Common Weakness Enumeration

The product allocates a reusable resource or group of resources on behalf of an actor without imposing any restrictions on the size or number of resources that can be allocated, in violation of the intended security policy for that actor.

Code frequently has to work with limited resources, so programmers must be careful to ensure that resources are not consumed too quickly, or too easily. Without use of quotas, resource limits, or other protection mechanisms, it can be easy for an attacker to consume many resources by rapidly making many requests, or causing larger resources to be used than is needed. When too many resources are allocated, or if a single resource is too large, then it can prevent the code from working correctly, possibly leading to a denial of service.

This listing shows possible areas for which the given weakness could appear. These may be for specific named Languages, Operating Systems, Architectures, Paradigms, Technologies, or a class of such platforms. The platform is listed along with how frequently the given weakness appears for that instance.

Languages

Class: Not Language-Specific (Often Prevalent)

High

Example 1

This code allocates a socket and forks each time it receives a new connection.

The program does not track how many connections have been made, and it does not limit the number of connections. Because forking is a relatively expensive operation, an attacker would be able to cause the system to run out of CPU, processes, or memory by making a large number of connections. Alternatively, an attacker could consume all available connections, preventing others from accessing the system remotely.

Example 2

In the following example a server socket connection is used to accept a request to store data on the local file system using a specified filename. The method openSocketConnection establishes a server socket to accept requests from a client. When a client establishes a connection to this service the getNextMessage method is first used to retrieve from the socket the name of the file to store the data, the openFileToWrite method will validate the filename and open a file to write to on the local file system. The getNextMessage is then used within a while loop to continuously read data from the socket and output the data to the file until there is no longer any data from the socket.

This example creates a situation where data can be dumped to a file on the local file system without any limits on the size of the file. This could potentially exhaust file or disk resources and/or limit other clients' ability to access the service.

Example 3

In the following example, the processMessage method receives a two dimensional character array containing the message to be processed. The two-dimensional character array contains the length of the message in the first character array and the message body in the second character array. The getMessageLength method retrieves the integer value of the length from the first character array. After validating that the message length is greater than zero, the body character array pointer points to the start of the second character array of the two-dimensional character array and memory is allocated for the new body character array.

This example creates a situation where the length of the body character array can be very large and will consume excessive memory, exhausting system resources. This can be avoided by restricting the length of the second character array with a maximum length check

Also, consider changing the type from 'int' to 'unsigned int', so that you are always guaranteed that the number is positive. This might not be possible if the protocol specifically requires allowing negative values, or if you cannot control the return value from getMessageLength(), but it could simplify the check to ensure the input is positive, and eliminate other errors such as signed-to-unsigned conversion errors (CWE-195) that may occur elsewhere in the code.

Example 4

In the following example, a server object creates a server socket and accepts client connections to the socket. For every client connection to the socket a separate thread object is generated using the ClientSocketThread class that handles request made by the client through the socket.

In this example there is no limit to the number of client connections and client threads that are created. Allowing an unlimited number of client connections and threads could potentially overwhelm the system and system resources.

The server should limit the number of client connections and the client threads that are created. This can be easily done by creating a thread pool object that limits the number of threads that are generated.

Example 5

An unnamed web site allowed a user to purchase tickets for an event. A menu option allowed the user to purchase up to 10 tickets, but the back end did not restrict the actual number of tickets that could be purchased.

Example 6

Here the problem is that every time a connection is made, more memory is allocated. So if one just opened up more and more connections, eventually the machine would run out of memory.

This MemberOf Relationships table shows additional CWE Categories and Views that reference this weakness as a member. This information is often useful in understanding where a weakness fits within the context of external information sources.

The product provides an application for administrators to manage parts of the underlying operating system, but the application does not accurately identify all of the relevant entities or resources that exist in the OS; that is, the application's model of the OS's state is inconsistent with the OS's actual state.

Many products provide web-based applications or other interfaces for managing the underlying operating system. This is common with cloud, network access devices, home networking, and other systems. When the management tool does not accurately represent what is in the OS - such as user accounts - then the administrator might not see suspicious activities that would be noticed otherwise.

