rxvt-unicode "insecure" setting [Was: terminal emulators' processing of escape sequences]

[Ian Zimmerman <itz () primate net>]

This is a bit tangential to this particular thread, but clearly
security-related.

Quoting "man urxvt", section "RESOURCES":

 insecure: boolean

  Enables "insecure" mode. Rxvt-unicode offers some escape sequences
  that echo arbitrary strings like the icon name or the locale. This
  could be abused if somebody gets 8-bit-clean access to your display,
  whether through a mail client displaying mail bodies unfiltered or
  through write(1) or any other means. Therefore, these sequences are
  disabled by default. (Note that many other terminals, including xterm,
  have these sequences enabled by default, which doesn't make it safer,
  though).

  You can enable them by setting this boolean resource or specifying
  -insecure as an option. At the moment, this enables display-answer,
  locale, findfont, icon label and window title requests.

My ~/.Xresources file, trimmed.  I am sure it is getting read, because
of the cutchars, colors and geometry settings:

 Rxvt.background: seashell
 Rxvt.color10: green4
 Rxvt.color11: orange2
 Rxvt.color14: cyan4
 Rxvt.color2: green3
 Rxvt.color3: orange
 Rxvt.color6: cyan3
 Rxvt.cutchars: '"!' &()*,;<=>?@[]^{|} #$%+-./:
 Rxvt.foreground: Gray40
 Rxvt.geometry: 103x36
 Rxvt.saveLines: 200
 Rxvt.visualBell: on
 URxvt.font: x:-misc-fixed-medium-r-semicondensed--13-*-*-*-*-*-iso10646-1
 URxvt.perl-ext-common:
 URxvt.insecure: false

Finally, a chunk of my ~/.bashrc:

 # If this is an xterm set the title to something informative
 case "$TERM" in
         xterm*|rxvt*)
             PROMPT_COMMAND="echo -ne \"\E]0; $$ ${LOGNAME}@${HOSTNAME}:\${PWD} \a\""
             ;;
         *)
             ;;
 esac

And ... it works!

Why?

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