1. Home
  2. CWE Database
  3. CWE-787
Base · Medium

CWE-787: Out-of-bounds Write

The product writes data past the end, or before the beginning, of the intended buffer.

CWE-787 · Base Level ·17 CVEs ·7 Mitigations

Description

The product writes data past the end, or before the beginning, of the intended buffer.

Potential Impact

Integrity

Modify Memory, Execute Unauthorized Code or Commands

Availability

DoS: Crash, Exit, or Restart

Other

Unexpected State

Demonstrative Examples

The following code attempts to save four different identification numbers into an array.
Bad
int id_sequence[3];
                     /* Populate the id array. */
                     id_sequence[0] = 123;id_sequence[1] = 234;id_sequence[2] = 345;id_sequence[3] = 456;
Since the array is only allocated to hold three elements, the valid indices are 0 to 2; so, the assignment to id_sequence[3] is out of bounds.
In the following code, 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).
This code takes an IP address from the user and verifies that it is well formed. It 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. 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).
This code applies an encoding procedure to an input string and stores it into a buffer.
Bad
char * copy_input(char *user_supplied_string){
                        int i, dst_index;char *dst_buf = (char*)malloc(4*sizeof(char) * MAX_SIZE);if ( MAX_SIZE <= strlen(user_supplied_string) ){die("user string too long, die evil hacker!");}dst_index = 0;for ( i = 0; i < strlen(user_supplied_string); i++ ){
                              if( '&' == user_supplied_string[i] ){dst_buf[dst_index++] = '&';dst_buf[dst_index++] = 'a';dst_buf[dst_index++] = 'm';dst_buf[dst_index++] = 'p';dst_buf[dst_index++] = ';';}else if ('<' == user_supplied_string[i] ){
                                    
                                       
                                       /* encode to &lt; */
                                       
                                    
                                 }else dst_buf[dst_index++] = user_supplied_string[i];
                           }return dst_buf;
                     }
The programmer attempts to encode the ampersand character in the user-controlled string. However, the length of the string is validated before the encoding procedure is applied. Furthermore, the programmer assumes encoding expansion will only expand a given character by a factor of 4, while the encoding of the ampersand expands by 5. As a result, when the encoding procedure expands the string it is possible to overflow the destination buffer if the attacker provides a string of many ampersands.

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:

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].

Implementation Moderate

Replace unbounded copy functions with analogous functions that support length arguments, such as strcpy with strncpy. Create these if they are not available.

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 — 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
  • 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 IDDescription
CVE-2025-27363Font rendering library does not properly handle assigning a signed short value to an unsigned long (CWE-195), leading to an integer wraparound (CWE-190), c
CVE-2023-1017The reference implementation code for a Trusted Platform Module does not implement length checks on data, allowing for an attacker to write 2 bytes past the end of a buffer.
CVE-2021-21220Chain: insufficient input validation (CWE-20) in browser allows heap corruption (CWE-787), as exploited in the wild per CISA KEV.
CVE-2021-28664GPU kernel driver allows memory corruption because a user can obtain read/write access to read-only pages, as exploited in the wild per CISA KEV.
CVE-2020-17087Chain: integer truncation (CWE-197) causes small buffer allocation (CWE-131) leading to out-of-bounds write (CWE-787) in kernel pool, as exploited in the wild per CISA KEV.
CVE-2020-1054Out-of-bounds write in kernel-mode driver, as exploited in the wild per CISA KEV.
CVE-2020-0041Escape from browser sandbox using out-of-bounds write due to incorrect bounds check, as exploited in the wild per CISA KEV.
CVE-2020-0968Memory corruption in web browser scripting engine, as exploited in the wild per CISA KEV.
CVE-2020-0022chain: mobile phone Bluetooth implementation does not include offset when calculating packet length (CWE-682), leading to out-of-bounds write (CWE-787)
CVE-2019-1010006Chain: compiler optimization (CWE-733) removes or modifies code used to detect integer overflow (CWE-190), allowing out-of-bounds write (CWE-787).
CVE-2009-1532malformed inputs cause accesses of uninitialized or previously-deleted objects, leading to memory corruption
CVE-2009-0269chain: -1 value from a function call was intended to indicate an error, but is used as an array index instead.
CVE-2002-2227Unchecked length of SSLv2 challenge value leads to buffer underflow.
CVE-2007-4580Buffer underflow from a small size value with a large buffer (length parameter inconsistency, CWE-130)
CVE-2007-4268Chain: integer signedness error (CWE-195) passes signed comparison, leading to heap overflow (CWE-122)

Showing 15 of 17 observed examples.

CWE-119: Improper Restriction of Operations within the Bounds of a Memory Buffer

The product performs operations on a memory buffer, but it reads from or writes to a memory location outside the buffer'...

Taxonomy Mappings

  • ISA/IEC 62443: Part 3-3 — Req SR 3.5
  • ISA/IEC 62443: Part 4-1 — Req SI-1
  • ISA/IEC 62443: Part 4-1 — Req SI-2
  • ISA/IEC 62443: Part 4-1 — Req SVV-1
  • ISA/IEC 62443: Part 4-1 — Req SVV-3
  • ISA/IEC 62443: Part 4-2 — Req CR 3.5

Frequently Asked Questions

What is CWE-787?

CWE-787 (Out-of-bounds Write) is a software weakness identified by MITRE's Common Weakness Enumeration. It is classified as a Base-level weakness. The product writes data past the end, or before the beginning, of the intended buffer.

How can CWE-787 be exploited?

Attackers can exploit CWE-787 (Out-of-bounds Write) to modify memory, execute unauthorized code or commands. This weakness is typically introduced during the Implementation phase of software development.

How do I prevent CWE-787?

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-787?

CWE-787 is classified as a Base-level weakness (Medium abstraction). It has been observed in 17 real-world CVEs.