For example, numerous systems utilize a web front-end for administrative control. They also offer the ability to add, alter, and drop users with various privileges as it relates to the functionality of the system. A potential architectural weakness may exist where the user information reflected in the web interface does not mirror the users in the underlying operating system. Many web UI or REST APIs use the underlying operating system for authentication; the system's logic may also track an additional set of user capabilities within configuration files and datasets for authorization capabilities. When there is a discrepancy between the user information in the UI or REST API's interface system and the underlying operating system's user listing, this may introduce a weakness into the system. For example, if an attacker compromises the OS and adds a new user account - a "ghost" account - then the attacker could escape detection if the management tool does not list the newly-added account.

This discrepancy could be exploited in several ways:

• A rogue admin could insert a new account into a system that will persist if they are terminated or wish to take action on a system that cannot be directly associated with them.
• An attacker can leverage a separate command injection attack available through the web interface to insert a ghost account with shell privileges such as ssh.
• An attacker can leverage existing web interface APIs, manipulated in such a way that a new user is inserted into the operating system, and the user web account is either partially created or not at all.
• An attacker could create an admin account which is viewable by an administrator, use this account to create the ghost account, delete logs and delete the first created admin account.

Many of these attacker scenarios can be realized by leveraging separate vulnerabilities related to XSS, command injection, authentication bypass, or logic flaws on the various systems.

This listing shows possible areas for which the given weakness could appear. These may be for specific named Languages, Operating Systems, Architectures, Paradigms, Technologies, or a class of such platforms. The platform is listed along with how frequently the given weakness appears for that instance.

Languages	Class: Not Language-Specific (Undetermined Prevalence)
Operating Systems	Class: Not OS-Specific (Undetermined Prevalence)
Technologies	Class: Web Based (Undetermined Prevalence)

Example 1

Suppose that an attacker successfully gains root privileges on a Linux system and adds a new 'user2' account:

This new user2 account would not be noticed on the web interface, if the interface does not refresh its data of available users.

It could be argued that for this specific example, an attacker with root privileges would be likely to compromise the admin tool or otherwise feed it with false data. However, this example shows how the discrepancy in critical data can help attackers to escape detection.

This MemberOf Relationships table shows additional CWE Categories and Views that reference this weakness as a member. This information is often useful in understanding where a weakness fits within the context of external information sources.

The product's architecture contains too many - or too few - horizontal layers.

This issue makes it more difficult to maintain the product, which indirectly affects security by making it more difficult or time-consuming to find and/or fix vulnerabilities. It also might make it easier to introduce vulnerabilities.

While the interpretation of "expected range" may vary for each product or developer, CISQ recommends a default minimum of 4 layers and maximum of 8 layers.

This MemberOf Relationships table shows additional CWE Categories and Views that reference this weakness as a member. This information is often useful in understanding where a weakness fits within the context of external information sources.

The product does not properly control situations in which an adversary can cause the product to consume or produce excessive resources without requiring the adversary to invest equivalent work or otherwise prove authorization, i.e., the adversary's influence is "asymmetric."

This can lead to poor performance due to "amplification" of resource consumption, typically in a non-linear fashion. This situation is worsened if the product allows malicious users or attackers to consume more resources than their access level permits.

This listing shows possible areas for which the given weakness could appear. These may be for specific named Languages, Operating Systems, Architectures, Paradigms, Technologies, or a class of such platforms. The platform is listed along with how frequently the given weakness appears for that instance.

Languages	Class: Not Language-Specific (Undetermined Prevalence)
Operating Systems	Class: Not OS-Specific (Undetermined Prevalence)
Architectures	Class: Not Architecture-Specific (Undetermined Prevalence)
Technologies	Class: Not Technology-Specific (Undetermined Prevalence) Class: Client Server (Undetermined Prevalence)

Example 1

This code listens on a port for DNS requests and sends the result to the requesting address.

This code sends a DNS record to a requesting IP address. UDP allows the source IP address to be easily changed ('spoofed'), thus allowing an attacker to redirect responses to a target, which may be then be overwhelmed by the network traffic.

Example 2

This function prints the contents of a specified file requested by a user.

This code first reads a specified file into memory, then prints the file if the user is authorized to see its contents. The read of the file into memory may be resource intensive and is unnecessary if the user is not allowed to see the file anyway.

