In this attack, the target software is given input that the adversary knows will be modified and expanded in size during processing. This attack relies on the target software failing to anticipate that the expanded data may exceed some internal limit, thereby creating a buffer overflow.
Likelihood Of Attack
Medium
Typical Severity
High
Relationships
This table shows the other attack patterns and high level categories that are related to this attack pattern. These relationships are defined as ChildOf and ParentOf, and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as CanFollow, PeerOf, and CanAlsoBe are defined to show similar attack patterns that the user may want to explore.
Nature
Type
ID
Name
ChildOf
Standard Attack Pattern - A standard level attack pattern in CAPEC is focused on a specific methodology or technique used in an attack. It is often seen as a singular piece of a fully executed attack. A standard attack pattern is meant to provide sufficient details to understand the specific technique and how it attempts to accomplish a desired goal. A standard level attack pattern is a specific type of a more abstract meta level attack pattern.
Identify target application: The adversary identifies a target application or program to perform the buffer overflow on. Adversaries often look for applications that accept user input and that perform manual memory management.
Experiment
Find injection vector: The adversary identifies an injection vector to deliver the excessive content to the targeted application's buffer.
Techniques
In this attack, the normal method of providing large user input does not work. The program performs bounds checking on the user input, but not the expanded user input. The adversary needs to provide input that they believe will be expanded by the program to overflow a buffer. To identify where this is possible, an adversary either needs to have knowledge of the inner workings of the program or use a disassembler and other reverse engineering tools to guide the search.
Craft overflow content: The adversary crafts the input to be given to the program. If the intent is to simply cause the software to crash, the input needs only to expand to an excessive quantity of random data. If the intent is to leverage the overflow for execution of arbitrary code, the adversary will craft input that expands in a way that not only overflows the targeted buffer but does so in such a way that the overwritten return address is replaced with one of the adversaries' choosing which points to code injected by the adversary.
Techniques
Create specific files and directories on the system and then give input using path traversal shortcuts to those directories that could expand past an input buffer.
Exploit
Overflow the buffer: Using the injection vector, the adversary gives the crafted input to the program, overflowing the buffer.
Prerequisites
The program expands one of the parameters passed to a function with input controlled by the user, but a later function making use of the expanded parameter erroneously considers the original, not the expanded size of the parameter.
The expanded parameter is used in the context where buffer overflow may become possible due to the incorrect understanding of the parameter size (i.e. thinking that it is smaller than it really is).
Skills Required
[Level: High]
Finding this particular buffer overflow may not be trivial. Also, stack and especially heap based buffer overflows require a lot of knowledge if the intended goal is arbitrary code execution. Not only that the adversary needs to write the shell code to accomplish their goals, but the adversary also needs to find a way to get the program execution to jump to the planted shell code. There also needs to be sufficient room for the payload. So not every buffer overflow will be exploitable, even by a skilled adversary.
Resources Required
Access to the program source or binary. If the program is only available in binary then a disassembler and other reverse engineering tools will be helpful.
Consequences
This table specifies different individual consequences associated with the attack pattern. The Scope identifies the security property that is violated, while the Impact describes the negative technical impact that arises if an adversary succeeds in their attack. The Likelihood provides information about how likely the specific consequence is expected to be seen relative to the other consequences in the list. For example, there may be high likelihood that a pattern will be used to achieve a certain impact, but a low likelihood that it will be exploited to achieve a different impact.
Scope
Impact
Likelihood
Confidentiality
Access Control
Authorization
Gain Privileges
Availability
Unreliable Execution
Integrity
Modify Data
Confidentiality
Integrity
Availability
Execute Unauthorized Commands
Confidentiality
Read Data
Mitigations
Ensure that when parameter expansion happens in the code that the assumptions used to determine the resulting size of the parameter are accurate and that the new size of the parameter is visible to the whole system
Example Instances
Attack Example: FTP glob()
The glob() function in FTP servers has been susceptible to attack as a result of incorrect resizing. This is an ftpd glob() Expansion LIST Heap Overflow Vulnerability. ftp daemon contains a heap-based buffer overflow condition. The overflow occurs when the LIST command is issued with an argument that expands into an oversized string after being processed by glob().
This buffer overflow occurs in memory that is dynamically allocated. It may be possible for adversaries to exploit this vulnerability and execute arbitrary code on the affected host.
To exploit this, the adversary must be able to create directories on the target host.
The glob() function is used to expand short-hand notation into complete file names. By sending to the FTP server a request containing a tilde (~) and other wildcard characters in the pathname string, a remote adversary can overflow a buffer and execute arbitrary code on the FTP server to gain root privileges. Once the request is processed, the glob() function expands the user input, which could exceed the expected length. In order to exploit this vulnerability, the adversary must be able to create directories on the FTP server.
Buffer overflow in the glob implementation in libc in NetBSD-current before 20050914, and NetBSD 2.* and 3.* before 20061203, as used by the FTP daemon, allows remote authenticated users to execute arbitrary code via a long pathname that results from path expansion.
The limit computation of an internal buffer was done incorrectly. The size of the buffer in byte was used as element count, even though the elements of the buffer are 2 bytes long. Long expanded path names would therefore overflow the buffer.
A Related Weakness relationship associates a weakness with this attack pattern. Each association implies a weakness that must exist for a given attack to be successful. If multiple weaknesses are associated with the attack pattern, then any of the weaknesses (but not necessarily all) may be present for the attack to be successful. Each related weakness is identified by a CWE identifier.
Description
This table shows the other attack patterns and high level categories that are related to this attack pattern. These relationships are defined as ChildOf and ParentOf, and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as CanFollow, PeerOf, and CanAlsoBe are defined to show similar attack patterns that the user may want to explore.
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