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Improper Validation of Array Index

ID: 129Date: (C)2012-05-14   (M)2017-11-16
Type: weaknessStatus: DRAFT
Abstraction Type: Base


The product uses untrusted input when calculating or using an array index, but the product does not validate or incorrectly validates the index to ensure the index references a valid position within the array.

Likelihood of Exploit: High

Applicable Platforms
Language: C
Language: Often
Language: C++
Language: Often
Language Class: Language-independent

Time Of Introduction

  • Implementation

Related Attack Patterns

Common Consequences

ScopeTechnical ImpactNotes
DoS: crash / exit / restart
Use of an index that is outside the bounds of an array will very likely result in the corruption of relevant memory and perhaps instructions, leading to a crash, if the values are outside of the valid memory area.
Modify memory
If the memory corrupted is data, rather than instructions, the system will continue to function with improper values.
Modify memory
Read memory
Use of an index that is outside the bounds of an array can also trigger out-of-bounds read or write operations, or operations on the wrong objects; i.e., "buffer overflows" are not always the result. This may result in the exposure or modification of sensitive data.
Execute unauthorized code or commands
If the memory accessible by the attacker can be effectively controlled, it may be possible to execute arbitrary code, as with a standard buffer overflow and possibly without the use of large inputs if a precise index can be controlled.
DoS: crash / exit / restart
Execute unauthorized code or commands
Read memory
Modify memory
A single fault could allow either an overflow (CWE-788) or underflow (CWE-786) of the array index. What happens next will depend on the type of operation being performed out of bounds, but can expose sensitive information, cause a system crash, or possibly lead to arbitrary code execution.

Detection Methods

Automated Static Analysis
This weakness can often be detected using automated static analysis tools. Many modern tools use data flow analysis or constraint-based techniques to minimize the number of false positives.
Automated static analysis generally does not account for environmental considerations when reporting out-of-bounds memory operations. This can make it difficult for users to determine which warnings should be investigated first. For example, an analysis tool might report array index errors that originate from command line arguments in a program that is not expected to run with setuid or other special privileges.
Automated Dynamic Analysis
This weakness can be detected using dynamic tools and techniques that interact with the software using large test suites with many diverse inputs, such as fuzz testing (fuzzing), robustness testing, and fault injection. The software's operation may slow down, but it should not become unstable, crash, or generate incorrect results.
Black Box
Black box methods might not get the needed code coverage within limited time constraints, and a dynamic test might not produce any noticeable side effects even if it is successful.

Potential Mitigations

Architecture and Design
Input Validation
Libraries or Frameworks
Use an input validation framework such as Struts or the OWASP ESAPI Validation API. If you use Struts, be mindful of weaknesses covered by the CWE-101 category.
Architecture and Design
 For any security checks that are performed on the client side, ensure that these checks are duplicated on the server side, in order to avoid CWE-602. Attackers can bypass the client-side checks by modifying values after the checks have been performed, or by changing the client to remove the client-side checks entirely. Then, these modified values would be submitted to the server.
Even though client-side checks provide minimal benefits with respect to server-side security, they are still useful. First, they can support intrusion detection. If the server receives input that should have been rejected by the client, then it may be an indication of an attack. Second, client-side error-checking can provide helpful feedback to the user about the expectations for valid input. Third, there may be a reduction in server-side processing time for accidental input errors, although this is typically a small savings.
Language Selection
Use a language that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid.
For example, Ada allows the programmer to constrain the values of a variable and languages such as Java and Ruby will allow the programmer to handle exceptions when an out-of-bounds index is accessed.
Environment Hardening
Use a feature like Address Space Layout Randomization (ASLR) [R.129.3] [R.129.4].
Defense in Depth
This is not a complete solution. However, it forces the attacker to guess an unknown value that changes every program execution. In addition, an attack could still cause a denial of service, since the typical response is to exit the application.
Environment Hardening
Use a CPU and operating system that offers Data Execution Protection (NX) or its equivalent [R.129.4] [R.129.5].
Defense in Depth
This is not a complete solution, since buffer overflows could be used to overwrite nearby variables to modify the software's state in dangerous ways. In addition, it cannot be used in cases in which self-modifying code is required. Finally, an attack could still cause a denial of service, since the typical response is to exit the application.
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.
When accessing a user-controlled array index, use a stringent range of values that are within the target array. Make sure that you do not allow negative values to be used. That is, verify the minimum as well as the maximum of the range of acceptable values.
 Be especially careful to validate all input when invoking code that crosses language boundaries, such as from an interpreted language to native code. This could create an unexpected interaction between the language boundaries. Ensure that you are not violating any of the expectations of the language with which you are interfacing. For example, even though Java may not be susceptible to buffer overflows, providing a large argument in a call to native code might trigger an overflow.
Architecture and Design
Environment Hardening
Run your code using the lowest privileges that are required to accomplish the necessary tasks [R.129.6]. 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.
Architecture and Design
Sandbox or Jail
Run the code in a "jail" or similar sandbox environment that enforces strict boundaries between the process and the operating system. This may effectively restrict which files can be accessed in a particular directory or which commands can be executed by the software.
OS-level examples include the Unix chroot jail, AppArmor, and SELinux. In general, managed code may provide some protection. For example, java.io.FilePermission in the Java SecurityManager allows the software to specify restrictions on file operations.
This may not be a feasible solution, and it only limits the impact to the operating system; the rest of the application may still be subject to compromise.
Be careful to avoid CWE-243 and other weaknesses related to jails.
The effectiveness of this mitigation depends on the prevention capabilities of the specific sandbox or jail being used and might only help to reduce the scope of an attack, such as restricting the attacker to certain system calls or limiting the portion of the file system that can be accessed.

