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Re: A problem with "convert" - security related

Cross-posted to 2 groups.

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Às 22:03 de 10/11/18, Paul escreveu:
> Paulo da Silva wrote:
>> Às 08:34 de 10/11/18, stepore escreveu:
>>> On 11/09/2018 06:35 PM, Paulo da Silva wrote:
>>>
>>>
...
>> My problem is mainly to understand what are the risks of such utility
>> (convert) for having those restrictions in the policy file.
>> In particular, why is (or may be) it "dangerous" allowing write to pdf
>> files!
...

> 
> The reference to Ghostscript, comes from the ability to modify
> files in the file system using PostScript.
> 
> The fix many years ago, was to add a parameter to the command
> line for a gs run, which would prevent that. The scripts that come
> with the GhostScript package, have been using that parameter
> for more than ten years.
> 
> There should be no danger creating a file, as when Imagemagick
> does it. If Imagemagick calls an already-installed gs, one with
> compromised files in the installation, then just about anything
> could happen. (This is true of doing anything on a computer.
> Maybe my copy of "listdir" has been modified by a bad actor...)
> 
> When you run "convert", you're not running it elevated,
> so from a process point of view, the risk should be smaller.
> It should not be able to modify anything in the root account
> directory.
> 
> *******
> 
> https://www.mankier.com/1/gs
> 
>    Safer Mode
> 
>    The -dSAFER option disables the "deletefile" and "renamefile"
>    operators and prohibits opening piped commands ("%pipe%cmd") ...
> 
> These would be issues with later usage of .ps or .pdf files,
> and how any viewer or printing process handles documents.
> 
> In general, since PDF can also contain Javascript, there is
> more than one attack surface present. In Acrobat (reader), there is
> a tick box to turn off support for Javascript when viewing PDFs.
> 
> The PDF that "convert" makes, should be a pretty simple construct,
> as all it needs to do is make an image stream from the image file,
> and place page information around the top and bottom of the
> page definition. There should be nothing about the file itself
> which is dangerous.
> 
If I correctly read your post I think that those issues only occur
during reads, not creating/writing new pdf's from images.

Thanks

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A problem with "convert" - security related Paulo da Silva <p_s_d_a_s_i_l_v_a_ns@netcabo.pt> - 2018-11-10 02:35 +0000
  Re: A problem with "convert" - security related Paul <nospam@needed.invalid> - 2018-11-10 00:15 -0500
  Re: A problem with "convert" - security related stepore <linshine@here.now> - 2018-11-10 00:34 -0800
    Re: A problem with "convert" - security related Paul <nospam@needed.invalid> - 2018-11-10 07:32 -0500
    Re: A problem with "convert" - security related Paulo da Silva <p_s_d_a_s_i_l_v_a_ns@netcabo.pt> - 2018-11-10 17:19 +0000
      Re: A problem with "convert" - security related Paul <nospam@needed.invalid> - 2018-11-10 17:03 -0500
        Re: A problem with "convert" - security related Paulo da Silva <p_s_d_a_s_i_l_v_a_ns@netcabo.pt> - 2018-11-11 02:45 +0000
          Re: A problem with "convert" - security related Paul <nospam@needed.invalid> - 2018-11-10 23:26 -0500
            Re: A problem with "convert" - security related "Carlos E. R." <robin_listas@es.invalid> - 2018-11-12 14:17 +0100
              Re: A problem with "convert" - security related Paul <nospam@needed.invalid> - 2018-11-12 09:21 -0500
                Re: A problem with "convert" - security related "Carlos E. R." <robin_listas@es.invalid> - 2018-11-12 17:09 +0100

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