Adobe Reader Javascript Printf Buffer Overflow

Advisory ID Internal
CORE-2008-0526

1. Advisory Information

Title: Adobe Reader Javascript Printf Buffer Overflow
Advisory ID: CORE-2008-0526
Advisory URL: http://www.coresecurity.com/content/adobe-reader-buffer-overflow
Date published: 2008-11-04
Date of last update: 2008-11-04
Vendors contacted: Adobe
Release mode: Coordinated release

2. Vulnerability Information

Class: Buffer overflow
Remotely Exploitable: Yes (client-side)
Locally Exploitable: Yes
Bugtraq ID: 30035
CVE Name: CVE-2008-2992

3. Vulnerability Description

Adobe Reader is arguably the world's most ubiquitous electronic document sharing application. The software can be used to view, search, digitally sign, verify, print, and collaborate on Adobe PDF files, and includes scripting functionality to allow for extended customization and extensibility.

Adobe Reader suffers from a stack buffer overflow when parsing specially crafted (invalid) PDF files. The vulnerability is caused due to a boundary error when parsing format strings containing a floating point specifier in the "util.printf()" JavaScript function. Successful exploitation of the vulnerability requires that users open a maliciously crafted PDF file thereby allowing attackers to gain access to vulnerable systems and assume the privileges of a user running Acrobat Reader. Adobe Reader version 9, which was released in June 2008, is not vulnerable to the reported problem.

4. Vulnerable packages

  • Adobe Reader 8.1.2
  • Acrobat 8.1.2

5. Non-vulnerable packages

  • Adobe Reader 9
  • Acrobat 9
  • Adobe Reader 8.1.3
  • Acrobat 8.1.3

6. Vendor Information, Solutions and Workarounds

Adobe will issue a security update that addresses the vulnerable version 8.1.2 of Reader.

Alternatively, a possible workaround for this vulnerability is to disable JavaScript in Adobe Reader and Acrobat (in the software's Edit/Preferences menu). Disabling JavaScript will prevent the issue, although it will also prevent many basic Acrobat and Reader workflows from properly functioning.

7. Credits

The CVE-2008-2992 vulnerability was discovered by Damian Frizza from the CORE IMPACT Exploit Writers Team at Core Security Technologies while investigating the feasibility of exploiting a bug in Foxit Reader that had been disclosed in May 2008. The CVE-2008-1104 vulnerability was discovered in Foxit Reader by Dyon Balding from Secunia Research.

8. Technical Description / Proof of Concept Code

While investigating the feasibility of exploiting the vulnerability previously disclosed in Foxit Reader (CVE-2008-1104) [1] we found that Adobe Reader was affected by the same bug. After an initial examination of the involved implementation bug, it was believed that although present, the problem was apparently not exploitable in Adobe Reader due to the use of two structured exception handlers in the program. The primary difference between the Adobe and Foxit applications is the manner in which they perform security checks, and at first glance, it seemed as if the bug was not exploitable in Reader, since there was no way to control the program's first exception handler.

However, upon further examination of the code, we found that another overflow occurs before the call to the involved code is made in relation to the previously known vulnerability. This new problem was identified in the way vulnerable versions of Adobe Reader implement the JavaScript util.printf() function. The function first converts the argument it receives to a String, using only the first 16 digits of the argument and padding the rest with a fixed value of "0" (0x30). By passing an overly long and properly formatted command to the function it is possible to overwrite the program's memory and control its execution flow.

A specifically crafted PDF file that embeds JavaScript code to manipulate the program's memory allocation pattern and trigger the vulnerability can allow an attacker to execute arbitrary code with the privileges of a user running the Adobe Reader application.

We now present the details of the vulnerability. The original Secunia advisory concerning Foxit Reader stated: "the vulnerability is caused due to a boundary error when parsing format strings containing a floating point specifier in the util.printf() JavaScript function." While researching this bug we found that the following Javascript code triggers the bug:

 var num = 1.2 util.printf("%5000f",num)

 

These two simple Javascript lines cause the byte 0x20 to be copied 5000 times on the stack. This allows to take control of the exception handler, and also to trigger an exception when trying to write in the section that comes after the stack.

By filling somehow the heap until the address 0x20202020, for example by doing a heap spray from the Javascript, an exploit can be made for Foxit Reader.

After a Proof of Concept PDF file was crafted, we tried to open it with Adobe Reader 8.1.2. The application closed without warning, without crash, simply closed.

By disassembling the DLL library Escript.api, we reached this code:

 238AF9C5 PUSH EDI 238AF9C6 PUSH 20 238AF9C8 PUSH ESI 238AF9C9 CALL MSVCR80.memset

 

This is exactly the same bug, where EDI is the size to be copied, controlled by the attacker, and ESI is the destination, pointing to a buffer in the stack.

