.

Just a bug in how Valve hooks into the OS text renderer.

As long as the text appears in the chat window, Steam will crash. This is even if it's appearing from the last time it was sent inside the history.

We have spent some time testing this.
The strings do not work on MacOS (Sierra is the version we tested on), iOS, Linux, and Android. In fact, the strings appear different on all three platforms.
It will crash on Windows.
If you are a Linux user - We really need your help on this part. We could not test this due to none of us having a proper Linux computer. Please PM me. Read up. So now, the only platform is Windows that this affects. Thanks for helping, /u/BlueShellOP

A recent post is currently very popular and has garnered some controversy and bans being handled. We thank /u/cpguy5089 for being the first to bring this to the subreddit's attention. Now, after looking into it for these two days, we have decided to make an official announcement.

At the time being, **we encourage everyone to update to the Steam BETA Client just released. http://steamcommunity.com/groups/SteamClientBeta/announcements/detail/562230352012157129. This will potentially fix the texts being sent. It was just pushed out a few minutes ago.

We encourage everyone to be careful at this time. Thank you for reading. We apologize for taking so long to look into it.
- /r/Steam Moderators

Edit: If you send the strings in the comments or it is found that you are sending the strings to others, you will be permanently banned from the subreddit.
EDIT 2: THIS HAS BEEN FIXED. IT HAS BEEN PATCHED SERVER SIDE.