By default, ASP.NET servers store the HttpSessionState object, its attributes and any objects they reference in memory. This model limits active session state to what can be accommodated by the system memory of a single machine. In order to expand capacity beyond these limitations, servers are frequently configured to persistent session state information, which both expands capacity and permits the replication across multiple machines to improve overall performance. In order to persist its session state, the server must serialize the HttpSessionState object, which requires that all objects stored in it be serializable.

In order for the session to be serialized correctly, all objects the application stores as session attributes must declare the [Serializable] attribute. Additionally, if the object requires custom serialization methods, it must also implement the ISerializable interface.

Example 1: The following class adds itself to the session, but since it is not serializable, the session cannot be serialized correctly.

public class DataGlob {
   String GlobName;
   String GlobValue;

   public void AddToSession(HttpSessionState session) {
     session["glob"] = this;
   }
}

If multiple threads are trying to obtain a lock on a resource, calling sleep() while holding a lock can cause all of the other threads to wait for the resource to be released, which can result in degraded performance and deadlock.

Example 1: The following code calls sleep() while holding a lock.

ReentrantLock rl = new ReentrantLock();
...
rl.lock();
Thread.sleep(500);
...
rl.unlock();

Many talented individuals have spent a great deal of time pondering ways to make double-checked locking work in order to improve performance. None have succeeded.

Example 1: At first blush it may seem that the following bit of code achieves thread safety while avoiding unnecessary synchronization.

if (fitz == null) {
  synchronized (this) {
    if (fitz == null) {
      fitz = new Fitzer();
    }
  }
}
return fitz;

The programmer wants to guarantee that only one Fitzer() object is ever allocated, but does not want to pay the cost of synchronization every time this code is called. This idiom is known as double-checked locking.

Unfortunately, it does not work, and multiple Fitzer() objects can be allocated. See The "Double-Checked Locking is Broken" Declaration for more details [1].

After a thread signals other threads waiting on a mutex, it must unlock the mutex by calling pthread_mutex_unlock() before another thread can begin running. If the signaling thread fails to unlock the mutex, the pthread_cond_wait() call in the second thread will not return and the thread will not execute.

Example 1: The following code signals another thread waiting on a mutex by calling pthread_cond_signal(), but fails to unlock the mutex the other thread is waiting on.

   ...
   pthread_mutex_lock(&count_mutex);

   // Signal waiting thread
   pthread_cond_signal(&count_threshold_cv);
   ...

Applications require temporary files so frequently that many different mechanisms exist for creating them in the C Library and Windows(R) API. Most of these functions are vulnerable to various forms of attacks.
Example: The following code uses a temporary file for storing intermediate data gathered from the network before it is processed.

...
if (tmpnam_r(filename)){
	FILE* tmp = fopen(filename,"wb+");
	while((recv(sock,recvbuf,DATA_SIZE, 0) > 0)&&(amt!=0))
		amt = fwrite(recvbuf,1,DATA_SIZE,tmp);
}
...

This otherwise unremarkable code is vulnerable to a number of different attacks because it relies on an insecure method for creating temporary files. The vulnerabilities introduced by this function and others are described in the following sections. The most egregious security problems related to temporary file creation have occurred on Unix-based operating systems, but Windows applications have parallel risks. This section includes a discussion of temporary file creation on both Unix and Windows systems.

Methods and behaviors can vary between systems, but the fundamental risks introduced by each are reasonably constant. See the Recommendations section for information about safe core language functions and advice regarding a secure approach to creating temporary files.

The functions designed to aid in the creation of temporary files can be broken into two groups based on whether they simply provide a filename or actually open a new file.