The program generates an exception here:

 78144AFF REP STOS DWORD PTR ES:[EDI]

 

inside the code of the memset function, when trying to write in the section that comes after the stack.

We examined the active SEH (Structured Exception Handlers), and found that in this case (unlike the Foxit case), we have two SEH:

 Address SE handler 0012EE70 EScript.238F6F95 0012F140 20202020

 

One has been completely overwritten by us, and the other not. The code of the first handler is:

 238F6F95 MOV EDX,DWORD PTR SS:[ESP+8] 238F6F99 LEA EAX,DWORD PTR DS:[EDX+C] 238F6F9C MOV ECX,DWORD PTR DS:[EDX-58] 238F6F9F XOR ECX,EAX 238F6FA1 CALL EScript.23806D28 Security Cookie Check 1 238F6FA6 MOV ECX,DWORD PTR DS:[EDX+22C] 238F6FAC XOR ECX,EAX 238F6FAE CALL EScript.23806D28 Security Cookie Check 2 238F6FB3 MOV EAX,EScript.2391B54C 238F6FB8 JMP MSVCR80.__CxxFrameHandler3

 

When the exception is generated, this handler takes the control and is charged of checking two security cookies. One of them has been overwritten, so the execution jumps directly to ExitProcess.

Until now, the difference between the two bugs is only the cookies check. In Foxit we have the following structure on the stack:

 Memset Buffer RET ... ... SEH ... ...

 

Whereas in Adobe Reader we basically have:

 SEH1 --> Security Cookie Check ... Memset Buffer SecurityCookie RET ... SEH2 --> Overwritten with 0x20202020

 

At first sight, it seems that this bug in not exploitable, since there is no way to control the first handler. But... looking at the code, we found that before the vulnerable call to memset, another overflow occurs. As a first step, the program transforms the argument received by the function util.printf() to a String here:

 238AF8D1 FSTP QWORD PTR SS:[ESP] 238AF8D4 CALL DWORD PTR DS:[MSVCR80._fcvt]

 

The function returns only the first 16 digits, and the rest is padded with "0" (0x30). This is copied to the stack here:

 238AF946 MOV EAX,DWORD PTR SS:[EBP-28] 238AF949 MOV AL,BYTE PTR DS:[EAX+EDX] 238AF94C MOV BYTE PTR DS:[ESI+EDI],AL # copy to the stack 238AF94F INC EDI 238AF950 INC EDX 238AF951 CMP EDX,ECX

 

After this copy is completed, the stack has the following disposition:

 Float To String Buffer ... ... SEH1 --> Overwritten with 0x30303030 ... ... Memset Buffer ... ... SEH2 --> Overwritten with 0x20202020 ... ...

 

If we call the vulnerable function with a long enough number, we can avoid the cookies check and jump to the address 0x30303030. By allocating memory somehow and copying our shellcode there, we can execute arbitrary code in the context of the application.

The following Python code generates a PoC Javascript code:

 fill = 276 * '8' script=""" var num = 12999999999999999999_FILL_ util.printf("%45000f",num) """ script = script.replace('_FILL_', fill) print script

 

By embedding the generated script in a PDF file, we got an Access violation when executing [30303030].

