[pmg-devel] [PATCH pmg-api v2] fix #3734: scrub 'url' from style tags/attributes

[Thomas Lamprecht]

On 25.11.21 15:14, Dominik Csapak wrote:
> if 'view images' for the quarantine is disabled, it is expected that
> *no* images will be loaded. but in addition to img (src/href/etc.)
> also css can load external images via the 'url' directive
> 
> since html scrubber does not parse/iterate over css, we simply remove
> the url+protocol part of those tags/attributes. this technically leaves behind
> invalid css, but the browsers should cope with that.
> (we cannot 'cleanly' remove without much more effort because of quoting)
> 
> also we have to scrub the style tags in 'dump_html' since HTML::Scrubber
> does not have a way to modify the *content* of a tag, only the
> attributes...
>

I found two issues (see inline), I got fully commited followups here, but I did
not push that yet so we can quick check tomorrow if its ok for you and that I
missed nothing else.

> Signed-off-by: Dominik Csapak <d.csapak at proxmox.com>
> ---
> changes from v1:
> * replace url with ___ and protocol:// with _ instead of removing
> * move sub out and use the reference
> * always pass $cid_hash and only use it in the function when
>   $view_images is set
> * improve comment to show what 'dump_html' does
> 
> @thomas: a note to our off-list discussion regarding url-encoding the
> protocol: you *could* do it, but the browser does not recognize it as
> a protocol and interprets it as a relative url, so we're safe on
> this regard

thx for checking!

> 
>  src/PMG/HTMLMail.pm | 31 +++++++++++++++++++++++++++----
>  1 file changed, 27 insertions(+), 4 deletions(-)
> 
> diff --git a/src/PMG/HTMLMail.pm b/src/PMG/HTMLMail.pm
> index b69a596..c5c94bf 100644
> --- a/src/PMG/HTMLMail.pm
> +++ b/src/PMG/HTMLMail.pm
> @@ -15,8 +15,26 @@ use HTML::Scrubber;
>  use PMG::Utils;
>  use PMG::MIMEUtils;
>  
> +# $value is a ref to a string scalar
> +my sub remove_urls {
> +    my ($value) = @_;

$$value can be undef here, so I added a 
return if !defined $$value;

to avoid a ugly warning like:

pmgproxy[164923]: Use of uninitialized value in substitution (s///) at /usr/share/perl5/PMG/HTMLMail.pm line 22.
pmgproxy[164923]: Use of uninitialized value in substitution (s///) at /usr/share/perl5/PMG/HTMLMail.pm line 25.

every time one loads a mail where this happens.

> -- 8< snip 8< --

> @@ -137,7 +159,7 @@ sub getscrubber {
>  	    span => 1,
>  	    src => $viewimages ? qr{^(?!(?:java)?script)}i : 0,
>  	    start => 1,
> -	    style => 1,
> +	    style => $viewimages ? 1 : \remove_urls_from_attr,

this actually does not works as expected, to get the callback functionality we need
to set it at the "rules" one, not here at the "default" one, which really just is the
boolean default for that attribute type, and setting it to an code ref-makes may call
it once but due to returning undef it just plainly disables the tag, which is way more
scrubbing than we want to achieve.