CWE-805: Buffer Access with Incorrect Length Value

Description

The product uses a sequential operation to read or write a buffer, but it uses an incorrect length value that causes it to access memory that is outside of the bounds of the buffer.

When the length value exceeds the size of the destination, a buffer overflow could occur.

Potential Impact

Integrity, Confidentiality, Availability

Read Memory, Modify Memory, Execute Unauthorized Code or Commands

Availability

Modify Memory, DoS: Crash, Exit, or Restart, DoS: Resource Consumption (CPU)

Demonstrative Examples

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

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);
                     }

This function allocates a buffer of 64 bytes to store the hostname under the assumption that the maximum length value of hostname is 64 bytes, however there is no guarantee that the hostname will not be larger than 64 bytes. If an attacker specifies an address which resolves to a very large hostname, then the function may overwrite sensitive data or even relinquish control flow to the attacker.

Note that this example also contains an unchecked return value (CWE-252) that can lead to a NULL pointer dereference (CWE-476).

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

Bad

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).

In the following example, the source character string is copied to the dest character string using the method strncpy.

Bad

...char source[21] = "the character string";char dest[12];strncpy(dest, source, sizeof(source)-1);...

However, in the call to strncpy the source character string is used within the sizeof call to determine the number of characters to copy. This will create a buffer overflow as the size of the source character string is greater than the dest character string. The dest character string should be used within the sizeof call to ensure that the correct number of characters are copied, as shown below.

Good

...char source[21] = "the character string";char dest[12];strncpy(dest, source, sizeof(dest)-1);...

In this example, the method outputFilenameToLog outputs a filename to a log file. The method arguments include a pointer to a character string containing the file name and an integer for the number of characters in the string. The filename is copied to a buffer where the buffer size is set to a maximum size for inputs to the log file. The method then calls another method to save the contents of the buffer to the log file.

Bad

#define LOG_INPUT_SIZE 40
                     
                     // saves the file name to a log file
                     int outputFilenameToLog(char *filename, int length) {
                        int success;
                           
                           // buffer with size set to maximum size for input to log file
                           char buf[LOG_INPUT_SIZE];
                           
                           // copy filename to buffer
                           strncpy(buf, filename, length);
                           
                           // save to log file
                           success = saveToLogFile(buf);
                           return success;
                     }

However, in this case the string copy method, strncpy, mistakenly uses the length method argument to determine the number of characters to copy rather than using the size of the local character string, buf. This can lead to a buffer overflow if the number of characters contained in character string pointed to by filename is larger then the number of characters allowed for the local character string. The string copy method should use the buf character string within a sizeof call to ensure that only characters up to the size of the buf array are copied to avoid a buffer overflow, as shown below.

Good

...
                     // copy filename to buffer
                     strncpy(buf, filename, sizeof(buf)-1);...

Mitigations & Prevention

Requirements

Use a language that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid. For example, many languages that perform their own memory management, such as Java and Perl, are not subject to buffer overflows. Other languages, such as Ada and C#, typically provide overflow protection, but the protection can be disabled by the programmer. Be wary that a language's interface to native code may still be subject to ove

Architecture and Design

Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid. Examples include the Safe C String Library (SafeStr) by Messier and Viega [REF-57], and the Strsafe.h library from Microsoft [REF-56]. These libraries provide safer versions of overflow-prone string-handling functions.

OperationBuild and Compilation Defense in Depth

Use automatic buffer overflow detection mechanisms that are offered by certain compilers or compiler extensions. Examples include: the Microsoft Visual Studio /GS flag, Fedora/Red Hat FORTIFY_SOURCE GCC flag, StackGuard, and ProPolice, which provide various mechanisms including canary-based detection and range/index checking. D3-SFCV (Stack Frame Canary Validation) from D3FEND [REF-1334] discusses canary-based detection in detail.

Implementation

Consider adhering to the following rules when allocating and managing an application's memory:

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.

OperationBuild and Compilation Defense in Depth

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 mo

Operation Defense in Depth

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].

Architecture and DesignOperation

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 product or its environment. For example, database applications rarely need to run as the database administrator, especially in day-to-day operations.

Architecture and DesignOperation Limited

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

Detection Methods

Automated Static Analysis High — 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
Automated Dynamic Analysis Moderate — This weakness can be detected using dynamic tools and techniques that interact with the product using large test suites with many diverse inputs, such as fuzz testing (fuzzing), robustness testing, and fault injection. The product's operation may slow down, but it should not become unstable, crash,
Manual Analysis — Manual analysis can be useful for finding this weakness, but it might not achieve desired code coverage within limited time constraints. This becomes difficult for weaknesses that must be considered for all inputs, since the attack surface can be too large.
Automated Dynamic Analysis Moderate — Use tools that are integrated during compilation to insert runtime error-checking mechanisms related to memory safety errors, such as AddressSanitizer (ASan) for C/C++ [REF-1518].

Real-World CVE Examples

CVE ID	Description
CVE-2011-1959	Chain: large length value causes buffer over-read (CWE-126)
CVE-2011-1848	Use of packet length field to make a calculation, then copy into a fixed-size buffer
CVE-2011-0105	Chain: retrieval of length value from an uninitialized memory location
CVE-2011-0606	Crafted length value in document reader leads to buffer overflow
CVE-2011-0651	SSL server overflow when the sum of multiple length fields exceeds a given value
CVE-2010-4156	Language interpreter API function doesn't validate length argument, leading to information exposure

Taxonomy Mappings

CERT C Secure Coding: ARR38-C — Guarantee that library functions do not form invalid pointers

Frequently Asked Questions

What is CWE-805?

CWE-805 (Buffer Access with Incorrect Length Value) is a software weakness identified by MITRE's Common Weakness Enumeration. It is classified as a Base-level weakness. The product uses a sequential operation to read or write a buffer, but it uses an incorrect length value that causes it to access memory that is outside of the bounds of the buffer.

How can CWE-805 be exploited?

Attackers can exploit CWE-805 (Buffer Access with Incorrect Length Value) to read memory, modify memory, execute unauthorized code or commands. This weakness is typically introduced during the Implementation phase of software development.

How do I prevent CWE-805?

Key mitigations include: Use a language that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid. For example, many languages that perform their own memory

What is the severity of CWE-805?

CWE-805 is classified as a Base-level weakness (Medium abstraction). It has been observed in 6 real-world CVEs.