Buffer Overflow via Symbolic Links
Attack Pattern ID: 45 (Detailed Attack Pattern Completeness: Complete)Typical Severity: HighStatus: Draft
+ Description


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.

+ 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


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.

+ Payload Activation Impact

The most common is remote code execution.

+ Related Weaknesses
CWE-IDWeakness NameWeakness Relationship Type
120Buffer Copy without Checking Size of Input ('Classic Buffer Overflow')Targeted
285Improper Access Control (Authorization)Secondary
302Authentication Bypass by Assumed-Immutable DataTargeted
118Improper Access of Indexable Resource ('Range Error')Targeted
119Failure to Constrain Operations within the Bounds of a Memory BufferTargeted
74Failure to Sanitize Data into a Different Plane ('Injection')Targeted
20Improper Input ValidationTargeted
680Integer Overflow to Buffer OverflowTargeted
697Insufficient ComparisonTargeted
+ Related Attack Patterns
NatureTypeIDNameDescriptionView(s) this relationship pertains toView\(s\)
ChildOfAttack PatternAttack Pattern100Overflow Buffers 
Mechanism of Attack (primary)1000
+ Related Security Principles
  • Reluctance to trust

+ Purposes
  • Penetration
  • Exploitation
+ CIA Impact
Confidentiality Impact: HighIntegrity Impact: HighAvailability Impact: High
+ Technical Context
Architectural Paradigms
+ References
G. Hoglund and G. McGraw. "Exploiting Software: How to Break Code". Addison-Wesley. February 2004.

CWE - Buffer Errors

+ Content History
G. Hoglund and G. McGraw. Exploiting Software: How to Break Code. Addison-Wesley, February 2004.Cigital, Inc2007-03-01
Eric DalciCigital, Inc2007-02-13Fleshed out content to CAPEC schema from the original descriptions in "Exploiting Software"
Sean BarnumCigital, Inc2007-03-05Review and revise
Richard StruseVOXEM, Inc2007-03-26Review and feedback leading to changes in Name and Description
Sean BarnumCigital, Inc2007-04-13Modified pattern content according to review and feedback