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 DescriptionDescription Summary The software does not check the return value from a method or
function, which can prevent it from detecting unexpected states and
conditions.
Extended Description Two common programmer assumptions are "this function call can never fail" and "it doesn't matter if this function call fails". If an attacker can force the function to fail or otherwise return a value that is not expected, then the subsequent program logic could lead to a vulnerability, because the software is not in a state that the programmer assumes. For example, if the program calls a function to drop privileges but does not check the return code to ensure that privileges were successfully dropped, then the program will continue to operate with the higher privileges. Time of Introduction
Applicable PlatformsLanguages All Common Consequences
Likelihood of ExploitLow Demonstrative ExamplesExample 1 Consider the following code: (Bad Code) C char buf[10], cp_buf[10]; fgets(buf, 10, stdin); strcpy(cp_buf, buf); The programmer expects that when fgets() returns, buf will contain a null-terminated string of length 9 or less. But if an I/O error occurs, fgets() will not null-terminate buf. Furthermore, if the end of the file is reached before any characters are read, fgets() returns without writing anything to buf. In both of these situations, fgets() signals that something unusual has happened by returning NULL, but in this code, the warning will not be noticed. The lack of a null terminator in buf can result in a buffer overflow in the subsequent call to strcpy(). Example 2 The following code does not check to see if memory allocation succeeded before attempting to use the pointer returned by malloc(). (Bad Code) C buf = (char*) malloc(req_size); strncpy(buf, xfer, req_size); The traditional defense of this coding error is: "If my program runs out of memory, it will fail. It doesn't matter whether I handle the error or simply allow the program to die with a segmentation fault when it tries to dereference the null pointer." This argument ignores three important considerations: - Depending upon the type and size of the application, it may be possible to free memory that is being used elsewhere so that execution can continue. - It is impossible for the program to perform a graceful exit if required. If the program is performing an atomic operation, it can leave the system in an inconsistent state. - The programmer has lost the opportunity to record diagnostic information. Did the call to malloc() fail because req_size was too large or because there were too many requests being handled at the same time? Or was it caused by a memory leak that has built up over time? Without handling the error, there is no way to know. Example 3 The following code loops through a set of users, reading a private data file for each user. The programmer assumes that the files are always 1 kilobyte in size and therefore ignores the return value from Read(). If an attacker can create a smaller file, the program will recycle the remainder of the data from the previous user and handle it as though it belongs to the attacker. (Bad Code) Java char[] byteArray = new char[1024]; for (IEnumerator i=users.GetEnumerator(); i.MoveNext()
;i.Current()) { String userName = (String) i.Current();
String pFileName = PFILE_ROOT + "/" + userName;
StreamReader sr = new StreamReader(pFileName);
sr.Read(byteArray,0,1024);//the file is always 1k bytes
sr.Close();
processPFile(userName, byteArray);
} (Bad Code) Java FileInputStream fis; byte[] byteArray = new byte[1024]; for (Iterator i=users.iterator(); i.hasNext();) { String userName = (String) i.next();
String pFileName = PFILE_ROOT + "/" + userName;
FileInputStream fis = new FileInputStream(pFileName);
fis.read(byteArray); // the file is always 1k bytes
fis.close();
processPFile(userName, byteArray);
Example 4 The following code does not check to see if the string returned by getParameter() is null before calling the member function compareTo(), potentially causing a NULL dereference. (Bad Code) Java String itemName = request.getParameter(ITEM_NAME); if (itemName.compareTo(IMPORTANT_ITEM)) { ...
} ... The following code does not check to see if the string returned by theItem property is null before calling the member function Equals(), potentially causing a NULL dereference. string itemName = request.Item(ITEM_NAME); (Bad Code) if (itemName.Equals(IMPORTANT_ITEM)) { ...
} ... The traditional defense of this coding error is: "I know the requested value will always exist because.... If it does not exist, the program cannot perform the desired behavior so it doesn't matter whether I handle the error or simply allow the program to die dereferencing a null value." But attackers are skilled at finding unexpected paths through programs, particularly when exceptions are involved. Example 5 The following code shows a system property that is set to null and later dereferenced by a programmer who mistakenly assumes it will always be defined. (Bad Code) System.clearProperty("os.name"); ... String os = System.getProperty("os.name"); if (os.equalsIgnoreCase("Windows 95")) System.out.println("Not
supported"); The traditional defense of this coding error is: "I know the requested value will always exist because.... If it does not exist, the program cannot perform the desired behavior so it doesn't matter whether I handle the error or simply allow the program to die dereferencing a null value." But attackers are skilled at finding unexpected paths through programs, particularly when exceptions are involved. Example 6 The following VB.NET code does not check to make sure that it has read 50 bytes from myfile.txt. This can cause DoDangerousOperation() to operate on an unexpected value. (Bad Code) Dim MyFile As New FileStream("myfile.txt", FileMode.Open,
FileAccess.Read, FileShare.Read) Dim MyArray(50) As Byte MyFile.Read(MyArray, 0, 50) DoDangerousOperation(MyArray(20)) In .NET, it is not uncommon for programmers to misunderstand Read() and related methods that are part of many System.IO classes. The stream and reader classes do not consider it to be unusual or exceptional if only a small amount of data becomes available. These classes simply add the small amount of data to the return buffer, and set the return value to the number of bytes or characters read. There is no guarantee that the amount of data returned is equal to the amount of data requested. Example 7 It is not uncommon for Java programmers to misunderstand read() and related methods that are part of many java.io classes. Most errors and unusual events in Java result in an exception being thrown. But the stream and reader classes do not consider it unusual or exceptional if only a small amount of data becomes available. These classes simply add the small amount of data to the return buffer, and set the return value to the number of bytes or characters read. There is no guarantee that the amount of data returned is equal to the amount of data requested. This behavior makes it important for programmers to examine the return value from read() and other IO methods to ensure that they receive the amount of data they expect. Observed Examples
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Page Last Updated:
October 29, 2009
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CWE is a Software Assurance strategic initiative sponsored by the National Cyber Security Division of the U.S. Department of Homeland Security. This Web site is hosted by The MITRE Corporation. Contact cwe@mitre.org for more information. |
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