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CWE-252: Unchecked Return Value

 
Unchecked Return Value
Weakness ID: 252 (Weakness Base)Status: Draft
+ Description

Description 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
  • Implementation
+ Applicable Platforms

Languages

All

+ Common Consequences
ScopeEffect
Availability
Integrity

Technical Impact: Unexpected state; DoS: crash / exit / restart

An unexpected return value could place the system in a state that could lead to a crash or other unintended behaviors.

+ Likelihood of Exploit

Low

+ Demonstrative Examples

Example 1

Consider the following code segment:

(Bad Code)
Example Language:
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

In the following example, it is possible to request that memcpy move a much larger segment of memory than assumed:

(Bad Code)
Example Language:
int returnChunkSize(void *) {
/* if chunk info is valid, return the size of usable memory,
* else, return -1 to indicate an error
*/
...
}
int main() {
...
memcpy(destBuf, srcBuf, (returnChunkSize(destBuf)-1));
...
}

If returnChunkSize() happens to encounter an error it will return -1. Notice that the return value is not checked before the memcpy operation (CWE-252), so -1 can be passed as the size argument to memcpy() (CWE-805). Because memcpy() assumes that the value is unsigned, it will be interpreted as MAXINT-1 (CWE-195), and therefore will copy far more memory than is likely available to the destination buffer (CWE-787, CWE-788).

Example 3

The following code does not check to see if memory allocation succeeded before attempting to use the pointer returned by malloc().

(Bad Code)
Example Language:
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 4

The following examples read a file into a byte array.

(Bad Code)
Example Language: C# 
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)
Example Language: 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);

The 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 treat it as though it belongs to the attacker.

Example 5

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)
Example Language: 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 6

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 7

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 8

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.

Example 9

This example takes an IP address from a user, verifies that it is well formed and then looks up the hostname and copies it into a buffer.

(Bad Code)
Example Language:
void host_lookup(char *user_supplied_addr){
struct hostent *hp;
in_addr_t *addr;
char hostname[64];
in_addr_t inet_addr(const char *cp);

/*routine that ensures user_supplied_addr is in the right format for conversion */
validate_addr_form(user_supplied_addr);
addr = inet_addr(user_supplied_addr);
hp = gethostbyaddr( addr, sizeof(struct in_addr), AF_INET);
strcpy(hostname, hp->h_name);
}

If an attacker provides an address that appears to be well-formed, but the address does not resolve to a hostname, then the call to gethostbyaddr() will return NULL. When this occurs, a NULL pointer dereference (CWE-476) will occur in the call to strcpy().

Note that this example is also vulnerable to a buffer overflow (see CWE-119).

Example 10

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

(Bad Code)
Example Language:
void f(pthread_mutex_t *mutex) {
pthread_mutex_lock(mutex);

/* access shared resource */

pthread_mutex_unlock(mutex);
}

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 them to higher levels.

(Good Code)
 
int f(pthread_mutex_t *mutex) {
int result;

result = pthread_mutex_lock(mutex);
if (0 != result)
return result;

/* access shared resource */

return pthread_mutex_unlock(mutex);
}
+ Observed Examples
ReferenceDescription
Unchecked return value leads to resultant integer overflow and code execution.
Program does not check return value when invoking functions to drop privileges, which could leave users with higher privileges than expected by forcing those functions to fail.
Program does not check return value when invoking functions to drop privileges, which could leave users with higher privileges than expected by forcing those functions to fail.
chain: unchecked return value can lead to NULL dereference
chain: unchecked return value (CWE-252) leads to free of invalid, uninitialized pointer (CWE-824).
+ Potential Mitigations

Phase: Implementation

Check the results of all functions that return a value and verify that the value is expected.

Effectiveness: High

Checking the return value of the function will typically be sufficient, however beware of race conditions (CWE-362) in a concurrent environment.

Phase: Implementation

Ensure that you account for all possible return values from the function.

Phase: Implementation

When designing a function, make sure you return a value or throw an exception in case of an error.

+ Background Details

Many functions will return some value about the success of their actions. This will alert the program whether or not to handle any errors caused by that function.

