Fix a Remote Code Execution (RCE) vulnerability identified in an
excellent report by Timo Mihaljov (@solita-timo-mihaljov).
You are vulnerable iff both:
1. You are using Nippy to serialize and deserialize data from an
UNTRUSTED SOURCE.
2. You have a vulnerable ("gadget") class on your classpath.
Notably Clojure <= 1.8 includes such a class [1].
Many other libraries do too, some examples at [2].
To prevent this risk, a Serialization whitelist has been added.
Any classes not *explicitly* authorized by the whitelist to use
Serialization will NOT be permitted to.
The default whitelist is EMPTY, meaning this is a BREAKING
change iff you make use of Nippy's Serialization support. In
this case, you'll need to update the whitelist for your needs.
For more info see the `*serializable-whitelist*` docstring.
[1] https://clojure.atlassian.net/browse/CLJ-2204
[2] https://github.com/frohoff/ysoserial
Further info below provided by Timo:
------------------------------------
Deserialization vulnerabilities are exploited by constructing objects of classes
whose constructors perform some action that's useful to the attacker. A class like
this is called a gadget, and a collection of such classes that can be combined to
reach the attacker's goal is called a gadget chain.
There are three prerequisites for exploiting a deserialization vulnerability:
1) The attacker must be able to control the deserialized data, for example,
by gaining write access to the data store where trusted parties serialize
data or by exploiting some other vulnerability on the other end of a
communications channel.
2) The deserializer must construct objects of classes specified in the
serialized data. In other words, the attacker must have full control over
which classes get instantiated.
3) The classpath must contain gadgets that can be combined into a gadget chain.
The vulnerable code is in Nippy's function `read-serializable`, which calls the
`readObject` method of `ObjectInputStream`.
I have only tested the PoC with the latest stable version, 2.14.0, but looking at
Nippy's Git history, I believe all versions starting with the following commit
are vulnerable:
commit 9448d2b3ce
[Thu Oct 24 13:47:25 2013 +0700]
For a user to be affected, they must:
1) use Nippy to serialize untrusted input, and
2) have a gadget chain on their classpath.
I suspect (but haven't verified) that using Nippy's encryption feature prevents
exploitation in some cases, but if it's used to encrypt the communications between
two systems, one compromised endpoint could send encrypted but
attacker-controlled data to the other.
Ysoserial [4] contains a list of some Java libraries with known gadget chains.
If any of those libraries can be found on the user's classpath, they are known
to be vulnerable. (Ysoserial's list is not exhaustive, so even if a user doesn't
have these particular libraries on their classpath, they may still have some
other gadget chains loaded.)
Unfortunately Clojure versions before 1.9 contained a gadget chain in the
standard library [5][6], so all Nippy users running Clojure 1.8 or earlier
are vulnerable. (Note that users of later Clojure versions may or may not
be vulnerable, depending on whether they have gadget chains from other
libraries on their classpath.)
[4] https://github.com/frohoff/ysoserial
[5] https://groups.google.com/forum/#!msg/clojure/WaL3hHzsevI/7zHU-L7LBQAJ
[6] https://clojure.atlassian.net/browse/CLJ-2204
Also toyed with:
- Possibility single var derefs at `freeze`/`thaw` call.
Abandoned since big change, and slower with opts destructuring.
- Possibility of consolidating all config into a single var.
Abandoned since breaking, and slower with opts destructuring.
Why?
- AES-GCM is faster and can be more secure, Ref. https://goo.gl/Dsc9mL, etc.
- AES-GCM is an authenticated[1] encryption mechanism, providing
automatic integrity checks. This is relevant to [#101].
What's the issue with #101?
- We compress then encrypt on freeze ; Reverse would make compression useless
- So we decrypt then decompress on thaw
Attempting CBC decryption with the wrong password will often but not
*always* throw. Meaning it's possible for decompression could be
attempted with a junk ba. And this can cause some decompressors to
fail in a destructive way, including large allocations (DDoS) or even
taking down the JVM in extreme cases.
Possible solutions?
- We could add our own HMAC, etc.
- And/or we could use something like AES-GCM which offers built-in
integrity and will throw an AEADBadTagException on failure.
There may indeed be reasons [2,3,4] to consider adding a custom HMAC -
and that's still on the cards for later.
But in the meantime, the overall balance of pros/cons seems to lean
in the direction of choosing AES-GCM as a reasonable default.
Note that the change in this commit is done in a backward-compatible
way using Nippy's versioned header: new payloads will be written using
AES-GCM by default. But old payloads already written using AES-CBC will
continue to be read using that scheme.
References
[1] https://en.wikipedia.org/wiki/Authenticated_encryption
[2] https://www.daemonology.net/blog/2009-06-24-encrypt-then-mac.html
[3] https://blog.cryptographyengineering.com/2011/12/04/matt-green-smackdown-watch-are-aead/
[4] HMAC vs AEAD integrity, https://crypto.stackexchange.com/q/24379
[5] AES-GCM vs HMAC-SHA256 integrity, https://crypto.stackexchange.com/q/30627
Motivation for changing this default:
v1 compatibility requires that in the event of a thaw failure, a fallback
attempt is made using v1 options. This must include an attempt at Snappy
decompression.
But the version of Snappy we're using has a major bug that can segfault +
crash the JVM when attempted against non-Snappy data:
https://github.com/dain/snappy/issues/20
I'd switch to an alternative Snappy implementation, but the only other
implementation I'm aware of uses JNI which can introduce troublesome
compatibility issues even for people who don't want the Snappy support.
Had hoped that the Snappy bug would eventually get fixed, but that's
looking unlikely.
Nippy v2 was released on July 22nd 2013 (2 years, 2 months ago) - so
am hoping that the majority of lib users will no longer have a need
for v1 data thaw support at this point.
For those that do, they can re-enable v1 thaw support with this flag.
If a better alternative solution ever presents (e.g. the Snappy bug
is fixed, an alternative implementation turns up, or we write a util
to reliably identify Snappy compressed data) - we can re-enable this
flag by default.
