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Date: Tue, 9 Jan 2018 09:23:02 +0100
From: Imre Rad <radimre83@...il.com>
To: bugtraq@...urityfocus.com
Subject: CVE-2017-17485: one more way of rce in jackson-databind when
 defaultTyping+objects are used

Jackson-databind is a popular library in Java for JSON
marshalling/unmarshalling.

It has a feature called default-typing: when the target class has some
polymorph fields inside (such as interfaces, abstract classes or the
Object base class), the library can include type info into the JSON
structure and use that info at unmarshalling. This can be dangerous
when the input is controlled by an attacker and the target class
contains a field of type Object or something general (like
Comparable).

After the original discoveries (CVE-2017-7525) had been reported, the
author patched this attack surface with a blacklist, which was
incomplete (as by nature of blacklists).

I created a proof-of-concept project as a follow-up to demonstrate one
more way of exploitation: by abusing Spring classes via Jackson, this
could lead to remote code execution.

MITRE assigned CVE-2017-17485 to this vulnerability.

The project is available on Github:

https://github.com/irsl/jackson-rce-via-spel/


Affected versions:
The following ones (inclusive) and older: 2.9.3, 2.7.9.1, 2.8.10

Mitigation

The fixed versions 2.7.9.2, 2.8.11 and 2.9.3.1 (this latter is to be
released at time of writing these lines) expanded the blacklist once
again so that Spring application contexts cannot be instantiated
anymore.

The new major version (3.x) of Jackson-databind will address this
topic via a new API layer that provides a way to achieve
whitelisting-based serialization for these polymorph classes.



More info:

https://medium.com/@...towncoder/on-jackson-cves-dont-panic-here-is-what-you-need-to-know-54cd0d6e8062

https://adamcaudill.com/2017/10/04/exploiting-jackson-rce-cve-2017-7525/

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