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Improper Check for Unusual or Exceptional Conditions

ID: 754Date: (C)2012-05-14   (M)2017-11-07
Type: weaknessStatus: INCOMPLETE
Abstraction Type: Class


The software does not check or improperly checks for unusual or exceptional conditions that are not expected to occur frequently during day to day operation of the software.

Extended Description

The programmer may assume that certain events or conditions will never occur or do not need to be worried about, such as low memory conditions, lack of access to resources due to restrictive permissions, or misbehaving clients or components. However, attackers may intentionally trigger these unusual conditions, thus violating the programmer's assumptions, possibly introducing instability, incorrect behavior, or a vulnerability.

Note that this entry is not exclusively about the use of exceptions and exception handling, which are mechanisms for both checking and handling unusual or unexpected conditions.

Likelihood of Exploit: Medium

Applicable Platforms
Language Class: Language-independent

Time Of Introduction

  • Implementation

Related Attack Patterns

Common Consequences

ScopeTechnical ImpactNotes
DoS: crash / exit / restart
Unexpected state
The data which were produced as a result of a function call could be in a bad state upon return. If the return value is not checked, then this bad data may be used in operations, possibly leading to a crash or other unintended behaviors.

Detection Methods

Automated Static Analysis
Automated static analysis may be useful for detecting unusual conditions involving system resources or common programming idioms, but not for violations of business rules.
Manual Dynamic Analysis
Identify error conditions that are not likely to occur during normal usage and trigger them. For example, run the program under low memory conditions, run with insufficient privileges or permissions, interrupt a transaction before it is completed, or disable connectivity to basic network services such as DNS. Monitor the software for any unexpected behavior. If you trigger an unhandled exception or similar error that was discovered and handled by the application's environment, it may still indicate unexpected conditions that were not handled by the application itself.

Potential Mitigations

Language Selection
Use a language that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid.
Choose languages with features such as exception handling that force the programmer to anticipate unusual conditions that may generate exceptions. Custom exceptions may need to be developed to handle unusual business-logic conditions. Be careful not to pass sensitive exceptions back to the user (CWE-209, CWE-248).
 Check the results of all functions that return a value and verify that the value is expected.
Checking the return value of the function will typically be sufficient, however beware of race conditions (CWE-362) in a concurrent environment.
 If using exception handling, catch and throw specific exceptions instead of overly-general exceptions (CWE-396, CWE-397). Catch and handle exceptions as locally as possible so that exceptions do not propagate too far up the call stack (CWE-705). Avoid unchecked or uncaught exceptions where feasible (CWE-248).
Using specific exceptions, and ensuring that exceptions are checked, helps programmers to anticipate and appropriately handle many unusual events that could occur.
 Ensure that error messages only contain minimal details that are useful to the intended audience, and nobody else. The messages need to strike the balance between being too cryptic and not being cryptic enough. They should not necessarily reveal the methods that were used to determine the error. Such detailed information can be used to refine the original attack to increase the chances of success.
If errors must be tracked in some detail, capture them in log messages - but consider what could occur if the log messages can be viewed by attackers. Avoid recording highly sensitive information such as passwords in any form. Avoid inconsistent messaging that might accidentally tip off an attacker about internal state, such as whether a username is valid or not.
Exposing additional information to a potential attacker in the context of an exceptional condition can help the attacker determine what attack vectors are most likely to succeed beyond DoS.
Input Validation
Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a whitelist of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does.
When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, "boat" may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as "red" or "blue."
Do not rely exclusively on looking for malicious or malformed inputs (i.e., do not rely on a blacklist). A blacklist is likely to miss at least one undesirable input, especially if the code's environment changes. This can give attackers enough room to bypass the intended validation. However, blacklists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.
 Performing extensive input validation does not help with handling unusual conditions, but it will minimize their occurrences and will make it more difficult for attackers to trigger them.
Architecture and Design
 If the program must fail, ensure that it fails gracefully (fails closed). There may be a temptation to simply let the program fail poorly in cases such as low memory conditions, but an attacker may be able to assert control before the software has fully exited. Alternately, an uncontrolled failure could cause cascading problems with other downstream components; for example, the program could send a signal to a downstream process so the process immediately knows that a problem has occurred and has a better chance of recovery.
Architecture and Design
 Use system limits, which should help to prevent resource exhaustion. However, the software should still handle low resource conditions since they may still occur.

Sometimes, when a return value can be used to indicate an error, an unchecked return value is a code-layer instance of a missing application-layer check for exceptional conditions. However, return values are not always needed to communicate exceptional conditions. For example, expiration of resources, values passed by reference, asynchronously modified data, sockets, etc. may indicate exceptional conditions without the use of a return value.

Related CWETypeViewChain
CWE-754 ChildOf CWE-889 Category CWE-888  

Demonstrative Examples   (Details)

  1. Consider the following code segment: (Demonstrative Example Id DX-7)
  2. In the following C/C++ example the method outputStringToFile opens a file in the local filesystem and outputs a string to the file. The input parameters output and filename contain the string to output to the file and the name of the file respectively.
  3. In the following Java example the method readFromFile uses a FileReader object to read the contents of a file. The FileReader object is created using the File object readFile, the readFile object is initialized using the setInputFile method. The setInputFile method should be called before calling the readFromFile method.
  4. 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. (Demonstrative Example Id DX-12)
  5. The following code does not check to see if memory allocation succeeded before attempting to use the pointer returned by malloc(). (Demonstrative Example Id DX-8)
  6. 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. (Demonstrative Example Id DX-10)
  7. 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. (Demonstrative Example Id DX-9)
  8. 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. (Demonstrative Example Id DX-11)
  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. (Demonstrative Example Id DX-1)

Observed Examples

  1. CVE-2007-3798 : Unchecked return value leads to resultant integer overflow and code execution.
  2. CVE-2006-4447 : 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.
  3. CVE-2006-2916 : 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.

For more examples, refer to CVE relations in the bottom box.

White Box Definitions

Black Box Definitions

Taxynomy Mappings

CERT C++ Secure Coding MEM32-CPP
Detect and handle memory allocation errors
CERT C++ Secure Coding ERR39-CPP
Guarantee exception safety
CERT C Secure Coding MEM32-C
Detect and handle memory allocation errors


  1. Mark Dowd John McDonald Justin Schuh .The Art of Software Security Assessment 1st Edition. Addison Wesley. Section:'Chapter 7, "Program Building Blocks" Page 341'. Published on 2006.
  2. Mark Dowd John McDonald Justin Schuh .The Art of Software Security Assessment 1st Edition. Addison Wesley. Section:'Chapter 1, "Exceptional Conditions," Page 22'. Published on 2006.
  3. Michael Howard David LeBlanc John Viega .24 Deadly Sins of Software Security. McGraw-Hill. Section:'"Sin 11: Failure to Handle Errors Correctly." Page 183'. Published on 2010.
  4. Frank Kim .Top 25 Series - Rank 15 - Improper Check for Unusual or Exceptional Conditions. SANS Software Security Institute. 2010-03-15.

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