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Adobe Acrobat and PDF security: no improvements for 2 years

Adobe Acrobat and PDF security: no improvements for 2 years.
Software released in 2003 contains vulnerabilities disclosured in 2001

July 8, 2003


SUMMARY
======================================================================

In early 2001, we have discovered a serious security flaw in Adobe
Acrobat and Adobe Acrobat Reader. In July'2001, we've briefly
described it in "eBook Security: Theory and Practice" speech on DefCon
security conference. Since there was no reaction from Adobe (though
Adobe representative has attended the conference), we have reported
this vulnerability to CERT in September'2002 (after more than a year),
still not disclosing technical details to the public. Only in
March'2003, CERT Vulnerability Note (VU#549913) has been published,
and after a week, Adobe has responded officially (for the first time)
issuing the Vendor Statement (JSHA-5EZQGZ), promising to fix the
problem in new versions of Adobe Acrobat and Adobe Reader software
expected in the second quarter of 2003. When these versions became
available, we have found that though some minor improvements have been
made, the whole Adobe security model is still very vulnerable, and so
sent a follow-up to both CERT and Adobe. Both parties failed to
respond. Below is the full story.


CONTACT INFORMATION
======================================================================

 Name   : ElcomSoft Co.Ltd.
 E-mail   : info@elcomsoft.com
 Web   : http://www.elcomsoft.com
 Phone / fax  : +1 866 448-2703


HISTORY
======================================================================

Adobe Systems Inc is referred as "Vendor"
ElcomSoft Co.Ltd. is referred as "Reporter"

  07/16/2001: "eBook Security: Theory and Practice" presentation on
              DefCon 9:
              http://www-2.cs.cmu.edu/~dst/Adobe/Gallery/defcon.ppt
  06/13/2002: Report sent to vendor
              (PASSKEY:75DF62C56A7DE9F888256BCB0001DF72)
  09/10/2002: Report sent to CERT
  10/08/2002: More detailed acknowledgment sent to reporter
  10/08/2002: Initial attempt to contact vendor via web feedback
  10/18/2002: Follow up to PR contact(s); point of contact initiated
  10/21/2002: Authentication loop closed; technical details sent
  10/29/2002: Ack asked for and received; further details sent
              related to report
  11/21/2002: Reporter asks for status update
  11/26/2002: Ping from reporter
  11/26/2002: Follow up with vendor to get status of report
  11/27/2002: Ack from vendor PR contact asserting more info soon
  11/28/2002: Follow up to vendor again asking for confirmation of
              details;
              let the vendor know reporter is willing to wait if
              details and solution acknowledged
  12/02/2002: Conversation with vendor contact verifying details of
              issue; mention made of issue being resolved in next
              release
  12/04/2002: Initial date identified for potential publication of the
              report
  12/09/2002: Vendor replies that their response is undergoing legal
              review
  12/18/2002: Reporter asks for status update; notes 45-day disclosure
              period over
  12/18/2002: Ack reporter
  12/18/2002: Ping vendor for written response again
  01/05/2003: Reporter asks for status update
  01/14/2003: Ack reporter; tentatively set publication date for 01/20
  01/20/2003: Reporter ack
  01/21/2003: Private CERT Vulnerability Card published with draft
              status
  03/19/2003: CERT Vulnerability Note (VU#549913) published:
              http://www.kb.cert.org/vuls/id/549913
  03/25/2003: Vendor Statement (JSHA-5EZQGZ) published:
              http://www.kb.cert.org/vuls/id/JSHA-5EZQGZ
  07/02/2003: Updated vulnerability report by reporter to CERT
  07/04/2003: Updated vulnerability report sent by reporter to vendor


TECHNICAL DETAILS
======================================================================

Description of the vulnerability
--------------------------------

  Adobe Acrobat Reader supports plug-ins, i.e. additional modules that
  extend the functionality of Adobe Acrobat and Adobe Acrobat Reader;
  plug-ins SDK and plug-ins certification (signing) mechanism are
  provided. By design, Adobe Acrobat (and Reader) should load only
  digitally signed plug-ins, while the key (for signing) is provided
  by Adobe itself -- to developers who has signed a special agreement
  with Adobe. Besides, some plug-ins are signed by Adobe using their
  own private Key, and there is a 'certified' (so-called 'trusted')
  mode in Acrobat, when only Adobe-certified plug-ins are being
  loaded.

  However, the implementation of certification mechanism is weak, and
  it is easy to write a plug-in that will look like one certified by
  Adobe, and so will be loaded even in 'certified' mode. Such plug-in
  can execute ANY code -- i.e. perform file operations (read, write,
  execute etc), access Windows Registry etc.

  At 03/25/2003, vendor (Adobe Systems Inc) issued "Vendor Statement",
  confirming the existence of the vulnerability:
  http://www.kb.cert.org/vuls/id/JSHA-5EZQGZ

  There, vendor notes:

  "The security mechanism for loading certified plug-ins will be
  updated in an upcoming release of Adobe Acrobat and Adobe Acrobat
  Reader available in the second quarter of 2003."

