[CVE-2015-7577] Nested attributes rejection proc bypass in Active Record.

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Aaron Patterson

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Jan 25, 2016, 2:33:20 PM1/25/16
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Nested attributes rejection proc bypass in Active Record.

There is a vulnerability in how the nested attributes feature in Active Record
handles updates in combination with destroy flags when destroying records is
disabled. This vulnerability has been assigned the CVE identifier CVE-2015-7577.

Versions Affected: 3.1.0 and newer
Not affected: 3.0.x and older
Fixed Versions: 5.0.0.beta1.1, 4.2.5.1, 4.1.14.1, 3.2.22.1

Impact
------
When using the nested attributes feature in Active Record you can prevent the
destruction of associated records by passing the `allow_destroy: false` option
to the `accepts_nested_attributes_for` method. However due to a change in the
commit [a9b4b5d][1] the `_destroy` flag prevents the `:reject_if` proc from
being called because it assumes that the record will be destroyed anyway.

However this isn't true if `:allow_destroy` is false so this leads to changes
that would have been rejected being applied to the record. Attackers could use
this do things like set attributes to invalid values and to clear all of the
attributes amongst other things. The severity will be dependent on how the
application has used this feature.

All users running an affected release should either upgrade or use one of
the workarounds immediately.

Releases
--------
The FIXED releases are available at the normal locations.

Workarounds
-----------
If you can't upgrade, please use the following monkey patch in an initializer
that is loaded before your application:

```
$ cat config/initializers/nested_attributes_bypass_fix.rb
module ActiveRecord
module NestedAttributes
private

def reject_new_record?(association_name, attributes)
will_be_destroyed?(association_name, attributes) || call_reject_if(association_name, attributes)
end

def call_reject_if(association_name, attributes)
return false if will_be_destroyed?(association_name, attributes)

case callback = self.nested_attributes_options[association_name][:reject_if]
when Symbol
method(callback).arity == 0 ? send(callback) : send(callback, attributes)
when Proc
callback.call(attributes)
end
end

def will_be_destroyed?(association_name, attributes)
allow_destroy?(association_name) && has_destroy_flag?(attributes)
end

def allow_destroy?(association_name)
self.nested_attributes_options[association_name][:allow_destroy]
end
end
end
```

Patches
-------
To aid users who aren't able to upgrade immediately we have provided patches for
the two supported release series. They are in git-am format and consist of a
single changeset.

* 3-2-nested-attributes-reject-if-bypass.patch - Patch for 3.2 series
* 4-1-nested-attributes-reject-if-bypass.patch - Patch for 4.1 series
* 4-2-nested-attributes-reject-if-bypass.patch - Patch for 4.2 series
* 5-0-nested-attributes-reject-if-bypass.patch - Patch for 5.0 series

Please note that only the 4.1.x and 4.2.x series are supported at present. Users
of earlier unsupported releases are advised to upgrade as soon as possible as we
cannot guarantee the continued availability of security fixes for unsupported
releases.

Credits
-------
Thank you to Justin Coyne for reporting the problem and working with us to fix it.

[1]: https://github.com/rails/rails/commit/a9b4b5da7c216e4464eeb9dbd0a39ea258d64325

--
Aaron Patterson
http://tenderlovemaking.com/
3-2-nested-attributes-reject-if-bypass.patch
4-1-nested-attributes-reject-if-bypass.patch
4-2-nested-attributes-reject-if-bypass.patch
5-0-nested-attributes-reject-if-bypass.patch
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