[pmg-devel] [PATCH pmg-api v2 2/2] ruledb: content-type: add flag for matching only based on magic/content

[Friedrich Weber]

On 18/02/2025 14:54, Stoiko Ivanov wrote:
> our current content-type matching is sensibly quite cautious in
> matching if any available information indicates a potential match:
> * mime-type detection based on file contents
> * mime-type detection based on file suffix
> * content-type header
> 
> Sometimes this can lead to surprises (e.g. when a MUA sets the
> filetype of a pdf to application/octet-stream (the default type if no
> information is available), or a filter for zip-files matching
> docx-files.
> 
> This change gives users the option to restrict matching only on the
> content as detected by xdg_mime_get_mime_type_for_data.
> 
> This is a fix for the intial request in #2691 and addresses the
> suggestion from Friedrich from:
> https://bugzilla.proxmox.com/show_bug.cgi?id=5618#c2


Thanks for tackling this! I think having a flag like only-content makes
sense.

I tested this a bit and there seems to be one issue, steps to reproduce:

- add a What object with a Content Type Filter for application/pdf,
enable the new "Ignore header information" flag

- create a rule that blocks incoming mails matching this What object

- send an email with a random 1K blob as attachment that sets
Content-Type: application/pdf and some non-descriptive filename for the
attachment:

swaks --from [...] --to [...] --server [...] --attach-type
application/pdf --attach-name foo.bin --attach <(dd if=/dev/urandom
bs=1k count=1)

The email is blocked by the rule. But I would expect it to be accepted,
because the `xdg_mime_get_mime_type_for_data` shouldn't recognize the
random blob as PDF, and the user-provided Content-Type application/pdf
should be ignored.

I think the email is accepted because the magic ct [1] defaults to the
user-provided Content-Type and since `xdg_mime_get_mime_type_for_data`
returns application/octet-stream, we're keep it at the user-provided
Content-Type. I guess it would be nicer if the magic wouldn't default to
the user-provided Content-Type if "Ignore header information" is
enabled, but I'm not sure how easily this can be done.

[1]
https://git.proxmox.com/?p=pmg-api.git;a=blob;f=src/PMG/Utils.pm;h=0b8945f245;hb=6bbc222#l623

> 
> matches on the other items can be created with Match Field objects
> (for the content-type header) and Filename (for the match based on the
> provided filename - combinations of those should give us the complete
> flexibility.
> 
> inspired by the changes for disclaimer released with PMG 8.1:
> 51d1507 ("fix #2430: ruledb disclaimer: make separator configurable")
> 
> Signed-off-by: Stoiko Ivanov <s.ivanov at proxmox.com>
> ---
> would be grateful for suggestions that are a better fit than 'only-content'!
> 
>  src/PMG/RuleDB/ContentTypeFilter.pm | 75 ++++++++++++++++++++++++++---
>  1 file changed, 68 insertions(+), 7 deletions(-)
> 
> diff --git a/src/PMG/RuleDB/ContentTypeFilter.pm b/src/PMG/RuleDB/ContentTypeFilter.pm
> index 0199311..550a880 100644
> --- a/src/PMG/RuleDB/ContentTypeFilter.pm
> +++ b/src/PMG/RuleDB/ContentTypeFilter.pm
> @@ -26,7 +26,7 @@ sub otype_text {
>  }
>  
>  sub new {
> -    my ($type, $fvalue, $ogroup) = @_;
> +    my ($type, $fvalue, $ogroup, $only_content) = @_;
>  
>      my $class = ref($type) || $type;
>  
> @@ -36,6 +36,7 @@ sub new {
>      }
>  
>      my $self = $class->SUPER::new('content-type', $fvalue, $ogroup);
> +    $self->{only_content} = $only_content;
>  
>      return $self;
>  }
> @@ -52,9 +53,50 @@ sub load_attr {
>  	$obj->{field_value} = $nt;
>      }
>  
> +    my $sth = $ruledb->{dbh}->prepare(
> +	"SELECT * FROM Attribut WHERE Object_ID = ?");
> +
> +    $sth->execute($id);
> +
> +    $obj->{only_content} = 0;
> +
> +    while (my $ref = $sth->fetchrow_hashref()) {
> +	if ($ref->{name} eq 'only_content') {
> +	    $obj->{only_content} = $ref->{value};
> +	}
> +    }
> +
> +    $sth->finish();
> +
> +    $obj->{id} = $id;
> +
> +    $obj->{digest} = Digest::SHA::sha1_hex( $id, $value, $ogroup, $obj->{only_content});
> +
>      return $obj;
>  }
>  
> +sub save {
> +    my ($self, $ruledb) = @_;
> +
> +    if (defined($self->{id})) {
> +	#update - clean old attribut entries
> +	$ruledb->{dbh}->do(
> +	    "DELETE FROM Attribut WHERE Object_ID = ?",
> +	    undef, $self->{id});
> +    }
> +
> +    $self->{id} = $self->SUPER::save($ruledb);
> +
> +    if (defined($self->{only_content})) {
> +	$ruledb->{dbh}->do(
> +	    "INSERT INTO Attribut (Value, Name, Object_ID) VALUES (?, 'only_content', ?) ".
> +	    "ON CONFLICT(Object_ID, Name) DO UPDATE SET Value = Excluded.Value ",
> +	    undef, $self->{only_content},  $self->{id});
> +    }
> +
> +    return $self->{id};
> +}
> +
>  sub parse_entity {
>      my ($self, $entity) = @_;
>  
> @@ -78,12 +120,16 @@ sub parse_entity {
>  
>  	my $glob_ct = $entity->{PMX_glob_ct};
>  
> -	if ($header_ct && $header_ct =~ m|$self->{field_value}|) {
> -	    push @$res, $id;
> -	} elsif ($magic_ct && $magic_ct =~ m|$self->{field_value}|) {
> -	    push @$res, $id;
> -	} elsif ($glob_ct && $glob_ct =~ m|$self->{field_value}|) {
> +	my $check_only_content = ${self}->{only_content} // 1;
> +
> +	if ($magic_ct && $magic_ct =~ m|$self->{field_value}|) {
>  	    push @$res, $id;
> +	} elsif (!$check_only_content) {
> +	    if ($header_ct && $header_ct =~ m|$self->{field_value}|) {
> +		push @$res, $id;
> +	    } elsif ($glob_ct && $glob_ct =~ m|$self->{field_value}|) {
> +		push @$res, $id;
> +	    }
>  	}
>      }
>  
> @@ -112,19 +158,34 @@ sub properties {
>  	    pattern => '[0-9a-zA-Z\/\\\[\]\+\-\.\*\_]+',
>  	    maxLength => 1024,
>  	},
> +	'only-content' => {
> +	    description => "use content-type from scanning only (ignore filename and header)",
> +	    type => 'boolean',
> +	    optional => 1,
> +	    default => 0,
> +	},
>      };
>  }
>  
>  sub get {
>      my ($self) = @_;
>  
> -    return { contenttype => $self->{field_value} };
> +    return {
> +	contenttype => $self->{field_value},
> +	'only-content' => $self->{only_content},
> +    };
>  }
>  
>  sub update {
>      my ($self, $param) = @_;
>  
>      $self->{field_value} = $param->{contenttype};
> +
> +    if (defined($param->{'only-content'}) && $param->{'only-content'} == 1) {
> +	$self->{only_content} = 1;
> +    } else {
> +	delete $self->{only_content};
> +    }
>  }
>  
>  1;