Out-of-bounds Write
Draft Base
Structure: Simple
Description
This vulnerability occurs when software incorrectly writes data outside the boundaries of its allocated memory buffer, either beyond the end or before the beginning.
An out-of-bounds write happens when a program fails to properly validate the target location for a write operation, such as copying data, assigning a value, or modifying memory. This can corrupt adjacent data structures, crash the application, or alter critical program logic, often leading to unpredictable behavior. It's a fundamental memory safety flaw commonly arising from incorrect pointer arithmetic, off-by-one errors, or using unsafe functions that don't check buffer sizes. Attackers frequently exploit this weakness to execute arbitrary code, escalate privileges, or cause denial-of-service. To prevent it, developers should use secure, bounds-checked functions, employ modern safe languages or libraries, and rigorously validate all indices and offsets before performing write operations. Static and dynamic analysis tools are also essential for catching these dangerous errors before deployment.
Common Consequences 3
Scope: Integrity
Impact: Modify MemoryExecute Unauthorized Code or Commands
Write operations could cause memory corruption. In some cases, an adversary can modify control data such as return addresses in order to execute unexpected code.
Scope: Availability
Impact: DoS: Crash, Exit, or Restart
Attempting to access out-of-range, invalid, or unauthorized memory could cause the product to crash.
Scope: Other
Impact: Unexpected State
Subsequent write operations can produce undefined or unexpected results.
Detection Methods 2
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 buffer overflows 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.Potential Mitigations 7
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, 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 overflows, even if the language itself is theoretically safe.
Phase: Architecture and Design
Strategy: Libraries or Frameworks
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.
Phase: OperationBuild and Compilation
Strategy: Environment Hardening
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.
Effectiveness: Defense in Depth
Phase: Implementation
Consider adhering to the following rules when allocating and managing an application's memory:
- Double check that the buffer is as large as specified.
- When using functions that accept a number of bytes to copy, such as strncpy(), be aware that if the destination buffer size is equal to the source buffer size, it may not NULL-terminate the string.
- Check buffer boundaries if accessing the buffer in a loop and make sure there is no danger of writing past the allocated space.
- If necessary, truncate all input strings to a reasonable length before passing them to the copy and concatenation functions.
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
Replace unbounded copy functions with analogous functions that support length arguments, such as strcpy with strncpy. Create these if they are not available.
Effectiveness: Moderate
Demonstrative Examples 7
The following code attempts to save four different identification numbers into an array.
Code Example:
Bad
C
cSince 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.
ID : DX-114
In the following code, it is possible to request that memcpy move a much larger segment of memory than assumed:Code Example:
Bad
C
c/* if chunk info is valid, return the size of usable memory,*
cIf returnChunkSize() happens to encounter an error it will return -1. Notice that the return value is not checked before the memcpy operation (Unchecked Return Value), so -1 can be passed as the size argument to memcpy() (Buffer Access with Incorrect Length Value). Because memcpy() assumes that the value is unsigned, it will be interpreted as MAXINT-1 (Signed to Unsigned Conversion Error), and therefore will copy far more memory than is likely available to the destination buffer (Out-of-bounds Write, Access of Memory Location After End of Buffer).
ID : DX-1
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.Code Example:
Bad
C
c/*routine that ensures user_supplied_addr is in the right format for conversion /
cThis 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 (Unchecked Return Value) that can lead to a NULL pointer dereference (NULL Pointer Dereference).
ID : DX-19
This code applies an encoding procedure to an input string and stores it into a buffer.Code Example:
Bad
C
c/* encode to < / } 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.
ID : DX-87
In the following C/C++ code, a utility function is used to trim trailing whitespace from a character string. The function copies the input string to a local character string and uses a while statement to remove the trailing whitespace by moving backward through the string and overwriting whitespace with a NUL character.Code Example:
Bad
C
c// copy input string to a temporary string* char message[length+1]; int index; for (index = 0; index < length; index++) { ``` message[index] = strMessage[index]; } message[index] = '\0';
c// return string without trailing whitespace* retMessage = message; return retMessage;}
However, this function can cause a buffer underwrite if the input character string contains all whitespace. On some systems the while statement will move backwards past the beginning of a character string and will call the isspace() function on an address outside of the bounds of the local buffer.
