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Re: FileUtils.normalize/isLeadingPath have bitten me


I don't have enough background on the expectations of the normalize method, so can't really say how it should behave. Plus, it gets used in relatively larger number of places as compared to isLeadingPath, so not too sure how it might impact other places if we do change its existing semantics.

However, looking at the FileUtils#isLeadingPath(...) implementation, I wonder why it even uses normalize. Given that the goal of that API (as stated in the javadoc) is to figure out if one path leads the other, to me that translates to being a check to see whether the "leading" param to that method is a parent of the "path". I suppose that can be implemented by using the java.io.File#getCanonicalFile() call on each of the params and then doing a iterative getParent() call on the canonical File returned for the "path" and seeing if it ends up leading to the canonical File returned for "leading". The call to java.io.File#getCanonicalFile() should take care of the ".", ".." and even symbolic link reference resolutions (which I guess is a good thing?). Do you think this has merits? Or is the expectation of the isLeadingPath API solely based on the names of that passed files rather than their actual resolved location on the filesystem? I haven't yet tried it out myself to have a more clearer understanding of how it will end up behaving in the context that we use this API.

-Jaikiran


On 28/06/18 7:17 PM, Stefan Bodewig wrote:
Hi all

while looking into https://bz.apache.org/bugzilla/show_bug.cgi?id=62502
I realized that I had a false expectation of what normalize would do in
a certain edge case.

If you feed into it a path with more "../" segments than can be
travelled up, like

FileUtils.normalize("/tmp/dest/../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../Temp/evil.txt")

it will realize it would go outside of the file system root and returns
a new File instance with the original path. I'm not sure what I had
expected (an exception maybe) but this is now what I had assumed, my
fault.

Then isLeadingPath calls normalize for both its arguments and ends up
seeing "/tmp/dest/" is a prefix of
"/tmp/dest/../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../../Temp/evil.txt"
and unzip allows the file to escape.

It seems to depend on the OS what it makes of the path, on Linux I
receive an exception but apparently Windows just swallows the excess ../
segments.

I'm not 100% sure how to fix this properly.

Is normalize doing the right thing? If so, we need to fix isLeadingPath
for example by simply always returning false if either normalized path
contains "../" segments (because that means the path cannot exist on
disk at all).

Or should the behavior of normalize change? This unit test snippet

         assertEquals("will not go outside FS root (but will not throw an exception either)",
                 new File(localize("/1/../../b")),
                 FILE_UTILS.normalize(localize("/1/../../b")));

makes me think we better leave it as is as it seems to be by design (and
I just have forgotten about it).

In either case, once we agree on the fix I propose to cut new releases
immediately.

Stefan

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