webbrowser module + urls ending in .py = a security hole?

[Blair P. Houghton]

I'm just learning Python, so bear with.

I was messing around with the webbrowser module and decided it was
pretty cool to have the browser open a URL from within a python script,
so I wrote a short script to open a local file the same way, using the
script file as an example target:

# browser-test.py
import webbrowser
import sys
pathname = sys.argv[0]
protocol = 'file://'
url = protocol + pathname
webbrowser.open (url)

And what I got, instead of a browser window with the text of my script,
was a sequence of DOS windows popping up and disappearing.

Apparently that's because either Windows (XP SP2) or the browser
(Firefox) was interpreting the .py file extension and running Python to
execute it.

So is this a known (mis)feature, and will it happen if I chance to use
webbrowser.open () on a remote .py file?

Because if so, it's a king-hell security hole.

--Blair

[Blair P. Houghton]

Oh, uh, Python version 2.4.2, in case you're wondering.

--Blair

[Peter Hansen]

Blair P. Houghton wrote:

I was messing around with the webbrowser module and decided it was
pretty cool to have the browser open a URL from within a python script,
so I wrote a short script to open a local file the same way, using the
script file as an example target:

# browser-test.py
import webbrowser
import sys
pathname = sys.argv[0]
protocol = 'file://'
url = protocol + pathname
webbrowser.open (url)

And what I got, instead of a browser window with the text of my script,
was a sequence of DOS windows popping up and disappearing.

Apparently that's because either Windows (XP SP2) or the browser
(Firefox) was interpreting the .py file extension and running Python to
execute it.

So is this a known (mis)feature, and will it happen if I chance to use
webbrowser.open () on a remote .py file?
What happens when you load a remote .py file using the web browser
directly? With Firefox on my machine, it just displays the file, as
expected, whether loaded via webbrowser.open () or not. Make sure you're
testing with the same browser that webbrowser loads (try a regular HTML
file first if you're not sure which that is).
Because if so, it's a king-hell security hole.

It might probably worth a warning in the docs, but it's no larger a
security hole than the browser itself already has. If your browser is
configured to load files of a given type directly into a particular
application without first checking with you if you want it to do so,
you're potentially screwed already.

But is Firefox really your default browser? The webbrowser module could
be loading Internet Explorer on your machine, and we all know just how
safe *that* is...

-Peter

[Fuzzyman]

It sounds like you're running on windows *and* that webbrowser.py just
uses ``os.startfile` `.

For html files (associated with your default browser) this will *do the
right thing*. For everything else, it will *do the wrong thing*.

I could well be wrong though...

All the best,
Fuzzyman
http://www.voidspace.org.uk/python/index.shtml

[jason.lai]

Does that only happen when you open file:// urls? You already have
local access from Python, so it'd be more concerning if it happened
with Python files on remote servers.

- Jason

[Blair P. Houghton]

I'm going to try it out on a remote server later today.

I did use this script to fetch remote HTML
(url='http://www.python.org' ) before I tired the remote file, and it
opened the webpage in Firefox.

I may also try to poke around in webbrowser.py, if possible, to see if
I can see whether it's selecting the executable for the given
extension, or passing it off to the OS. I would think, since Python is
not /supposed/ to have client-side scripting powers, that even when the
script is on the client this is bad behavior.

Just don't have the bandwidth, just now.

Anyone got a good regex that will always detect an extension that might
be considered a script? Or reject all but known non-scripted
extensions? Because wrapping the webbrowser.open () call would be the
workaround, and upgrading webbrowser.py would be a solution.

--Blair

[Blair P. Houghton]

Sorry...should read:

"I did use the script to fetch remote HTML
(url='http://www.python.org' ) before I tried the local file, and it
opened the webpage in Firefox."

Too many chars, too few fingers.

--Blair

[Peter Hansen]

Blair P. Houghton wrote:

I'm going to try it out on a remote server later today.
Don't bother. I've confirmed the behaviour you saw, and that it is not
what I'd expect either. My Firefox certainly isn't configured to run
..py scripts even when invoked with the "file:" protocol, so webbrowser
is almost certainly Doing Bad Things on Windows.

The relevant code from webbrowser.py shows this, confirming FuzzyMan's
suspicions:

class WindowsDefault:
def open(self, url, new=0, autoraise=1):
os.startfile(ur l)

def open_new(self, url):
self.open(url)
I may also try to poke around in webbrowser.py, if possible, to see if
I can see whether it's selecting the executable for the given
extension, or passing it off to the OS. I would think, since Python is
not /supposed/ to have client-side scripting powers, that even when the
script is on the client this is bad behavior.
I'd agree. I suspect this ought to be reported as a security flaw,
though it would be nice to know what the fix should be before doing so.
Anyone know a more suitable approach on Windows than just passing
things off to startfile()?
Just don't have the bandwidth, just now.

Anyone got a good regex that will always detect an extension that might
be considered a script? Or reject all but known non-scripted
extensions?

Would it be sufficient in your case merely to allow only .html files to
be loaded? Or URLs without .extensions? Or even just permit only the
http: protocol?

-Peter

[Peter Hansen]

Peter Hansen wrote:

I'd agree. I suspect this ought to be reported as a security flaw,
though it would be nice to know what the fix should be before doing so.
Anyone know a more suitable approach on Windows than just passing
things off to startfile()?

It appears the correct approach might be something along the lines of
reading the registry to find what application is configured for the
"HTTP" protocol (HKCR->HTTP->shell->open->command) and run that, passing
it the URL. I think that would do what most people expect, even when
the URL actually passed specifies the "file" protocol and not "http".

Thoughts?

-Peter

[olsongt]

Http protocol give the content-type in the http headers, so the
originating server determines how your browser is going to handle it,
not the client browser. I think the problem is that the 'file://'
protocol probably does use the registry keys above since it's not
getting any HTTP headers.