Merge_slashes and decoding of the path

On my site, I accept full URL-encoded URLs as part of the path, for
example:

http://www.mysite.com/search/http%3A%2F%2Fexample.com%2F

I recently moved my site to nginx and I found that it was decoding and
collapsing the slashes before passing it on to Passenger. It would pass
along the URL like this: http://www.mysite.com/search/http:/example.com/

I found the merge_slashes setting, and on setting it to off, Passenger
now
receives URLs like this:
http://www.mysite.com/search/http://example.com/ .
So the slashes are kept, but the path is still decoded. The nginx
documentation [1] says “However, for security considerations, it is
better
to avoid turning the compression off.”

What are the security considerations here? Why does nginx not allow the
encoded slashes to be passed through (like Apache does[2]), and if it
did
so, would that negate the security concerns?

[1]
http://nginx.org/en/docs/http/ngx_http_core_module.html#merge_slashes
[2] http://httpd.apache.org/docs/2.2/mod/core.html#allowencodedslashes

Hello!

On Mon, Nov 25, 2013 at 10:44:15AM -0600, Jason Barnabe wrote:

So the slashes are kept, but the path is still decoded. The nginx
documentation [1] says “However, for security considerations, it is better
to avoid turning the compression off.”

What are the security considerations here?

Example of a vulnerable configuration is given in the directive
description you’ve linked (http://nginx.org/r/merge_slashes):

: Note that compression is essential for the correct matching of
: prefix string and regular expression locations. Without it, the
: “//scripts/one.php” request would not match
:
: location /scripts/ {
: …
: }
: and might be processed as a static file. So it gets converted to
: “/scripts/one.php”.

That is, with merge_slashes switched off, restrictions like

location /protected/ {
    deny all;
}

can be easily bypassed by using a request to “//protected/file”.
It should be taken into account while writing a configuration for
a server with merge_slashes switched off.

Why does nginx not allow the
encoded slashes to be passed through (like Apache does[2]), and if it did
so, would that negate the security concerns?

While not decoding slashes is probably a better than not merging
them, it’s not really a good aproach either. This way, the

http://www.mysite.com/search/http%3A%2F%2Fexample.com%2F

URL becomes equivalent to

http://www.mysite.com/search/http%3A%252F%252Fexample.com%252F

which isn’t really consistent and may produce unexpected results.


Maxim D.
http://nginx.org/en/donation.html

On Mon, Nov 25, 2013 at 11:08 AM, Maxim D. [email protected]
wrote:

: and might be processed as a static file. So it gets converted to
a server with merge_slashes switched off.

I’m not sure that applies in my configuration, where I’m using Passenger
and have no “protected” locations, but I can see how this could lead to
problems.

http://www.mysite.com/search/http%3A%252F%252Fexample.com%252F

which isn’t really consistent and may produce unexpected results.

I don’t see how this would necessarily be the case, but I’ll defer to
your
knowledge. However, I don’t think we should let perfect be the enemy of
“better”, especially if the thing “better” would replace is a potential
security concern.

I do have this worked around in my app code (look for http:/, replace
with
http://) so it’s not a functional issue for me at the moment. It would
be a
nice feature if it could be done sensibly - should I file an issue in
trac?

Hello!

On Mon, Nov 25, 2013 at 11:30:46AM -0600, Jason Barnabe wrote:

: …
can be easily bypassed by using a request to “//protected/file”.

encoded slashes to be passed through (like Apache does[2]), and if it did

which isn’t really consistent and may produce unexpected results.

I don’t see how this would necessarily be the case, but I’ll defer to your
knowledge. However, I don’t think we should let perfect be the enemy of
“better”, especially if the thing “better” would replace is a potential
security concern.

E.g., consider a configuration using an imaginary “decode_slashes”
directive:

decode_slashes off;

location / {
    proxy_pass http://backend;
}

location /protected/ {
    auth_basic ...
    proxy_pass http://backend;
}

A request to “/protected%2Ffile” would be passed to a backend with
non-decoded slash, and if there is an nginx in default
configuration, %2F will be decoded there. That is, a
“/protected/file” will be returned, bypassing security
restrictions configured.

Given the above, I think that not decoding slashes isn’t a
solution. It may be useful in some configurations, but it isn’t
safe from security point of view either.

I do have this worked around in my app code (look for http:/, replace with
http://) so it’s not a functional issue for me at the moment. It would be a
nice feature if it could be done sensibly - should I file an issue in trac?

I don’t think that we need this feature due to inconsistencies it
introduce, see above.


Maxim D.
http://nginx.org/en/donation.html

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