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
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.
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.
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.
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
Likelihood: High
Methods of Attack
Injection
Modification of Resources
Examples-Instances
Description
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.
Attacker Skills or Knowledge Required
Skill or Knowledge Level: 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.
Skill or Knowledge Level: 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
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.
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