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simplify how munin-cgi-graph is called to reduce the attack surface area

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Seems like if REQUEST_METHOD is set to GET, then we can drop two redundant ways the query string is given. munin-cgi-graph itself reads the environment variables only, but its calls to Perl's CGI::param will look at the command line if REQUEST_METHOD is not used, otherwise it uses environment variables like CGI used to work.

Since this is all behind admin auth anyway, there isn't a public vulnerability. #914 was opened without comment which lead me to notice the redundancy and worry about a vulnerability, before I realized this is admin-only anyway.

The vulnerability was created by 6d6f3ea391.

See #914.

This is the v0.19b hotfix commit.
This commit is contained in:
Joshua Tauberer 2016-08-19 12:42:43 -04:00
parent 7c9f3e0b23
commit a14b17794b
2 changed files with 8 additions and 9 deletions
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2
CHANGELOG.md
View File

@ -1,6 +1,8 @@
CHANGELOG
=========
A remote code execution vulnerability is corrected in how the munin system monitoring graphs are generated for the control panel. The vulnerability involves an administrative user visiting a carefully crafted URL.
v0.19a (August 18, 2016)
------------------------

15
management/daemon.py
View File

@ -541,10 +541,9 @@ def munin_cgi(filename):
headers based on parameters in the requesting URL. All output is written
to stdout which munin_cgi splits into response headers and binary response
data.
munin-cgi-graph reads environment variables as well as passed input to determine
munin-cgi-graph reads environment variables to determine
what it should do. It expects a path to be in the env-var PATH_INFO, and a
querystring to be in the env-var QUERY_STRING as well as passed as input to the
command.
querystring to be in the env-var QUERY_STRING.
munin-cgi-graph has several failure modes. Some write HTTP Status headers and
others return nonzero exit codes.
Situating munin_cgi between the user-agent and munin-cgi-graph enables keeping
@ -552,7 +551,7 @@ def munin_cgi(filename):
support infrastructure like spawn-fcgi.
"""
COMMAND = 'su - munin --preserve-environment --shell=/bin/bash -c /usr/lib/munin/cgi/munin-cgi-graph "%s"'
COMMAND = 'su - munin --preserve-environment --shell=/bin/bash -c /usr/lib/munin/cgi/munin-cgi-graph'
# su changes user, we use the munin user here
# --preserve-environment retains the environment, which is where Popen's `env` data is
# --shell=/bin/bash ensures the shell used is bash
@ -564,12 +563,10 @@ def munin_cgi(filename):
query_str = request.query_string.decode("utf-8", 'ignore')
env = {'PATH_INFO': '/%s/' % filename, 'QUERY_STRING': query_str}
cmd = COMMAND % query_str
env = {'PATH_INFO': '/%s/' % filename, 'REQUEST_METHOD': 'GET', 'QUERY_STRING': query_str}
code, binout = utils.shell('check_output',
cmd.split(' ', 5),
# Using a maxsplit of 5 keeps the last 2 arguments together
input=query_str.encode('UTF-8'),
COMMAND.split(" ", 5),
# Using a maxsplit of 5 keeps the last arguments together
env=env,
return_bytes=True,
trap=True)