CWE 770

Manual Static Analysis

Manual static analysis can be useful for finding this weakness, but it might not achieve desired code coverage within limited time constraints. If denial-of-service is not considered a significant risk, or if there is strong emphasis on consequences such as code execution, then manual analysis may not focus on this weakness at all.

Fuzzing

While fuzzing is typically geared toward finding low-level implementation bugs, it can inadvertently find uncontrolled resource allocation problems. This can occur when the fuzzer generates a large number of test cases but does not restart the targeted software in between test cases. If an individual test case produces a crash, but it does not do so reliably, then an inability to limit resource allocation may be the cause.

When the allocation is directly affected by numeric inputs, then fuzzing may produce indications of this weakness.

Effectiveness: Opportunistic

Automated Dynamic Analysis

Certain automated dynamic analysis techniques may be effective in producing side effects of uncontrolled resource allocation problems, especially with resources such as processes, memory, and connections. The technique may involve generating a large number of requests to the software within a short time frame. Manual analysis is likely required to interpret the results.

Automated Static Analysis

Specialized configuration or tuning may be required to train automated tools to recognize this weakness.

Automated static analysis typically has limited utility in recognizing unlimited allocation problems, except for the failure to release program-independent system resources such as files, sockets, and processes, or unchecked arguments to memory. For system resources, automated static analysis may be able to detect circumstances in which resources are not released after they have expired, or if too much of a resource is requested at once, as can occur with memory. Automated analysis of configuration files may be able to detect settings that do not specify a maximum value.

Automated static analysis tools will not be appropriate for detecting exhaustion of custom resources, such as an intended security policy in which a bulletin board user is only allowed to make a limited number of posts per day.

Phase: Requirements

Clearly specify the minimum and maximum expectations for capabilities, and dictate which behaviors are acceptable when resource allocation reaches limits.

Phase: Architecture and Design

Limit the amount of resources that are accessible to unprivileged users. Set per-user limits for resources. Allow the system administrator to define these limits. Be careful to avoid CWE-410.

Phase: Architecture and Design

Design throttling mechanisms into the system architecture. The best protection is to limit the amount of resources that an unauthorized user can cause to be expended. A strong authentication and access control model will help prevent such attacks from occurring in the first place, and it will help the administrator to identify who is committing the abuse. The login application should be protected against DoS attacks as much as possible. Limiting the database access, perhaps by caching result sets, can help minimize the resources expended. To further limit the potential for a DoS attack, consider tracking the rate of requests received from users and blocking requests that exceed a defined rate threshold.

Phase: Architecture and Design

Mitigation of resource exhaustion attacks requires that the target system either:

• recognizes the attack and denies that user further access for a given amount of time, typically by using increasing time delays

• uniformly throttles all requests in order to make it more difficult to consume resources more quickly than they can again be freed.

The first of these solutions is an issue in itself though, since it may allow attackers to prevent the use of the system by a particular valid user. If the attacker impersonates the valid user, he may be able to prevent the user from accessing the server in question.

The second solution can be difficult to effectively institute -- and even when properly done, it does not provide a full solution. It simply requires more resources on the part of the attacker.

Phase: Architecture and Design

Ensure that protocols have specific limits of scale placed on them.

Phase: Implementation

Ensure that all failures in resource allocation place the system into a safe posture.

Phase: Implementation

For system resources when using C, consider using the getrlimit() function included in the sys/resources library in order to determine how many files are currently allowed to be opened for the process.

Phase: Operation

Use resource-limiting settings provided by the operating system or environment. For example, setrlimit() can be used to set limits for certain types of resources. However, this is not available on all operating systems.

Ensure that your application performs the appropriate error checks and error handling in case resources become unavailable (CWE-703).

Vulnerability theory is largely about how behaviors and resources interact. "Resource exhaustion" can be regarded as either a consequence or an attack, depending on the perspective. This entry is an attempt to reflect one of the underlying weaknesses that enable these attacks (or consequences) to take place.

"Resource exhaustion" (CWE-400) is currently treated as a weakness, although it is more like a category of weaknesses that all have the same type of consequence. While this entry treats CWE-400 as a parent in view 1000, the relationship is probably more appropriately described as a chain.