Adobe Acrobat/Acrobat Reader are programs for creating and/or viewing
documents in Adobe Portable Document Format (PDF). More information is
available at http://www.adobe.com/products/acrobat/.
Exploitation of a buffer overflow vulnerability in the ActiveX component
packaged with Adobe Systems Inc.'s Acrobat/Acrobat Reader allows remote
attackers to execute arbitrary code.
The problem specifically exists upon retrieving a link of the following
GET /any_existing_dir/any_existing_pdf.pdf%00[long string] HTTP/1.1
Where [long string] is a malicious crafted long string containing
acceptable URI characters. The request must be made to a web server that
truncates the request at the null byte (%00), otherwise an invalid file
name is specified and a "file not found" page will be returned. Example
web servers that truncate the requested URI include Microsoft IIS and
Netscape Enterprise. Though the requested URI is truncated for the
purposes of locating the file the long string is still passed to the
Adobe ActiveX component responsible for rendering the page. This in turn
triggers a buffer overflow within RTLHeapFree() allowing for an attacker
to overwrite an arbitrary word in memory. The responsible instructions
from RTLHeapFree() are shown here:
0x77F83AE5 MOV EAX,[EDI+8]
0x77F83AE8 MOV ECX,[EDI+C]
0x77F83AED MOV [ECX],EAX
The register EDI contains a pointer to a user-supplied string. The
attacker therefore has control over both the ECX and EAX registers used
in the shown MOV instruction.
Successful exploitation allows remote attackers to utilize the arbitrary
word overwrite to redirect the flow of control and eventually take
control of the affected system. Code execution will occur under the
context of the user that instantiated the vulnerable version of Adobe
An attacker does not need to establish a malicious web site as
exploitation can occur by adding malicious content to the end of any
embedded link and referencing any Microsoft IIS or Netscape Enterprise
web server. Clicking on a direct malicious link is also not required as
it may be embedded within an IMAGE tag, an IFRAME or an auto-loading
Successful exploitation requires that a payload be written such that
certain areas of the input are URI acceptable. This includes initial
injected instructions as well as certain overwritten addresses. This
increases the complexity of successful exploitation. While not trivial,
exploitation is definitely plausible.
iDEFENSE has confirmed the existence of this vulnerability in Adobe
Acrobat 5.0.5, specifically, pdf.ocx version 188.8.131.522. It is suspected
that all current versions of Adobe Acrobat/Acrobat Reader are affected
by this vulnerability.
Change Adobe Acrobat/Acrobat Reader settings to prevent PDF files from
automatically opening when accessed via a web browser. When prompted,
first save the file to disk before opening thereby closing the
exploitation vector described.
This can be accomplished using the following steps:
1. Open Adobe Acrobat/Acrobat Reader
2. Go to Edit --> Preferences
3. Uncheck the "Display PDF in browser" setting
4. Click OK
iDEFENSE brought this vulnerability to the attention of the vendor
according to the publicized timeline. However, the vendor appears to
have attempted to silently fix this vulnerability without coordinating
public disclosure of the issue. Moreover, the vendor does not appear to
have publicly posted details of the security fix to inform clients of
the risks posed by unpatched versions of the software.
Adobe has stated that the vulnerability was patched in Adobe Acrobat
Reader 6.0.2. However, iDEFENSE has tested proof of concept exploit code
that will cause the latest version of Adobe Acrobat Reader (6.0.2) to
crash. Adobe has not provided details on the status of a fix for Adobe
The Common Vulnerabilities and Exposures (CVE) project has assigned the
name CAN-2004-0629 to this issue. This is a candidate for inclusion in
the CVE list (http://cve.mitre.org), which standardizes names for
04/19/2004 Initial vendor notification
04/19/2004 iDEFENSE clients notified
04/19/2004 Initial vendor response
06/07/2004 Approximate release date of Adobe Acrobat Reader 6.0.2
08/13/2004 Public disclosure
Rafel Ivgi (the_insider[at]mail.com) is credited with this discovery.
Get paid for vulnerability research
Copyright © 2004 Verisign, Inc.
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