Improper Validation of Array Index
Draft Variant
Structure: Simple
Description
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.
Common Consequences 5
Scope: IntegrityAvailability
Impact: DoS: Crash, Exit, or 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.
Scope: Integrity
Impact: Modify Memory
If the memory corrupted is data, rather than instructions, the system will continue to function with improper values.
Scope: ConfidentialityIntegrity
Impact: Modify MemoryRead 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.
Scope: IntegrityConfidentialityAvailability
Impact: 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.
Scope: IntegrityAvailabilityConfidentiality
Impact: DoS: Crash, Exit, or RestartExecute Unauthorized Code or CommandsRead MemoryModify Memory
A single fault could allow either an overflow (Access of Memory Location After End of Buffer) or underflow (Access of Memory Location Before Start of Buffer) 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 3
Automated Static AnalysisHigh
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 9
Phase: Architecture and Design
Strategy: Input Validation
Use an input validation framework such as Struts or the OWASP ESAPI Validation API. Note that using a framework does not automatically address all input validation problems; be mindful of weaknesses that could arise from misusing the framework itself (Improper Use of Validation Framework).
Phase: 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 Client-Side Enforcement of Server-Side Security. 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.
Phase: Requirements
Strategy: 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.
Phase: OperationBuild and Compilation
Strategy: Environment Hardening
Run or compile the software using features or extensions that randomly arrange the positions of a program's executable and libraries in memory. Because this makes the addresses unpredictable, it can prevent an attacker from reliably jumping to exploitable code.
Examples include Address Space Layout Randomization (ASLR) [REF-58] [REF-60] and Position-Independent Executables (PIE) [REF-64]. Imported modules may be similarly realigned if their default memory addresses conflict with other modules, in a process known as "rebasing" (for Windows) and "prelinking" (for Linux) [REF-1332] using randomly generated addresses. ASLR for libraries cannot be used in conjunction with prelink since it would require relocating the libraries at run-time, defeating the whole purpose of prelinking.
For more information on these techniques see D3-SAOR (Segment Address Offset Randomization) from D3FEND [REF-1335].
Effectiveness: Defense in Depth
Phase: Operation
Strategy: Environment Hardening
Use a CPU and operating system that offers Data Execution Protection (using hardware NX or XD bits) or the equivalent techniques that simulate this feature in software, such as PaX [REF-60] [REF-61]. These techniques ensure that any instruction executed is exclusively at a memory address that is part of the code segment.
For more information on these techniques see D3-PSEP (Process Segment Execution Prevention) from D3FEND [REF-1336].
Effectiveness: Defense in Depth
Phase: Implementation
Strategy: Input Validation
Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list 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. This 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, denylists 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.
Phase: Implementation
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.
Phase: Architecture and DesignOperation
Strategy: Environment Hardening
Run your code using the lowest privileges that are required to accomplish the necessary tasks [REF-76]. 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.
Phase: Architecture and DesignOperation
Strategy: 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 Creation of chroot Jail Without Changing Working Directory and other weaknesses related to jails.
Effectiveness: Limited
Demonstrative Examples 6
In the code snippet below, an untrusted integer value is used to reference an object in an array.
Code Example:
Bad
Java
javaIf index is outside of the range of the array, this may result in an ArrayIndexOutOfBounds Exception being raised.
ID : DX-34
The following example takes a user-supplied value to allocate an array of objects and then operates on the array.Code Example:
Bad
Java
javaThis example attempts to build a list from a user-specified value, and even checks to ensure a non-negative value is supplied. If, however, a 0 value is provided, the code will build an array of size 0 and then try to store a new Widget in the first location, causing an exception to be thrown.
ID : DX-100
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 methodCode Example:
Bad
C
c// check that the array index is less than the maximum*
ccHowever, this method only verifies that the given array index is less than the maximum length of the array but does not check for the minimum value (Numeric Range Comparison Without Minimum Check). This will allow a negative value to be accepted as the input array index, which will result in reading data before the beginning of the buffer (Buffer Under-read) and may allow access to sensitive memory. The input array index should be checked to verify that is within the maximum and minimum range required for the array (Improper Validation of Array Index). In this example the if statement should be modified to include a minimum range check, as shown below.
Code Example:
Good
C
c// check that the array index is within the correct*
cID : DX-134
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.Code Example:
Bad
C
/* capture the sizes of all messages / int getsizes(int sock, int count, int *sizes) { ``` ... char buf[BUFFER_SIZE]; int ok; int num, size;
c// continue read from socket until buf only contains '.'* if (DOTLINE(buf)) ``` break; else if (sscanf(buf, "%d %d", &num, &size) == 2) sizes[num - 1] = size; } ... }
In this example the message number retrieved from the buffer could be a value that is outside the allowable range of indices for the array and could possibly be a negative number. Without proper validation of the value to be used for the array index an array overflow could occur and could potentially lead to unauthorized access to memory addresses and system crashes. The value of the array index should be validated to ensure that it is within the allowable range of indices for the array as in the following code.
