OWASP ESAPI Security Advisory: MAC Bypass in ESAPI Symmetric Encryption

From: Kevin W. Wall <kevin.w.wall@gmail.com>
To: Bugtraq <bugtraq@securityfocus.com>
Subject: OWASP ESAPI Security Advisory: MAC Bypass in ESAPI Symmetric Encryption

OWASP ESAPI for Java          Security Advisory #1          The OWASP Foundation

                   MAC Bypass in ESAPI Symmetric Encryption

Category:       Symmetric cryptography
Module:         ESAPI (Encryptor interface)
Announced:      2013-08-23 via ESAPI-Dev mailing list

Credits:        Philippe Arteau
Affects:        ESAPI 2.0GA, ESAPI 2.0.1
Corrected:      2013-09-02 in ESAPI 2.1.0 (cryptographic version 20130830)
Google Issue #: 306
CWE:            CWE-310 (Cryptographic Issues)
CVE Identifier: CVE-2013-5679
CVSS Severity   CVSS v2 Base Score:             5.6
  (version 2.0)     Impact Subscore:            7.8
                    Exploitability Subscore:    3.9
                CVSS Temporal Score:            4.4
                CVSS Environmental Score:       4.4
                    Modified Impact Subscore:   7.8
                    Overall CVSS Score:         4.4
                CVSS v2 Vector

                    NOTE: Because ESAPI is an API, the context of how it is
                          used greatly affects the Exploitability
metrics scoring
                          component. You are encouraged to calculate a
score that
                          reflects you use of ESAPI encryption. The
above CVSSv2 vector
                          reflects what appears to be the most common
use of ESAPI
                          encryption which is to encrypt Java properties via

OWASP ESAPI (The OWASP Enterprise Security API) is a free, open source, web
application security control library that makes it easier for programmers
to write lower-risk applications. The ESAPI for Java library is designed
to make it easier for programmers to retrofit security into existing
applications. ESAPI for Java also serves as a solid foundation for new

One of the sets of security controls provided by ESAPI for Java is wrappers
around cryptographic primitives to provide authenticated encryption even
when the underlying cryptographic primitives in the JCE provider that
you are using does not supply these directly.

Problem Description
If you are using ESAPI symmetric crypto with CBC mode, PKCS#7 padding (called
PKCS5Padding in Java), and an HMAC for authenticity, then it may be possible
for an adversary to bypass the authenticity check by setting the MAC length
to 0 and the MAC to null. The configuration could result in an exploitable
vulnerability depending on the context for encryption and what degree an
attacker can tamper with the serialized ciphertext. CBC mode with PKCS#7
padding and HMAC is the default configuration for ESAPI Java.

Only some ESAPI systems installations are exploitable when using the
default configuration. To be exploitable, an attacker would require the
ability to modify ciphertext either at rest or in transit. For example,
consider the use of ESAPI encryption for encrypting Java properties via
EncryptedProperties. Because properties files are generally not externally
accessible, padding oracle attacks against encrypted property values would
have to come from an insider. Additionally, the insider would require read
access to the file containing the encrypted properties.  On the other
hand, use of ESAPI Encryptor to encrypt HTTP query parameters or HTTP
cookies would be directly accessible to attackers (or at least authenticated
attackers) thus increasing the risk of a successful attack. Because of
these variations based on the context of how ESAPI encryption might be
used, the accuracy of the CVSSv2 scores may not accurately reflect your
use of ESAPI; you are encouraged to calculate your own scores based on
your specific application\u2019s usage using a CVSSv2 calculator.

Because a MAC bypass is possible, authenticity of the ciphertext cannot be
guaranteed with the default ESAPI configuration. Consequently, this exposure
may allow a successful padding oracle attack against the default ESAPI
cryptosystem configuration and hence result in an exploitable vulnerability
that could result in a loss of confidentiality or a bypass of the
authentication or authorization system.

Switch to an authenticated encryption cipher mode such as CCM or GCM,
which are supplied by the SunJCE provider in Oracle JDK 7. For JDK 6
and earlier, you will need to use an alternate JCE provider such
as Bouncy Castle.

Upgrade your ESAPI for Java library to version 2.1.0 or higher.
1. Download the relevant patch from the location below from:
2. Verify the SHA1 checksum is cfc2604798fbc11fdd0758ffcc931e324693a162.
   (Checksum listed at the bottom of the page of the above URL.)
3. Install the patch.
   a. Unzip the "esapi-2.1.0-dist.zip" file
   b. Drop the new ESAPI jar, esapi-2.1.0.jar, into your WEB-INF/lib
      directory or place it somewhere else in your CLASSPATH.

Correction Details
The details of the correction are in the ESAPI 2.1.0 release notes
and more details are in Google Issue #306
including a JUnit test based on the Philippe Arteau\u2019s original proof-of-concept
code and discussion of how to tell if the security of your system may have
been compromised as a result of this exposure.

Note that ESAPI release 2.1.0 only addresses Arteau\u2019s first scenario,
the MAC bypass, in his original disclosure. Depending on your
configuration in ESAPI.properties, it make also be possible for
an attacker to manipulate the cipher transformation (e.g.,
changing the cipher mode from CBC to OFB or padding scheme) to
adverse effect. Note that attempts to change the cipher mode
would not affect the default ESAPI configuration as the only
supported non-authenticated cipher mode in the default ESAPI
configuration is CBC. (See the ESAPI property
"Encryptor.cipher_modes.additional_allowed" in ESAPI.properties
for further details.) The future ESAPI 2.1.1 release will address
these scenarios.

Additional Precautions
Once you have downloaded and installed the fixed version of the ESAPI jar,
check if your system had been compromised.

To check if your system had been compromised from results of this
exposure, locate your ESAPI logs and look for the following
security event string (which will all be on a single line) in
your ESAPI log file:

    Cannot validate MAC as it was never computed and stored.
    Decryption result may be garbage even when decryption succeeds.

Note that this log event can originate from two different causes
so its presence is not 100% conclusive of a security breach. One
possible cause is from a client using the deprecated
Encryptor.decrypt(String) method (that was in ESAPI 2.0GA and
2.0.1 and carried over from ESAPI 1.4, now removed in ESAPI 2.1.0)
and the method was used with the wrong encryption key. The other
way that this security event could appear in the ESAPI logs is if
the MAC length was set to 0 bytes and the MAC itself was set to
null as demonstrated in the proof-of-concept code. It is advised
that you look back through your ESAPI logs at least back to
August 21, 2013 which is when Philippe Arteau first posted an
announcement of this vulnerability to the public.

If you conclude that your system has been compromised via this issue,
you should take the extra precaution of generating all new symmetric
encryption keys that you have been using, including the Encryptor.MasterKey
in your ESAPI.properties file if you have been using that. (Of course, if
you plan to do so, if you have persisted any encrypted data using the old
keys, you will first want to decrypt that data so it could be re-encrypted
with the replacement keys just as if you were doing a key change operation.)

The reason that you might want to consider taking this additonal precaution is
that this vulnerability could lead to a padding oracle attack against your
system using the ESAPI 2.0 symmetric encryption--but only if you were using
CBC mode and a MAC, which are the default configuration. As Duong and Rizzo
showed us, it is possible to use a padding oracle attack to eventually divulge
the secret encryption key.

Google Issue #: 306
Contact details: Kevin W. Wall <kevin.w.wall@gmail.com>

-kevin wall
Blog: http://off-the-wall-security.blogspot.com/
NSA: All your crypto bit are belong to us.

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