Group 1 - "Unique" Filenames:

The first group of C Library and WinAPI functions designed to help with the process of creating temporary files do so by generating a unique file name for a new temporary file, which the program is then supposed to open. This group includes C Library functions like tmpnam(), tempnam(), mktemp() and their C++ equivalents prefaced with an _ (underscore) as well as the GetTempFileName() function from the Windows API. This group of functions suffers from an underlying race condition on the filename chosen. Although the functions guarantee that the filename is unique at the time it is selected, there is no mechanism to prevent another process or an attacker from creating a file with the same name after it is selected but before the application attempts to open the file. Beyond the risk of a legitimate collision caused by another call to the same function, there is a high probability that an attacker will be able to create a malicious collision because the filenames generated by these functions are not sufficiently randomized to make them difficult to guess.

If a file with the selected name is created, then depending on how the file is opened the existing contents or access permissions of the file may remain intact. If the existing contents of the file are malicious in nature, an attacker may be able to inject dangerous data into the application when it reads data back from the temporary file. If an attacker pre-creates the file with relaxed access permissions, then data stored in the temporary file by the application may be accessed, modified or corrupted by an attacker. On Unix based systems an even more insidious attack is possible if the attacker pre-creates the file as a link to another important file. Then, if the application truncates or writes data to the file, it may unwittingly perform damaging operations for the attacker. This is an especially serious threat if the program operates with elevated permissions.

Finally, in the best case the file will be opened with a call to open() using the O_CREAT and O_EXCL flags or to CreateFile() using the CREATE_NEW attribute, which will fail if the file already exists and therefore prevent the types of attacks described previously. However, if an attacker is able to accurately predict a sequence of temporary file names, then the application may be prevented from opening necessary temporary storage causing a denial of service (DoS) attack. This type of attack would not be difficult to mount given the small amount of randomness used in the selection of the filenames generated by these functions.

Group 2 - "Unique" Files:

The second group of C Library functions attempts to resolve some of the security problems related to temporary files by not only generating a unique file name, but also opening the file. This group includes C Library functions like tmpfile() and its C++ equivalents prefaced with an _ (underscore), as well as the slightly better-behaved C Library function mkstemp().

The tmpfile() style functions construct a unique filename and open it in the same way that fopen() would if passed the flags "wb+", that is, as a binary file in read/write mode. If the file already exists, tmpfile() will truncate it to size zero, possibly in an attempt to assuage the security concerns mentioned earlier regarding the race condition that exists between the selection of a supposedly unique filename and the subsequent opening of the selected file. However, this behavior clearly does not solve the function's security problems. First, an attacker may pre-create the file with relaxed access-permissions that will likely be retained by the file opened by tmpfile(). Furthermore, on Unix based systems if the attacker pre-creates the file as a link to another important file, the application may use its possibly elevated permissions to truncate that file, thereby doing damage on behalf of the attacker. Finally, if tmpfile() does create a new file, the access permissions applied to that file will vary from one operating system to another, which can leave application data vulnerable even if an attacker is unable to predict the filename to be used in advance.

Finally, mkstemp() is a reasonably safe way to create temporary files. It will attempt to create and open a unique file based on a filename template provided by the user combined with a series of randomly generated characters. If it is unable to create such a file, it will fail and return -1. On modern systems the file is opened using mode 0600, which means the file will be secure from tampering unless the user explicitly changes its access permissions. However, mkstemp() still suffers from the use of predictable file names and can leave an application vulnerable to denial of service attacks if an attacker causes mkstemp() to fail by predicting and pre-creating the filenames to be used.

The longer a session stays open, the larger the window of opportunity an attacker has to compromise user accounts. While a session remains active, an attacker may be able to brute-force a user's password, crack a user's wireless encryption key, or commandeer a session from an open browser. Longer session timeouts can also prevent memory from being released and eventually result in a denial of service if a sufficiently large number of sessions are created.

Example 1: Code in the following example sets a negative value for the maximum inactive interval resulting in a session that remains active indefinitely.

...
HttpSession sesssion = request.getSession(true);
sesssion.setMaxInactiveInterval(-1);
...

It is never a good idea for a web application to attempt to shut down the application container. A call to a termination method is probably part of leftover debug code or code imported from a non-J2EE application.