CWE-362 使用共享资源的并发执行不恰当同步问题（竞争条件）

Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition')

结构: Simple

Abstraction: Class

状态: Draft

被利用可能性: Medium

基本描述

The program contains a code sequence that can run concurrently with other code, and the code sequence requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence that is operating concurrently.

扩展描述

This can have security implications when the expected synchronization is in security-critical code, such as recording whether a user is authenticated or modifying important state information that should not be influenced by an outsider.

A race condition occurs within concurrent environments, and is effectively a property of a code sequence. Depending on the context, a code sequence may be in the form of a function call, a small number of instructions, a series of program invocations, etc.

A race condition violates these properties, which are closely related:

A race condition exists when an "interfering code sequence" can still access the shared resource, violating exclusivity. Programmers may assume that certain code sequences execute too quickly to be affected by an interfering code sequence; when they are not, this violates atomicity. For example, the single "x++" statement may appear atomic at the code layer, but it is actually non-atomic at the instruction layer, since it involves a read (the original value of x), followed by a computation (x+1), followed by a write (save the result to x).

The interfering code sequence could be "trusted" or "untrusted." A trusted interfering code sequence occurs within the program; it cannot be modified by the attacker, and it can only be invoked indirectly. An untrusted interfering code sequence can be authored directly by the attacker, and typically it is external to the vulnerable program.

相关缺陷

• cwe_Nature: ChildOf cwe_CWE_ID: 691 cwe_View_ID: 1000 cwe_Ordinal: Primary

适用平台

Language: [{'cwe_Name': 'C', 'cwe_Prevalence': 'Sometimes'}, {'cwe_Name': 'C++', 'cwe_Prevalence': 'Sometimes'}, {'cwe_Name': 'Java', 'cwe_Prevalence': 'Sometimes'}, {'cwe_Class': 'Language-Independent', 'cwe_Prevalence': 'Undetermined'}]

Paradigm: {'cwe_Name': 'Concurrent Systems Operating on Shared Resources', 'cwe_Prevalence': 'Often'}

常见的影响

检测方法

Black Box

White Box

DM-2 Automated Dynamic Analysis

Automated Static Analysis - Binary or Bytecode

Dynamic Analysis with Automated Results Interpretation

Dynamic Analysis with Manual Results Interpretation

Manual Static Analysis - Source Code

Automated Static Analysis - Source Code

Architecture or Design Review

可能的缓解方案

Architecture and Design

策略:

In languages that support it, use synchronization primitives. Only wrap these around critical code to minimize the impact on performance.

Architecture and Design

策略:

Use thread-safe capabilities such as the data access abstraction in Spring.

Architecture and Design

策略:

Minimize the usage of shared resources in order to remove as much complexity as possible from the control flow and to reduce the likelihood of unexpected conditions occurring. Additionally, this will minimize the amount of synchronization necessary and may even help to reduce the likelihood of a denial of service where an attacker may be able to repeatedly trigger a critical section (CWE-400).

Implementation

策略:

When using multithreading and operating on shared variables, only use thread-safe functions.

Implementation

策略:

Use atomic operations on shared variables. Be wary of innocent-looking constructs such as "x++". This may appear atomic at the code layer, but it is actually non-atomic at the instruction layer, since it involves a read, followed by a computation, followed by a write.

Implementation

策略:

Use a mutex if available, but be sure to avoid related weaknesses such as CWE-412.

Implementation

策略:

Avoid double-checked locking (CWE-609) and other implementation errors that arise when trying to avoid the overhead of synchronization.

Implementation

策略:

Disable interrupts or signals over critical parts of the code, but also make sure that the code does not go into a large or infinite loop.

Implementation

策略:

Use the volatile type modifier for critical variables to avoid unexpected compiler optimization or reordering. This does not necessarily solve the synchronization problem, but it can help.

MIT-17 ['Architecture and Design', 'Operation']

策略: Environment Hardening

Run your code using the lowest privileges that are required to accomplish the necessary tasks [REF-76]. If possible, create isolated accounts with limited privileges that are only used for a single task. That way, a successful attack will not immediately give the attacker access to the rest of the software or its environment. For example, database applications rarely need to run as the database administrator, especially in day-to-day operations.

示例代码

例

This code could be used in an e-commerce application that supports transfers between accounts. It takes the total amount of the transfer, sends it to the new account, and deducts the amount from the original account.

bad Perl

A race condition could occur between the calls to GetBalanceFromDatabase() and SendNewBalanceToDatabase().

Suppose the balance is initially 100.00. An attack could be constructed as follows:

attack Other

At this stage, the attacker should have a balance of 19.00 (due to 81.00 worth of transfers), but the balance is 99.00, as recorded in the database.

To prevent this weakness, the programmer has several options, including using a lock to prevent multiple simultaneous requests to the web application, or using a synchronization mechanism that includes all the code between GetBalanceFromDatabase() and SendNewBalanceToDatabase().

例

The following function attempts to acquire a lock in order to perform operations on a shared resource.

bad C

However, the code does not check the value returned by pthread_mutex_lock() for errors. If pthread_mutex_lock() cannot acquire the mutex for any reason, the function may introduce a race condition into the program and result in undefined behavior.

In order to avoid data races, correctly written programs must check the result of thread synchronization functions and appropriately handle all errors, either by attempting to recover from them or reporting it to higher levels.

good

分析过的案例

Notes

Maintenance The relationship between race conditions and synchronization problems (CWE-662) needs to be further developed. They are not necessarily two perspectives of the same core concept, since synchronization is only one technique for avoiding race conditions, and synchronization can be used for other purposes besides race condition prevention. Research Gap Race conditions in web applications are under-studied and probably under-reported. However, in 2008 there has been growing interest in this area. Research Gap Much of the focus of race condition research has been in Time-of-check Time-of-use (TOCTOU) variants (CWE-367), but many race conditions are related to synchronization problems that do not necessarily require a time-of-check. Research Gap From a classification/taxonomy perspective, the relationships between concurrency and program state need closer investigation and may be useful in organizing related issues.

分类映射

相关攻击模式

• CAPEC-26
• CAPEC-29

引用

• REF-44 24 Deadly Sins of Software Security

• REF-349 volatile - Multithreaded Programmer's Best Friend

• REF-350 Thread-safe webapps using Spring

• REF-351 Prevent race conditions

• REF-352 Race Conditions, Files, and Security Flaws; or the Tortoise and the Hare Redux

• REF-353 Secure Programming for Linux and Unix HOWTO

• REF-354 Discovering and Exploiting Named Pipe Security Flaws for Fun and Profit

• REF-355 On Race Vulnerabilities in Web Applications

• REF-356 Avoiding Race Conditions and Insecure File Operations

• REF-357 Top 25 Series - Rank 25 - Race Conditions

• REF-76 Least Privilege