| Attack Pattern ID | Pattern Abstraction: Detailed 45 |
| Typical Severity | High |
| Description | Summary This type of attack leverages the use of symbolic links to cause buffer overflows. An attacker can try to create or manipulate a symbolic link file such that its contents result in out of bounds data. When the target software processes the symbolic link file, it could potentially overflow internal buffers with insufficient bounds checking. Attack Execution Flow 1- The attacker creates or modifies a symbolic link pointing to a resources (e.g., file, directory). The content of the symbolic link file includes out-of-bounds (e.g. excessive length) data. 2- The target host consumes the data pointed to by the symbolic link file. The target host may either intentionally expect to read a symbolic link or it may be fooled by the replacement of the original resource and read the attacker's symbolic link. 3- While consuming the data, the target host does not check for buffer boundary which can lead to a buffer overflow. If the content of the data is controlled by the attacker, this is an avenue for remote code execution.
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| Attack Prerequisites | The attacker can create symbolic link on the target host. The target host does not perform correct boundary checking while consuming data from a ressources. |
| Typical Likelihood of Exploit |
High
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| Methods of Attack | - Injection
- Modification of Resources
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| Examples-Instances | Description Attack Example: Overflow with Symbolic Links in EFTP Server
The EFTP server has a buffer overflow that can be exploited if an attacker uploads a .lnk (link) file that contains more than 1,744 bytes. This is a classic example of an indirect buffer overflow. First the attacker uploads some content (the link file) and then the attacker causes the client consuming the data to be exploited. In this example, the ls command is exploited to compromise the server software.
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| Attacker Skill or Knowledge Required | Low : An attacker can simply overflow a buffer by inserting a long string into an attacker-modifiable injection vector. The result can be a DoS.
High : Exploiting a buffer overflow to inject malicious code into the stack of a software system or even the heap can require a higher skill level. |
| Probing Techniques | The attacker will look for temporary files in the world readable directories. Those temporary files are often created and read by the system. The attacker will look for Symbolic link or link target file that she can overide. |
| Indicators-Warnings of Attack | An attacker creating or modifying Symbolic links is a potential signal of attack in progress. An attacker deleting temporary files can also be a sign that the attacker is trying to replace legitimate resources with malicious ones. |
| Solutions and Mitigations | Pay attention to the fact that the ressource you read from can be a replaced by a Symbolic link. You can do a Symlink check before reading the file and decide that this is not a legitimate way of accessing the resource. Because Symlink can be modified by an attacker, make sure that the ones you read are located in protected directories. Pay attention to the resource pointed to by your symlink links (See attack pattern named "Forced Symlink race"), they can be replaced by malicious resources. Always check the size of the input data before copying to a buffer. Use a language or compiler that performs automatic bounds checking. Use an abstraction library to abstract away risky APIs. Not a complete solution. Compiler-based canary mechanisms such as StackGuard, ProPolice and the Microsoft Visual Studio /GS flag. Unless this provides automatic bounds checking, it is not a complete solution. Use OS-level preventative functionality. Not a complete solution. |
| Attack Motivation-Consequences | - Denial of Service
- Run Arbitrary Code
- Information Leakage
- Data Modification
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| Context Description | Content-Based Buffer Overflow
Data files are ubiquitous. They are used to store everything from documents to content media and critical computer settings. Every file has an inherent format that often encompasses special information such as file length, media type, and which fonts are boldface, all encoded directly in the data file. The attack vector against data files like these is simple: Mess up the data file and wait for some unsuspecting user to open it.
Some kinds of files are strikingly simple and others have complex binary structures and numerical data embedded in them. Sometimes the simple act of opening a complex file in a hex editor and tweaking a few bytes is enough to cause the (unsuspecting) program that consumes the file to crash and burn.
What’s really interesting from an attacker’s point of view is formatting data file-embedded poison pills in such a way that virus code is activated. A great example of this involved the Winamp program in which an overly long IDv3 tag would cause a buffer overflow. In the header of an MP3 file, there is a location where a normal text string can be placed. This is called the IDv3 tag, and if an overly long tag were to be supplied, Winamp would suffer a buffer overflow. This could be used by an attacker to construct malicious music files that attack the computer once they are opened in Winamp.
Access right to the symbolic link:
When a symlink is created there are no rights associated with it (this why you read them with rights lrwxrwxrwx). So
everybody can modify them even if the owner of the Symlink is root and if the user changing the Symbolic link has no right on the link target file. The relevant rights are on the linked target file. To prevent someone from modifying the symlink in the first place, the directory containing it should have limited access rights.
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| Injection Vector | The resource pointed to by the Symbolic link (e.g., file, directory, etc.) |
| Payload | The buffer overrun by the attacker. |
| Activation Zone | When the function returns control to the main program, it jumps to the return address portion of the stack frame. Unfortunately that return address may have been overwritten by the overflowed buffer and the address may contain a call to a privileged command or to a malicious code. |
| Payload Activation Impact | The most common is remote code execution. |
| Related Weaknesses | | CWE-ID | Weakness Name | Weakness Relationship Type |
|---|
| 120 | Unbounded Transfer ('Classic Buffer Overflow') | Targeted | | 285 | Missing or Inconsistent Access Control | Secondary | | 302 | Authentication Bypass by Assumed-Immutable Data | Targeted | | 118 | Range Errors | Targeted | | 119 | Failure to Constrain Operations within the Bounds of an Allocated Memory Buffer | Targeted | | 74 | Failure to Sanitize Data into a Different Plane (aka 'Injection') | Targeted | | 20 | Insufficient Input Validation | Targeted |
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| Related Security Principles | |
| Purpose | Penetration Exploitation |
| CIA Impact | | Confidentiality Impact | Integrity Impact | Availability Impact |
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| High | High | High |
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| Technical Context | | Architectural Paradigm | Framework | Platform | Language |
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| All | All | All | All |
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| References | G. Hoglund and G. McGraw. Exploiting Software: How to Break Code. Addison-Wesley, February 2004. CWE – Buffer Errors |
| Source | | Submission(s) |
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| Submitter | Organization | Date | Comment |
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| G. Hoglund and G. McGraw. Exploiting Software: How to Break Code. Addison-Wesley, February 2004. | Cigital, Inc | 2007-03-01 | |
| Modification(s) |
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| Modifier | Organization | Date | Comment |
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| Eric Dalci | Cigital, Inc | 2007-02-13 | Fleshed out content to CAPEC schema from the original descriptions in "Exploiting Software" | | Sean Barnum | Cigital, Inc | 2007-03-05 | Review and revise | | Richard Struse | VOXEM, Inc | 2007-03-26 | Review and feedback leading to changes in Name and Description | | Sean Barnum | Cigital, Inc | 2007-04-13 | Modified pattern content according to review and feedback |
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