gajim insecure file creation when using latex

[Nico Golde <oss-security+ml () ngolde de>]

Hi,
Gajim seems to support latex in instant messages. This is implemented by
dumping the content to a .tex template on disk and converting the result to an 
image. To prevent security problems, it is at least checking the input for 
dangerous latex commands such as \input (as far as I can see nothing is 
missing from this list).

However, it fails to create this temporary file in a secure manner:
From src/common/latex.py:
60 def get_tmpfile_name():
61         random.seed()
62         int_ = random.randint(0, 100)
63         return os.path.join(gettempdir(), 'gajimtex_' + int_.__str__())
...
113 def latex_to_image(str_):
114         result = None
115         exitcode = 0
116 
117         try:
118                 bg_str, fg_str = gajim.interface.get_bg_fg_colors()
119         except:
120                 # interface may not be available when we test latext at startup
121                 bg_str, fg_str = 'rgb 1.0 1.0 1.0', 'rgb 0.0 0.0 0.0'
122 
123         # filter latex code with bad commands
124         if check_blacklist(str_):
125                 # we triggered the blacklist, immediately return None
126                 return None
127 
128         tmpfile = get_tmpfile_name()
130         # build latex string
131         write_latex(os.path.join(tmpfile + '.tex'), str_)
and finally:
65 def write_latex(filename, str_):
66         texstr = '\\documentclass[12pt]{article}\\usepackage[dvips]{graphicx}'
67         texstr += '\\usepackage{amsmath}\\usepackage{amssymb}'
68         texstr += '\\pagestyle{empty}'
69         texstr += '\\begin{document}\\begin{large}\\begin{gather*}'
70         texstr += str_
71         texstr += '\\end{gather*}\\end{large}\\end{document}'
72 
73         file_ = open(filename, "w+")
74         file_.write(texstr)
75         file_.flush()
76         file_.close()

I think this is of pretty minor severity even though it still allows a local attacker
to overwrite files the victim has write access to with latex content by using symlinks
and latex IMs are used.

Cheers
Nico
-- 
Nico Golde - http://www.ngolde.de - nion () jabber ccc de - GPG: 0xA0A0AAAA
For security reasons, all text in this mail is double-rot13 encrypted.

Attachment: _bin
Description: