Date/time conversion specifiers are expanded after replacing buffer local variables in name of log files. In some cases, this can lead to an error in function strftime and a crash caused by the use of an uninitialized buffer.
Mitigation
You can unload the logger plugin, thus stopping recording of all buffers: /plugin unload logger.
Credit
The issue was discovered by Joseph Bisch.
WSA-2013-2: [IRC] Crash on send of unknown commands to IRC server.
Untrusted command for function hook_process could lead to execution of commands, because of shell expansions (so the problem is only caused by some scripts, not by WeeChat itself).
Mitigation
Remove/unload all scripts calling the API function hook_process.
WSA-2011-1: [IRC] Possible man-in-the-middle attack in TLS connection to IRC server.