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CAPEC-24: Filter Failure through Buffer Overflow

Filter Failure through Buffer Overflow
Definition in a New Window Definition in a New Window
Attack Pattern ID: 24
Abstraction: Detailed
Status: Draft
Completeness: Complete
Presentation Filter:
+ Summary

In this attack, the idea is to cause an active filter to fail by causing an oversized transaction. An attacker may try to feed overly long input strings to the program in an attempt to overwhelm the filter (by causing a buffer overflow) and hoping that the filter does not fail securely (i.e. the user input is let into the system unfiltered).

+ Attack Steps
  1. Survey: The attacker surveys the target application, possibly as a valid and authenticated user

    Spidering web sites for inputs that involve potential filtering

    Brute force guessing of filtered inputs

  1. Attempt injections: Try to feed overly long data to the system. This can be done manually or a dynamic tool (black box) can be used to automate this. An attacker can also use a custom script for that purpose.

    Brute force attack through black box penetration test tool.

    Fuzzing of communications protocols

    Manual testing of possible inputs with attack data.

  2. Monitor responses: Watch for any indication of failure occurring. Carefully watch to see what happened when filter failure occurred. Did the data get in?

    Boron tagging. Choose clear attack inputs that are easy to notice in output. In binary this is often 0xa5a5a5a5 (alternating 1s and 0s). Another obvious tag value is all zeroes, but it is not always obvious what goes wrong if the null values get into the data.

    Check Log files. An attacker with access to log files can look at the outcome of bad input.

  1. Abuse the system through filter failure: An attacker writes a script to consistently induce the filter failure.

    DoS through filter failure. The attacker causes the system to crash or stay down because of its failure to filter properly.

    Malicious code execution. An attacker introduces a malicious payload and executes arbitrary code on the target system.

    An attacker can use the filter failure to introduce malicious data into the system and leverage a subsequent SQL injection, Cross Site Scripting, Command Injection or similar weakness if it exists.

+ Attack Prerequisites
  • Ability to control the length of data passed to an active filter.

+ Typical Severity


+ Typical Likelihood of Exploit

Likelihood: High

+ Methods of Attack
  • Injection
+ Examples-Instances


Sending in arguments that are too long to cause the filter to fail open is one instantiation of the filter failure attack. The Taylor UUCP daemon is designed to remove hostile arguments before they can be executed. If the arguments are too long, however, the daemon fails to remove them. This leaves the door open for attack.


A filter is used by a web application to filter out characters that may allow the input to jump from the data plane to the control plane when data is used in a SQL statement (chaining this attack with the SQL injection attack). Leveraging a buffer overflow the attacker makes the filter fail insecurely and the tainted data is permitted to enter unfiltered into the system, subsequently causing a SQL injection.


Audit Truncation and Filters with Buffer Overflow. Sometimes very large transactions can be used to destroy a log file or cause partial logging failures. In this kind of attack, log processing code might be examining a transaction in real-time processing, but the oversized transaction causes a logic branch or an exception of some kind that is trapped. In other words, the transaction is still executed, but the logging or filtering mechanism still fails. This has two consequences, the first being that you can run transactions that are not logged in any way (or perhaps the log entry is completely corrupted). The second consequence is that you might slip through an active filter that otherwise would stop your attack.

+ 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

Try to feed very long data as input to the program and watch for any indication that a failure has occurred. Then see if input has been admitted into the system.

Some dynamic analysis tools may be helpful here to determine whether failure can be induced by feeding overly long inputs strings into the system.

+ Indicators-Warnings of Attack

Many exceptions are thrown by the application's filter modules in a short period of time. Check the logs. See if the probes are coming from the same IP address.

+ Obfuscation Techniques

An attacker may temporally space out their probes.

An attacker may perform probes from different IP addresses.

+ Solutions and Mitigations

Make sure that ANY failure occurring in the filtering or input validation routine is properly handled and that offending input is NOT allowed to go through. Basically make sure that the vault is closed when failure occurs.

Pre-design: Use a language or compiler that performs automatic bounds checking.

Pre-design through Build: 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.

Operational: Use OS-level preventative functionality. Not a complete solution.

Design: Use an abstraction library to abstract away risky APIs. Not a complete solution.

+ Attack Motivation-Consequences
ScopeTechnical ImpactNote
Modify memory
Execute unauthorized code or commands
Run Arbitrary Code
Bypass protection mechanism
DoS: crash / exit / restart
+ Injection Vector

Web form, URL, File, Command line, Network socket, etc.

+ Payload

All of the data that just got into the system unfiltered becomes the payload.

+ Activation Zone

Since the input enters the system effectively unfiltered, it may be dangerous if used in a SQL statement (i.e. SQL injection), as part of the command executed on the target system (i.e. command injection), as part of the reflection API (i.e. reflection injection), placed in logs (i.e. log injection), or perhaps to overflow another buffer in the system and give the attacker ability to execute arbitrary code. A subsequent buffer overflow may not even be required for that as the original one may be leveraged if the attacker gets lucky, that is the payload is activated in the filter itself, which also becomes the activation zone.

+ Payload Activation Impact

Since no input validation is effectively performed in this situation, the impact of the attack may be a complete compromise of confidentiality, integrity, accountability and availability services.

+ Relevant Security Requirements

Input validation and filtering logic should fail securely (vault doors are closed)

+ Purposes
  • Penetration
+ CIA Impact
Confidentiality Impact: MediumIntegrity Impact: HighAvailability Impact: Medium
+ Technical Context
Architectural Paradigms
+ References
[R.24.1] [REF-2] G. Hoglund and G. McGraw. "Exploiting Software: How to Break Code". Addison-Wesley. February 2004.
[R.24.2] [REF-3] "Common Weakness Enumeration (CWE)". CWE-119: Buffer Errors. Draft. The MITRE Corporation. 2007. <http://cwe.mitre.org/data/definitions/119.html>.
+ Content History
CAPEC Content TeamThe MITRE Corporation2014-06-23Internal_CAPEC_Team
CAPEC Content TeamThe MITRE Corporation2017-01-09Updated Related_Attack_PatternsInternal

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Page Last Updated or Reviewed: July 31, 2017