CAPEC-45 - Buffer Overflow via Symbolic Links

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.

Severity

Likelihood

Confidentiality

Integrity

Availability

  • Attack Methods 2
  • Injection
  • Modification of Resources
  • Purposes 2
  • Penetration
  • Exploitation
  • Sec Principles 1
  • Reluctance to trust
  • Scopes 4
  • DoS: crash / exit / restart
  • Availability
  • Execute unauthorized code or commands
  • Availability
  • Integrity
  • Confidentiality
  • Read memory
  • Confidentiality
  • Modify memory
  • Integrity

Low level: 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 level: 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.

The attacker can create symbolic link on the target host.

The target host does not perform correct boundary checking while consuming data from a resources.

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 override.

Step 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..

Step 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 attackers' symbolic link..

Step 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..


Pay attention to the fact that the resource 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.