GetRight 5.2a '.grs' Skin File Buffer Overflow Explained

What this paper is

This paper details a security vulnerability in GetRight, a popular download manager at the time. Specifically, it describes how a specially crafted skin file (.grs) could cause a buffer overflow when loaded by GetRight. This overflow, occurring within the DUNZIP32.DLL library, could allow a remote attacker to execute arbitrary code on the victim's machine.

Simple technical breakdown

The vulnerability lies in how GetRight processes its skin files. These files contain configuration and graphical elements for customizing the application's appearance. The paper states that a malicious .grs file can be created to send more data than the program expects into a specific memory buffer. When this happens, the excess data spills over into adjacent memory locations, overwriting critical program instructions. By carefully crafting this overflowing data, an attacker can redirect the program's execution flow to their own malicious code, effectively taking control of the application and potentially the system. The vulnerable component is identified as DUNZIP32.DLL version 4.0.0.3.

Complete code and payload walkthrough

The provided exploit is a .grs file named c_skin.grs. Since this is a binary file and not traditional C code, we will analyze its structure and the likely intent of its contents based on the exploit description.

The exploit itself is the .grs file. The paper states that "When you copy or click this link, getright automaticly download and try to load crafted skin and will trigger buffer overflow". This implies that the .grs file contains data designed to trigger the overflow.

Exploit File: 677.grs (referred to as c_skin.grs in the paper)

Payload Analysis:

Since the exploit is a binary .grs file, we cannot perform a line-by-line code walkthrough as with source code. However, we can infer its purpose based on the exploit description and the nature of buffer overflows.

• Structure of a .grs file: A .grs file likely contains configuration data, image references, and possibly script-like instructions for GetRight's skinning engine.
• Triggering the Overflow: The exploit file is crafted to contain a large amount of data, specifically in a field or section that is copied into a fixed-size buffer within DUNZIP32.DLL. This data is likely a long string of repeating characters (e.g., 'A's) or a pattern designed to overwrite the return address on the stack.
• Shellcode/Payload: Embedded within this overflowing data is the actual shellcode. This shellcode is the malicious code that will be executed once the program's execution flow is redirected. The exact nature of the shellcode is not detailed in the paper, but typical payloads for this era might include:
  • A simple message box to confirm execution.
  • Code to download and execute another payload from a remote server.
  • Code to open a reverse shell connection back to the attacker.
  • Code to perform file operations or system reconnaissance.

Mapping list:

• .grs file content: Represents the crafted input that exploits the vulnerability.
• Excessive data within .grs: The component of the .grs file that causes the buffer overflow.
• Overwritten return address: The critical part of the stack that is modified by the overflowing data, pointing to the attacker's shellcode.
• Shellcode within the .grs file: The arbitrary code intended to be executed by the vulnerable application.

Unknowns:

• The precise structure of the .grs file format is not detailed.
• The exact size of the vulnerable buffer is not specified.
• The specific bytes of the shellcode are not provided in the paper or linked exploit.

Practical details for offensive operations teams

• Required Access Level: This exploit is designed to be triggered by a remote user. Therefore, no local access is initially required. The attacker needs a way to deliver the malicious .grs file to the target.
• Lab Preconditions:
  • A vulnerable version of GetRight (5.2a or prior) installed on a target machine.
  • The target machine must be accessible over a network where the .grs file can be delivered.
  • A method for the target to interact with the .grs file (e.g., clicking a link that downloads it, or receiving it via email and opening it).
• Tooling Assumptions:
  • A tool capable of crafting binary files, or a method to modify existing .grs files to inject malicious data and shellcode.
  • Knowledge of the target system's architecture (e.g., x86) to ensure shellcode compatibility.
  • A way to host or deliver the malicious .grs file.
• Execution Pitfalls:
  • Anti-virus/Intrusion Detection Systems (IDS): The .grs file itself might be flagged if its structure or content is suspicious. The shellcode within it is also a prime target for AV detection.
  • GetRight Updates: If the target has updated GetRight to a patched version, the exploit will fail.
  • DLL Version Mismatch: The exploit targets a specific version of DUNZIP32.DLL. If a different version is present, the overflow might not occur or might not be exploitable in the same way.
  • Shellcode Reliability: Shellcode can be fragile. It might fail to execute due to environmental factors, stack protection mechanisms (though less common in 2004), or incorrect assumptions about the target's memory layout.
  • Delivery Mechanism: The success hinges on the target actually loading the malicious .grs file. Social engineering or other delivery methods are crucial.
• Tradecraft Considerations:
  • Stealthy Delivery: Avoid obvious methods that would alert the user or security software.
  • Payload Customization: The shellcode should be tailored to the objective (e.g., a reverse shell for persistent access, or a simple command execution for a one-off task).
  • Obfuscation: If possible, obfuscate the .grs file or the shellcode to evade detection.