+ Relationships
NatureTypeIDNameView(s) this relationship pertains toView(s)Named Chain(s) this relationship pertains toChain(s)
ChildOfWeakness ClassWeakness Class227Improper Fulfillment of API Contract ('API Abuse')
Development Concepts (primary)699
Seven Pernicious Kingdoms (primary)700
ChildOfCategoryCategory389Error Conditions, Return Values, Status Codes
Development Concepts699
ChildOfCategoryCategory728OWASP Top Ten 2004 Category A7 - Improper Error Handling
Weaknesses in OWASP Top Ten (2004) (primary)711
ChildOfCategoryCategory742CERT C Secure Coding Section 08 - Memory Management (MEM)
Weaknesses Addressed by the CERT C Secure Coding Standard (primary)734
ChildOfWeakness ClassWeakness Class754Improper Check for Unusual or Exceptional Conditions
Research Concepts (primary)1000
ChildOfCategoryCategory847CERT Java Secure Coding Section 02 - Expressions (EXP)
Weaknesses Addressed by the CERT Java Secure Coding Standard (primary)844
ChildOfCategoryCategory876CERT C++ Secure Coding Section 08 - Memory Management (MEM)
Weaknesses Addressed by the CERT C++ Secure Coding Standard (primary)868
ChildOfCategoryCategory962SFP Secondary Cluster: Unchecked Status Condition
Software Fault Pattern (SFP) Clusters (primary)888
CanPrecedeWeakness BaseWeakness Base476NULL Pointer Dereference
Research Concepts1000
Unchecked Return Value to NULL Pointer Dereference690
StartsChainCompound Element: ChainCompound Element: Chain690Unchecked Return Value to NULL Pointer Dereference
Named Chains709
Unchecked Return Value to NULL Pointer Dereference690
MemberOfViewView884CWE Cross-section
CWE Cross-section (primary)884
PeerOfWeakness BaseWeakness Base273Improper Check for Dropped Privileges
Research Concepts1000
+ Taxonomy Mappings
Mapped Taxonomy NameNode IDFitMapped Node Name
7 Pernicious KingdomsUnchecked Return Value
CLASPIgnored function return value
OWASP Top Ten 2004A7CWE More SpecificImproper Error Handling
CERT C Secure CodingMEM32-CDetect and handle memory allocation errors
CERT Java Secure CodingEXP00-JDo not ignore values returned by methods
CERT C++ Secure CodingMEM32-CPPDetect and handle memory allocation errors
Software Fault PatternsSFP4Unchecked Status Condition
+ References
[REF-7] Mark Dowd, John McDonald and Justin Schuh. "The Art of Software Security Assessment". Chapter 7, "Program Building Blocks" Page 341.. 1st Edition. Addison Wesley. 2006.
[REF-11] M. Howard and D. LeBlanc. "Writing Secure Code". Chapter 20, "Checking Returns" Page 624. 2nd Edition. Microsoft. 2002.
[REF-17] Michael Howard, David LeBlanc and John Viega. "24 Deadly Sins of Software Security". "Sin 11: Failure to Handle Errors Correctly." Page 183. McGraw-Hill. 2010.
+ Content History
Submissions
Submission DateSubmitterOrganizationSource
7 Pernicious KingdomsExternally Mined
Contributions
Contribution DateContributorOrganizationSource
2010-04-30Martin SeborCisco Systems, Inc. Content
Provided Demonstrative Example and suggested CERT reference
Modifications
Modification DateModifierOrganizationSource
2008-09-08CWE Content TeamMITREInternal
updated Common_Consequences, Relationships, Other_Notes, Taxonomy_Mappings
2008-11-24CWE Content TeamMITREInternal
updated Relationships, Taxonomy_Mappings
2009-01-12CWE Content TeamMITREInternal
updated Background_Details, Demonstrative_Examples, Description, Observed_Examples, Other_Notes, Potential_Mitigations
2009-03-10CWE Content TeamMITREInternal
updated Relationships
2009-05-27CWE Content TeamMITREInternal
updated Demonstrative_Examples
2009-07-27CWE Content TeamMITREInternal
updated Demonstrative_Examples
2009-12-28CWE Content TeamMITREInternal
updated Common_Consequences, Demonstrative_Examples, References
2010-02-16CWE Content TeamMITREInternal
updated Demonstrative_Examples, Potential_Mitigations, References
2010-04-05CWE Content TeamMITREInternal
updated Demonstrative_Examples
2010-06-21CWE Content TeamMITREInternal
updated Demonstrative_Examples, References
2010-09-27CWE Content TeamMITREInternal
updated Observed_Examples
2010-12-13CWE Content TeamMITREInternal
updated Demonstrative_Examples
2011-06-01CWE Content TeamMITREInternal
updated Common_Consequences, Demonstrative_Examples, Relationships, Taxonomy_Mappings
2011-06-27CWE Content TeamMITREInternal
updated Common_Consequences
2011-09-13CWE Content TeamMITREInternal
updated Relationships, Taxonomy_Mappings
2012-05-11CWE Content TeamMITREInternal
updated Common_Consequences, References, Relationships
2014-06-23CWE Content TeamMITREInternal
updated Demonstrative_Examples, Potential_Mitigations
2014-07-30CWE Content TeamMITREInternal
updated Demonstrative_Examples, Relationships, Taxonomy_Mappings
Page Last Updated: July 30, 2014