  In June 2003, new versions of Adobe Reader (6.0) became available,
  but it is still vulnerable. The details are below.

  There are two runtime modes for which they are enabled to load and
  execute:

  - Non-certified mode
  - Certified mode

  Non-certified mode has not been changed: new versions of Adobe
  Acrobat and Adobe Reader still load all third party plug-ins that
  have old signatures, including "forged" ones (as described in
  VU#549913).

  Certified mode has been improved, as promised: now the digital
  signatures (enabling key) uses stronger algorithms, and so cannot be
  forged. However, the whole Acrobat security model is still weak, as
  described below.

  Acrobat/Reader could be running in "Certified" mode in two cases:

  - "Certified plug-ins only" option was turned on when Acrobat
    starts. In this case Acrobat/Reader 6 loads only plug-ins with new
    tamper-resistant certificates, so plug-in with "forged"
    certificate could not be loaded.

  - "Certified plug-ins only" option was turned off, but there is no
    uncertified plug-ins available to be loaded. In this case Adobe
    Acrobat loads all available plug-ins (including plug-ins without
    digital signature at all). Adobe Reader requires all plug-ins to
    be signed, but does not reject plug-in if it has old-style
    signature. If all loaded plug-ins are certified by Adobe by new
    (Acrobat 6+) certification mechanism, Acrobat/Reader automatically
    switches to "Certified" mode.

  Adobe Acrobat contains a special (internal) function that returns
  active "Certified" status (is all loaded plug-ins are certified or
  not). Let's call that function "CTIsCertifiedMode". Behavior of all
  Acrobat components that requires "Certified" mode is based on the
  value returned by this function.

  Therefore, if plug-in with "forged" certificate is loaded, it can
  patch the code of CTIsCertifiedMode function in memory, and so force
  Acrobat to believe that it works in "Certified" mode.

  It is not a big problem to find CTIsCertifiedMode in memory.
  Plug-ins gets access to Acrobat/Reader core functions through the
  set of tables called Host Function Tables (HFTs). One of such tables
  has the name "CoreTools". The functions referred by that table are
  not documented by Adobe, but one of the functions within CoreTools
  HFT is CTIsCertifiedMode.

  The impact of this vulnerability is described below.

The impact of the vulnerability
-------------------------------

  There are many Adobe Acrobat and Adobe Reader plug-ins that can load
  (by design) only in certified mode. One example is all documents
  protected with "Adobe DRM" security handler (so-called eBooks).
  Certified more assures that all other plug-ins, loaded with those
  ones, have been also certified by Adobe.

  However, using the vulnerability described above, the plug-in with
  forged signature can perform virtually everything, including but not
  limited to:

  - removing or modifying any restrictions (from copying text to
    Clipboard, printing etc) from the documents loaded into Adobe
    Acrobat or Adobe Reader;
  - remove any DRM (Digital Rights Management) schemes from PDF
    documents, regardless the encryption handler used -- WebBuy,
    InterTrust DocBox, Adobe DRM (EBX) etc;
  - modify or remove digital signatures used within a PDF document;
  - affect any/all other aspects of a document's confidentiality,
    integrity and authenticity.

Systems and configurations that are vulnerable
----------------------------------------------

  Software:           Adobe Acrobat 4.x
                      Adobe Acrobat 5.x
                      Adobe Acrobat 6.0
                      Adobe Acrobat Reader 4.x
                      Adobe Acrobat Reader 5.x
                      Adobe Reader 6.0

  Operating systems:  Windows 98
                      Windows ME
                      Windows 2000
                      Windows XP

Possible solutions
------------------

  Adobe Acrobat and Adobe Reader should NOT be able to load ANY
  plug-ins that have old (designed for versions 4 and 5) certificates.
  All plug-ins for version 6 should use new, improved signatures.
  Besides, Acrobat/Reader should verify the integrity of its own
  executable code in memory, refusing to run (or just to load
  plug-ins) if the code has been modified.


CONCLUSION
======================================================================

Unfortunately, Adobe does not pay much attention to vulnerability
report. The official response usually is:

"Adobe will evaluate this report, as we do any report we receive. For
security reasons, Adobe can't discuss the measures we take as a
result. Security is an ongoing effort. We are committed to
strengthening the security of our products by using sophisticated,
industry-standard levels of software encryption and working with the
software community, including 'White Hat' security experts, to
incorporate features to advance the quality of our products. However,
no software is 100 percent secure from determined hackers."

To implement reliable and secure solutions, it is not enough just to
"use sophisticated, industry-standard levels of software encryption"
- it is necessary to use them *properly*. It is well known that the
chain is as weak as it's weakest link.


0
vkatalov (1)
7/8/2003 10:56:04 AM
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