9. Report Timeline

  • 2008-05-27: Core Security Technologies notifies the vendor of the vulnerability, similar to the CVE-2008-1104 vulnerability affecting Foxit Reader.
  • 2008-05-27: Vendor acknowledges notification.
  • 2008-05-28: Core sends technical details of the vulnerability, and a Javascript PoC that triggers the bug.
  • 2008-05-28: Vendor confirms that they were aware of this vulnerability. Vendor states that it will be fixed in an update, and that the schedule for this update is still to be determined.
  • 2008-05-29: Core sends some corrections to the info provided, and asks to be updated about the progress for releasing a fixed version.
  • 2008-05-30: Vendor agrees to keep Core updated on their progress.
  • 2008-06-09: Core sends the advisory draft, and states that it plans to publish the advisory on June 23rd. Core also offers the vendor to include a vendor statement in the advisory.
  • 2008-06-09: Vendor states that the issue will be solved in an update for Adobe Reader 8.1.2, and in the next major release of Adobe Acrobat and Reader.
  • 2008-06-13: Core requests an estimated date for the release of the next major Adobe Reader version, and expresses its concerns respect to delaying the publication of the advisory, since it is straightforward for a potential attacker to rediscover the vulnerability and exploit it "in the wild", given its relation to the Foxit reader CVE-2008-1104 bug.
  • 2008-06-16: Vendor responds that it expects the Adobe Reader 8.1.2 update to occur in the July timeframe.
  • 2008-06-27: Core requests updated information concerning the release dates of Adobe Reader 9 and the 8.1.2 update.
  • 2008-06-28: Vendor responds that Adobe Reader 9 is scheduled for July 1st.
  • 2008-07-01: Vendor states that the 8.1.2 update is tentatively scheduled for an early fall release, and not in July as previously informed.
  • 2008-07-01: Adobe Reader 9 is publicly released.
  • 2008-07-01: Core communicates its intention to publish its security advisory on July 2nd, given that there is a fixed Adobe Reader version available, and that the vulnerability is closely related to a vulnerability already publicly known and exploited (the Foxit reader CVE-2008-1104 bug).
  • 2008-07-01: Vendor requests Core to hold off the publication of the advisory, because there is still no solution for Adobe Acrobat 8 customers besides paying for an upgrade to Acrobat 9.
  • 2008-07-01: Core requests additional information before revising the publication date: a list of vulnerable products and versions, a concrete date for the release of the Acrobat and Reader 8.1.2 update, and possible workarounds for this vulnerability.
  • 2008-07-02: Vendor confirms that the vulnerable products and versions are: Adobe Reader 8.1.2 and Acrobat 8.1.2.
  • 2008-07-02: Vendor confirms that disabling JavaScript will prevent the issue, although it will also prevent many basic Acrobat and Reader workflows from properly functioning. Vendor commits to providing an estimated release date for the Acrobat 8.1.2 update as soon as possible.
  • 2008-07-02: Core communicates that it will reschedule the publication of the advisory.
  • 2008-07-02: Vendor states that it is working with the team to get a more specific schedule.
  • 2008-07-08: Vendor states that it is still working on expediting the fix.
  • 2008-07-08: Core appreciates the updates, and asks for the CVE number assigned to this issue.
  • 2008-07-09: Vendor communicates the estimated release date of October 15th.
  • 2008-07-09: Vendor states that the release of an updated Acrobat version is still estimated for October 15th.
  • 2008-09-03: Core requests updated information.
  • 2008-09-03: Vendor responds that the release date is still estimated for October 15th.
  • 2008-10-03: Vendor informs that there has been a delay in the schedule for the upcoming Reader 8.1.3 release. Its current plan of record is now to target November 4th as the deployment date.
  • 2008-10-20: Core asks for confirmation of the release date.
  • 2008-10-20: Vendor responds that it is still on track to release Reader 8.1.3 on November 4th, and that it should have a Security Bulletin draft to review next week.
  • 2008-10-29: Vendor confirms the release of Reader 8.1.3 on November 4th.
  • 2008-10-30: Core sends Adobe the advisory draft, and communicates that the publication date (Nov 4th) is considered final.
  • 2008-10-30: Adobe acknowledges reception, and states that it will send its Security Bulletin draft to Core the following day (Oct 31).
  • 2008-11-04: Advisory CORE-2008-0526 is published.

10. References

[1] Foxit Reader "util.printf()" Buffer Overflow (CVE-2008-1104) - http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2008-1104

11. About CoreLabs

CoreLabs, the research center of Core Security Technologies, is charged with anticipating the future needs and requirements for information security technologies. We conduct our research in several important areas of computer security including system vulnerabilities, cyber attack planning and simulation, source code auditing, and cryptography. Our results include problem formalization, identification of vulnerabilities, novel solutions and prototypes for new technologies. CoreLabs regularly publishes security advisories, technical papers, project information and shared software tools for public use at: http://www.coresecurity.com/corelabs.

12. About Core Security Technologies

Core Security Technologies develops strategic solutions that help security-conscious organizations worldwide develop and maintain a proactive process for securing their networks. The company's flagship product, CORE IMPACT, is the most comprehensive product for performing enterprise security assurance testing. CORE IMPACT evaluates network, endpoint and end-user vulnerabilities and identifies what resources are exposed. It enables organizations to determine if current security investments are detecting and preventing attacks. Core Security Technologies augments its leading technology solution with world-class security consulting services, including penetration testing and software security auditing. Based in Boston, MA and Buenos Aires, Argentina, Core Security Technologies can be reached at 617-399-6980 or on the Web at http://www.coresecurity.com.

13. Disclaimer

The contents of this advisory are copyright (c) 2008 Core Security Technologies and (c) 2008 CoreLabs, and may be distributed freely provided that no fee is charged for this distribution and proper credit is given.

14. PGP/GPG Keys

This advisory has been signed with the GPG key of Core Security Technologies advisories team.