Where this was used and when

• Context: This vulnerability was relevant in the era of widespread internet usage and software downloads, where users frequently installed and configured applications like download managers. The exploit leverages a common file processing vulnerability.
• Approximate Years/Dates: The paper was published in December 2004. Therefore, this vulnerability was likely actively exploited or at least a known threat around 2004-2005. It's possible it existed and was exploitable for some time before public disclosure.

Defensive lessons for modern teams

• Input Validation is Paramount: Always validate the size and content of any data received from external sources, especially when it's being copied into fixed-size buffers. This is a fundamental principle that remains critical.
• Secure DLLs: Ensure that all third-party libraries and DLLs used by applications are up-to-date and patched. Vulnerabilities in shared components can affect multiple applications.
• File Format Parsing: Be extremely cautious when parsing complex file formats. Malformed files can easily lead to vulnerabilities. Implement robust parsing logic and error handling.
• Memory Safety: While modern languages and compilers offer better memory safety features, understanding buffer overflow principles is still important for legacy systems and low-level programming.
• Regular Patching: Keep all software, including applications and their dependencies, updated to the latest versions.
• Endpoint Detection and Response (EDR): Modern EDR solutions are designed to detect anomalous behavior, including attempts to overwrite return addresses or execute unexpected code, even if the initial exploit vector is obscure.

ASCII visual (if applicable)

This exploit involves a file being processed by an application, leading to a memory corruption event. An ASCII visual can illustrate the concept of a buffer overflow.

+---------------------+
| Application Memory  |
+---------------------+
|                     |
|  [Safe Data]        |
|                     |
+---------------------+ <-- Stack Pointer
| [Return Address]    |
+---------------------+ <-- Stack Frame Boundary
| [Local Variables]   |
|  +---------------+  |
|  | Vulnerable    |  |
|  | Buffer        |  |
|  +---------------+  |
|  [Other Data]       |
+---------------------+

--- After Malicious .grs is Loaded and Processed ---

+---------------------+
| Application Memory  |
+---------------------+
|                     |
|  [Safe Data]        |
|                     |
+---------------------+ <-- Stack Pointer
| [ATTACKER'S CODE]   | <-- Return Address Overwritten
+---------------------+ <-- Stack Frame Boundary
| [Local Variables]   |
|  +---------------+  |
|  | Overflowing   |  |
|  | Data (Shellcode)|  |
|  +---------------+  |
|  [Other Data]       |
+---------------------+

The diagram shows a typical stack layout. The vulnerable buffer is filled with data from the .grs file. When the data exceeds the buffer's capacity, it overflows and overwrites the return address. The attacker's code (shellcode) is placed where the return address used to be, so when the function returns, it jumps to the attacker's code instead of the legitimate return point.

Source references

• Paper: GetRight 5.2a - '.grs' Skin File Buffer Overflow
• Author: ATmaCA
• Published: 2004-12-06
• Exploit-DB URL: https://www.exploit-db.com/papers/677
• Raw Exploit URL: https://gitlab.com/exploit-database/exploitdb-bin-sploits/-/raw/main/bin-sploits/677.grs

Original Exploit-DB Content (Verbatim)

GetRight Skin File (*.grs) Buffer Overflow May Let Remote Users Run Arbitrary
Code

Application:  GetRight
             Headlight Software
             www.getright.com

Author:
ATmaCA <atmaca@prohack.net>

a remote user can create a malicious skin file (*.grs) that, when loaded by the
target user, will trigger a buffer overflow in DUNZIP32.DLL (4.0.0.3) and
potentially execute arbitrary code.

AFFECTED VERSION:
Versions verified to be vulnerable:
GetRight 5.2a and prior versions are affected.

Solutions:
There was no response.

Exploit:
https://gitlab.com/exploit-database/exploitdb-bin-sploits/-/raw/main/bin-sploits/677.grs (c_skin.grs)
When you copy or click this link, getright automaticly download and try to load
crafted skin and will trigger buffer overflow

# milw0rm.com [2004-12-06]