Example 3

The DTD and the very brief XML below illustrate what is meant by an XML bomb. The ZERO entity contains one character, the letter A. The choice of entity name ZERO is being used to indicate length equivalent to that exponent on two, that is, the length of ZERO is 2^0. Similarly, ONE refers to ZERO twice, therefore the XML parser will expand ONE to a length of 2, or 2^1. Ultimately, we reach entity THIRTYTWO, which will expand to 2^32 characters in length, or 4 GB, probably consuming far more data than expected.

Example 4

This example attempts to check if an input string is a "sentence" [REF-1164].

The regular expression has a vulnerable backtracking clause inside (\w+\s?)*$ which can be triggered to cause a Denial of Service by processing particular phrases.

To fix the backtracking problem, backtracking is removed with the ?= portion of the expression which changes it to a lookahead and the \2 which prevents the backtracking. The modified example is:

Note that [REF-1164] has a more thorough (and lengthy) explanation of everything going on within the RegEx.

Example 5

An adversary can cause significant resource consumption on a server by filtering the cryptographic algorithms offered by the client to the ones that are the most resource-intensive on the server side. After discovering which cryptographic algorithms are supported by the server, a malicious client can send the initial cryptographic handshake messages that contains only the resource-intensive algorithms. For some cryptographic protocols, these messages can be completely prefabricated, as the resource-intensive part of the handshake happens on the server-side first (such as TLS), rather than on the client side. In the case of cryptographic protocols where the resource-intensive part should happen on the client-side first (such as SSH), a malicious client can send a forged/precalculated computation result, which seems correct to the server, so the resource-intensive part of the handshake is going to happen on the server side. A malicious client is required to send only the initial messages of a cryptographic handshake to initiate the resource-consuming part of the cryptographic handshake. These messages are usually small, and generating them requires minimal computational effort, enabling a denial-of-service attack. An additional risk is the fact that higher key size increases the effectiveness of the attack. Cryptographic protocols where the clients have influence over the size of the used key (such as TLS 1.3 or SSH) are most at risk, as the client can enforce the highest key size supported by the server.

This MemberOf Relationships table shows additional CWE Categories and Views that reference this weakness as a member. This information is often useful in understanding where a weakness fits within the context of external information sources.

The product performs authentication based on the name of a resource being accessed, or the name of the actor performing the access, but it does not properly check all possible names for that resource or actor.

This listing shows possible areas for which the given weakness could appear. These may be for specific named Languages, Operating Systems, Architectures, Paradigms, Technologies, or a class of such platforms. The platform is listed along with how frequently the given weakness appears for that instance.

Languages

Class: Not Language-Specific (Undetermined Prevalence)

This MemberOf Relationships table shows additional CWE Categories and Views that reference this weakness as a member. This information is often useful in understanding where a weakness fits within the context of external information sources.

The authentication scheme or implementation uses key data elements that are assumed to be immutable, but can be controlled or modified by the attacker.

This listing shows possible areas for which the given weakness could appear. These may be for specific named Languages, Operating Systems, Architectures, Paradigms, Technologies, or a class of such platforms. The platform is listed along with how frequently the given weakness appears for that instance.

Languages

Class: Not Language-Specific (Undetermined Prevalence)

Example 1

In the following example, an "authenticated" cookie is used to determine whether or not a user should be granted access to a system.

Modifying the value of a cookie on the client-side is trivial, but many developers assume that cookies are essentially immutable.

This MemberOf Relationships table shows additional CWE Categories and Views that reference this weakness as a member. This information is often useful in understanding where a weakness fits within the context of external information sources.

A capture-replay flaw exists when the design of the product makes it possible for a malicious user to sniff network traffic and bypass authentication by replaying it to the server in question to the same effect as the original message (or with minor changes).

Capture-replay attacks are common and can be difficult to defeat without cryptography. They are a subset of network injection attacks that rely on observing previously-sent valid commands, then changing them slightly if necessary and resending the same commands to the server.

This listing shows possible areas for which the given weakness could appear. These may be for specific named Languages, Operating Systems, Architectures, Paradigms, Technologies, or a class of such platforms. The platform is listed along with how frequently the given weakness appears for that instance.

Languages

Class: Not Language-Specific (Undetermined Prevalence)

High

This MemberOf Relationships table shows additional CWE Categories and Views that reference this weakness as a member. This information is often useful in understanding where a weakness fits within the context of external information sources.