Vulnerability Note VU#689835

Adobe PDF viewers allow non-certified plug-ins to put viewers into Certified Mode

Overview

By default, Adobe PDF viewers will start up and load non-certified plug-ins installed in a local plug_ins directory. Adobe Reader plug-ins not certified by Adobe, if allowed to load, can execute arbitrary code in the process space of the running viewer. One incremental impact of such arbitrary code execution is to put the viewer into 'Certified Mode', allowing the circumvention of certain digital right management features such as printing, copying of text, etc.

I. Description

Adobe Acrobat is software designed to create and manipulate Portable Document Format (PDF) files. The Adobe Reader is a more widely-deployed free PDF viewer. Acrobat plug-ins are separate executable code modules designed to use the Acrobat SDK to work within the Acrobat framework and extend the functionality and features of Adobe's PDF viewers. These are typically dynamic libraries installed in a plug_ins directory (with the extension .api on Windows systems). Installed plug-ins run with the same execution privileges as the user running the Acrobat PDF viewer, but may cause other plug-ins to not be loaded at startup, depending on whether they are digitally signed by Adobe's certification key.

There are three primary cryptographic features in Adobe Acrobat and Adobe Reader products. These are:

Document digital signatures
Document encryption
Plug-in cryptographic verification

While the first two features do not have any reported weaknesses and are believed to cryptographically strong, the third feature is only as reliable as the security of the underlying operating system. In particular, malicious or altered plug-ins, like any code sharing memory with an application, are able to circumvent certain digital rights management features that prevent printing, copying of text, etc.

The vulnerability described in VU#549913: Adobe Acrobat PDF viewers contain flaw when loading and verifying plug-ins is still present in Adobe Acrobat 6.0 and Adobe Reader 6.0 when loading of non-certified plug-ins is allowed (the default setting). Since plug-ins can run arbitrary code, users of these products will want to make sure untrusted plug-ins are not installed or loaded. Because Version 6 certified plug-in are now verified using strong cryptography, enabling the 'Use Only Certified Plug-ins' option will ensure that only plug-ins legitimately signed and distributed by Adobe will load (see the checkbox in the 'Application Startup' area under menu item 'Edit->Preferences->Startup').

II. Impact

There are two classes of end-users affected by this report:

Consumers of Adobe Acrobat and Adobe Reader Products

Attackers that can convince users to download and install malicious programs (non-certified plug-ins) may be able to execute arbitrary code on the user's system. Executing arbitrary code may allow an attacker to display false information when reporting document information and circumvent digital rights management features that prevent printing, copying of text, etc. This can only happen via non-certified plug-ins installed in a plug_ins directory when the 'Use Only Certified Plug-ins' checkbox is turned off, the default state in Adobe Acrobat 6.0 and Adobe Reader 6.0.

Digital Content Providers

Digital content providers can not rely on plug-in cryptographic verification mechanisms to prevent attackers from gaining certain rights. These rights include printing, copying of text, and other digital-rights-management features when the attacker is able to access legitimately decrypted documents and the attacker has control of the local system. Note this can happen regardless of the plug-in architecture used. The ability for any application to protect such rights is dependent on the underlying operating system architecture, not application architecture.

III. Solution

Adobe has provided a statement regarding this issue, available here:

http://www.kb.cert.org/vuls/id/JSHA-5PAMS7

Workarounds

There are two classes of end-user response to this report:

Consumers of Adobe Acrobat and Adobe Reader Products

Be careful not to install untrusted software, including non-certified Adobe plug-ins (those not signed and deployed by Adobe), unless absolutely certain of the origin and integrity of such software. Unverified non-certified plug-ins can be removed from the plug-ins directory, and they will no longer load at startup.

If you desire additional protection, you may wish to set the certified-plug-ins-only feature. When the 'Use Only Certified Plug-ins' checkbox under 'Edit->Preferences->Startup' (under 'Application Startup') is enabled (not the default), non-certified plug-ins are prevented from loading at startup.

Finally, to prevent all plug-ins from loading when an Acrobat viewer starts, press the 'Shift' key while the application is starting.

Digital Content Providers

Do not rely on any digital rights management features enforced solely via non-certified plug-ins for critical content. When appropriate, use the stronger document signature and encryption features built into the Adobe Acrobat products.

Systems Affected

Vendor	Status	Date Updated
Adobe Systems Incorporated	Vulnerable	15-Jul-2003

References

http://www.kb.cert.org/vuls/id/549913
Public disclosure by Vladimir Katalov of ElcomSoft Co. Ltd.

Credit

Thanks to Vladimir Katalov of ElcomSoft Co. Ltd. for reporting this vulnerability to the CERT/CC. Thanks to Adobe Systems Incorporated for working with CERT/CC to help inform the Internet community about these issues.

This document was written by Cory F. Cohen and Jeffrey S. Havrilla.

Other Information

Date Public	07/08/2003
Date First Published	07/15/2003 04:54:12 PM
Date Last Updated	07/15/2003
CERT Advisory
CVE-ID(s)	CAN-2003-0142
NVD-ID(s)	CAN-2003-0142
US-CERT Technical Alerts
Metric	1.06
Document Revision	51

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