COMMON ATTACK PATTERN ENUMERATION AND CLASSIFICATIONLive MITRE record

CAPEC-10

Buffer Overflow via Environment Variables

This attack pattern involves causing a buffer overflow through manipulation of environment variables. Once the adversary finds that they can modify an environment variable, they may try to overflow associated buffers. This attack leverages implicit trust often placed in environment variables.

Abstraction

Detailed

Typical severity

High

Likelihood of attack

High

Understand the attack

What is this attack pattern?

This attack pattern involves causing a buffer overflow through manipulation of environment variables. Once the adversary finds that they can modify an environment variable, they may try to overflow associated buffers. This attack leverages implicit trust often placed in environment variables.

Extended description

Although the focus of this attack is putting excessive content into an environment variable that is loaded into a buffer, environment variables can be used to assist a classic buffer overflow attack as well. In the case where the buffer used in a traditional buffer overflow attack is not large enough to store the adversary's shell code, they will store the shell code in an environment variable and attempt to return to its address, rather than back into the data they wrote to the buffer.

Execution flow

How does the attack proceed, step by step?

1

Explore

Identify target application

The adversary identifies a target application or program to perform the buffer overflow on. In this attack the adversary looks for an application that loads the content of an environment variable into a buffer.
2

Experiment

Find injection vector

The adversary identifies an injection vector to deliver the excessive content to the targeted application's buffer.

Techniques used

  • Change the values of environment variables thought to be used by the application to contain excessive data. If the program is loading the value of the environment variable into a buffer, this could cause a crash and an attack vector will be found.

Craft overflow content

The adversary crafts the content to be injected. If the intent is to simply cause the software to crash, the content need only consist of an excessive quantity of random data. If the intent is to leverage the overflow for execution of arbitrary code, the adversary crafts the payload in such a way that the overwritten return address is replaced with one of the adversary's choosing.

Techniques used

  • Create malicious shellcode that will execute when the program execution is returned to it.
  • Use a NOP-sled in the overflow content to more easily "slide" into the malicious code. This is done so that the exact return address need not be correct, only in the range of all of the NOPs
3

Exploit

Overflow the buffer

Using the injection vector, the adversary injects the crafted overflow content into the buffer.

Requirements

What does the attack require?

Prerequisites

  • The application uses environment variables.
  • An environment variable exposed to the user is vulnerable to a buffer overflow.
  • The vulnerable environment variable uses untrusted data.
  • Tainted data used in the environment variables is not properly validated. For instance boundary checking is not done before copying the input data to a buffer.

Skills 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.

Resources required

Not stated in the MITRE record.

Detection

What are the indicators of the attack?

If the application does bound checking, it should fail when the data source is larger than the size of the destination buffer. If the application's code is well written, that failure should trigger an alert.

Impact

What does a successful attack lead to?

Unreliable Execution

Affected scopes: Availability

Execute Unauthorized Commands

Affected scopes: Confidentiality, Integrity, Availability

Read Data

Affected scopes: Confidentiality

Modify Data

Affected scopes: Integrity

Gain Privileges

Affected scopes: Confidentiality, Access Control, Authorization

Defence

How is it prevented and mitigated?

1
Do not expose environment variable to the user.
2
Do not use untrusted data in your environment variables.
3
Use a language or compiler that performs automatic bounds checking
4
There are tools such as Sharefuzz [ REF-2 ] which is an environment variable fuzzer for Unix that support loading a shared library. You can use Sharefuzz to determine if you are exposing an environment variable vulnerable to buffer overflow.

Real world

MITRE example instances

Example instance 1Detail
A buffer overflow in sccw allows local users to gain root access via the $HOME environmental variable. See also: CVE-1999-0906
Example instance 2Detail
A buffer overflow in the rlogin program involves its consumption of the $TERM environmental variable. See also: CVE-1999-0046