This weakness can precede uncontrolled memory allocation (CWE-789) in languages that automatically expand an array when an index is used that is larger than the size of the array, such as JavaScript.

Related CWETypeViewChain
CWE-129 ChildOf CWE-890 Category CWE-888  

Demonstrative Examples   (Details)

  1. In the code snippet below, an untrusted integer value is used to reference an object in an array.
  2. In the following code, the method retrieves a value from an array at a specific array index location that is given as an input parameter to the method (Demonstrative Example Id DX-100)
  3. In the following example the method displayProductSummary is called from a Web service servlet to retrieve product summary information for display to the user. The servlet obtains the integer value of the product number from the user and passes it to the displayProductSummary method. The displayProductSummary method passes the integer value of the product number to the getProductSummary method which obtains the product summary from the array object containing the project summaries using the integer value of the product number as the array index.
  4. The following example asks a user for an offset into an array to select an item. (Demonstrative Example Id DX-90)
  5. The following example retrieves the sizes of messages for a pop3 mail server. The message sizes are retrieved from a socket that returns in a buffer the message number and the message size, the message number (num) and size (size) are extracted from the buffer and the message size is placed into an array using the message number for the array index.
  6. The following example takes a user-supplied value to allocate an array of objects and then operates on the array. (Demonstrative Example Id DX-34)

Observed Examples

  1. CVE-2005-0369 : large ID in packet used as array index
  2. CVE-2001-1009 : negative array index as argument to POP LIST command
  3. CVE-2003-0721 : Integer signedness error leads to negative array index
  4. CVE-2004-1189 : product does not properly track a count and a maximum number, which can lead to resultant array index overflow.
  5. CVE-2007-5756 : Chain: device driver for packet-capturing software allows access to an unintended IOCTL with resultant array index error.
  6. CVE-2005-2456 : Chain: array index error (CWE-129) leads to deadlock (CWE-833)

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

White Box Definitions

Black Box Definitions

Taxynomy Mappings

CLASP  Unchecked array indexing
PLOVER  INDEX - Array index overflow
CERT C Secure Coding ARR00-C
Understand how arrays work
CERT C Secure Coding ARR30-C
Guarantee that array indices are within the valid range
CERT C Secure Coding ARR38-C
Do not add or subtract an integer to a pointer if the resulting value does not refer to a valid array element
CERT C Secure Coding INT32-C
Ensure that operations on signed integers do not result in overflow
CERT C++ Secure Coding INT10-CPP
Do not assume a positive remainder when using the % operator
CERT C++ Secure Coding INT32-CPP
Ensure that operations on signed integers do not result in overflow
CERT C++ Secure Coding ARR00-CPP
Understand when to prefer vectors over arrays
CERT C++ Secure Coding ARR30-CPP
Guarantee that array and vector indices are within the valid range
CERT C++ Secure Coding ARR38-CPP
Do not add or subtract an integer to a pointer or iterator if the resulting value does not refer to a valid element in the array or container


  1. M. Howard D. LeBlanc .Writing Secure Code 2nd Edition. Microsoft. Section:'Chapter 5, "Array Indexing Errors" Page 144'. Published on 2002.
  2. Jason Lam .Top 25 Series - Rank 14 - Improper Validation of Array Index. SANS Software Security Institute. 2010-03-12.
  3. Michael Howard .Address Space Layout Randomization in Windows Vista.
  4. .PaX.
  5. Microsoft .Understanding DEP as a mitigation technology part 1.
  6. Sean Barnum Michael Gegick .Least Privilege. Published on 2005-09-14.
  7. Michael Howard David LeBlanc John Viega .24 Deadly Sins of Software Security. McGraw-Hill. Section:'"Sin 5: Buffer Overruns." Page 89'. Published on 2010.

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