Code Example:
Good
C
/* capture the sizes of all messages / int getsizes(int sock, int count, int *sizes) { ``` ... char buf[BUFFER_SIZE]; int ok; int num, size;
c// continue read from socket until buf only contains '.'* if (DOTLINE(buf)) ``` break; else if (sscanf(buf, "%d %d", &num, &size) == 2) { if (num > 0 && num <= (unsigned)count) sizes[num - 1] = size; else
cID : DX-133
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.Code Example:
Bad
Java
// Method called from servlet to obtain product information* public String displayProductSummary(int index) {
javaIn this example the integer value used as the array index that is provided by the user may be outside the allowable range of indices for the array which may provide unexpected results or cause the application to fail. The integer value used for the array index should be validated to ensure that it is within the allowable range of indices for the array as in the following code.
Code Example:
Good
Java
// Method called from servlet to obtain product information* public String displayProductSummary(int index) {
javaAn alternative in Java would be to use one of the collection objects such as ArrayList that will automatically generate an exception if an attempt is made to access an array index that is out of bounds.
Code Example:
Good
Java
javaID : DX-90
The following example asks a user for an offset into an array to select an item.Code Example:
Bad
C
cThe programmer allows the user to specify which element in the list to select, however an attacker can provide an out-of-bounds offset, resulting in a buffer over-read (Buffer Over-read).
Observed Examples 6
CVE-2005-0369large ID in packet used as array index
CVE-2001-1009negative array index as argument to POP LIST command
CVE-2003-0721Integer signedness error leads to negative array index
CVE-2004-1189product does not properly track a count and a maximum number, which can lead to resultant array index overflow.
CVE-2007-5756Chain: device driver for packet-capturing software allows access to an unintended IOCTL with resultant array index error.
CVE-2005-2456Chain: array index error (Improper Validation of Array Index) leads to deadlock (Deadlock)
References 15
Writing Secure Code
Michael Howard and David LeBlanc
Microsoft Press
04-12-2002
ID: REF-7
Top 25 Series - Rank 14 - Improper Validation of Array Index
Jason Lam
SANS Software Security Institute
12-03-2010
ID: REF-96
Address Space Layout Randomization in Windows Vista
Michael Howard
ID: REF-58
Understanding DEP as a mitigation technology part 1
Microsoft
https://msrc.microsoft.com/blog/2009/06/understanding-dep-as-a-mitigation-technology-part-1/(2023-04-07)
ID: REF-61
Least Privilege
Sean Barnum and Michael Gegick
14-09-2005
ID: REF-76
24 Deadly Sins of Software Security
Michael Howard, David LeBlanc, and John Viega
McGraw-Hill
2010
ID: REF-44
Position Independent Executables (PIE)
Grant Murphy
Red Hat
28-11-2012
ID: REF-64
Automated Source Code Security Measure (ASCSM)
Object Management Group (OMG)
01-2016
ID: REF-962
The CLASP Application Security Process
Secure Software, Inc.
2005
ID: REF-18
Prelink and address space randomization
John Richard Moser
05-07-2006
https://lwn.net/Articles/190139/(2023-04-26)
ID: REF-1332
Jump Over ASLR: Attacking Branch Predictors to Bypass ASLR
Dmitry Evtyushkin, Dmitry Ponomarev, Nael Abu-Ghazaleh
2016
http://www.cs.ucr.edu/~nael/pubs/micro16.pdf(2023-04-26)
ID: REF-1333
Segment Address Offset Randomization (D3-SAOR)
D3FEND
2023
ID: REF-1335
Process Segment Execution Prevention (D3-PSEP)
D3FEND
2023
ID: REF-1336
Bypassing Browser Memory Protections: Setting back browser security by 10 years
Alexander Sotirov and Mark Dowd
2008
ID: REF-1337
Likelihood of Exploit
High
Applicable Platforms
Languages:
C : OftenC++ : OftenNot Language-Specific : Undetermined
Modes of Introduction
Implementation
Related Attack Patterns
Alternate Terms
out-of-bounds array index
index-out-of-range
array index underflow
Functional Areas
- Memory Management
Affected Resources
- Memory
Related Weaknesses
ChildOf:
Improper Input Validation (CWE-20)
CanPrecede:
Use of Out-of-range Pointer Offset (CWE-823)
Taxonomy Mapping
- CLASP
- PLOVER
- CERT C Secure Coding
- CERT C Secure Coding
- CERT C Secure Coding
- CERT C Secure Coding
- SEI CERT Perl Coding Standard
- OMG ASCSM
- Software Fault Patterns
Notes
RelationshipThis weakness can precede uncontrolled memory allocation (Memory Allocation with Excessive Size Value) in languages that automatically expand an array when an index is used that is larger than the size of the array, such as JavaScript.
TheoreticalAn improperly validated array index might lead directly to the always-incorrect behavior of "access of array using out-of-bounds index."