ID : DX-20
The following code allocates memory for a maximum number of widgets. It then gets a user-specified number of widgets, making sure that the user does not request too many. It then initializes the elements of the array using InitializeWidget(). Because the number of widgets can vary for each request, the code inserts a NULL pointer to signify the location of the last widget.Code Example:
Bad
C
cHowever, this code contains an off-by-one calculation error (Off-by-one Error). It allocates exactly enough space to contain the specified number of widgets, but it does not include the space for the NULL pointer. As a result, the allocated buffer is smaller than it is supposed to be (Incorrect Calculation of Buffer Size). So if the user ever requests MAX_NUM_WIDGETS, there is an out-of-bounds write (Out-of-bounds Write) when the NULL is assigned. Depending on the environment and compilation settings, this could cause memory corruption.
ID : DX-88
The following is an example of code that may result in a buffer underwrite. This code is attempting to replace the substring "Replace Me" in destBuf with the string stored in srcBuf. It does so by using the function strstr(), which returns a pointer to the found substring in destBuf. Using pointer arithmetic, the starting index of the substring is found.Code Example:
Bad
C
cIn the case where the substring is not found in destBuf, strstr() will return NULL, causing the pointer arithmetic to be undefined, potentially setting the value of idx to a negative number. If idx is negative, this will result in a buffer underwrite of destBuf.
Observed Examples 17
CVE-2025-27363Font rendering library does not properly handle assigning a signed short value to an unsigned long (Signed to Unsigned Conversion Error), leading to an integer wraparound (Integer Overflow or Wraparound), causing too small of a buffer (Incorrect Calculation of Buffer Size), leading to an out-of-bounds write (Out-of-bounds Write).
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 (Improper Input Validation) in browser allows heap corruption (Out-of-bounds Write), 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 (Numeric Truncation Error) causes small buffer allocation (Incorrect Calculation of Buffer Size) leading to out-of-bounds write (Out-of-bounds Write) 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 (Incorrect Calculation), leading to out-of-bounds write (Out-of-bounds Write)
CVE-2019-1010006Chain: compiler optimization (Compiler Optimization Removal or Modification of Security-critical Code) removes or modifies code used to detect integer overflow (Integer Overflow or Wraparound), allowing out-of-bounds write (Out-of-bounds Write).
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, Improper Handling of Length Parameter Inconsistency)
CVE-2007-4268Chain: integer signedness error (Signed to Unsigned Conversion Error) passes signed comparison, leading to heap overflow (Heap-based Buffer Overflow)
CVE-2009-2550Classic stack-based buffer overflow in media player using a long entry in a playlist
CVE-2009-2403Heap-based buffer overflow in media player using a long entry in a playlist
References 20
Smashing The Stack For Fun And Profit
Aleph One
Phrack
08-11-1996
ID: REF-1029
Writing Secure Code
Michael Howard and David LeBlanc
Microsoft Press
04-12-2002
ID: REF-7
Writing Secure Code
Michael Howard and David LeBlanc
Microsoft Press
04-12-2002
ID: REF-7
24 Deadly Sins of Software Security
Michael Howard, David LeBlanc, and John Viega
McGraw-Hill
2010
ID: REF-44
The Art of Software Security Assessment
Mark Dowd, John McDonald, and Justin Schuh
Addison Wesley
2006
ID: REF-62
The Art of Software Security Assessment
Mark Dowd, John McDonald, and Justin Schuh
Addison Wesley
2006
ID: REF-62
Buffer UNDERFLOWS: What do you know about it?
Vuln-Dev Mailing List
10-01-2004
https://seclists.org/vuln-dev/2004/Jan/22(2023-04-07)
ID: REF-90
Using the Strsafe.h Functions
Microsoft
ID: REF-56
Safe C String Library v1.0.3
Matt Messier and John Viega
http://www.gnu-darwin.org/www001/ports-1.5a-CURRENT/devel/safestr/work/safestr-1.0.3/doc/safestr.html(2023-04-07)
ID: REF-57
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
Position Independent Executables (PIE)
Grant Murphy
Red Hat
28-11-2012
ID: REF-64
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
Stack Frame Canary Validation (D3-SFCV)
D3FEND
2023
ID: REF-1334
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
Secure by Design Alert: Eliminating Buffer Overflow Vulnerabilities
Cybersecurity and Infrastructure Security Agency
12-02-2025
ID: REF-1477
Likelihood of Exploit
High
Applicable Platforms
Languages:
C : OftenC++ : OftenAssembly : Undetermined
Technologies:
ICS/OT : Often
Modes of Introduction
Implementation
Alternate Terms
Memory Corruption
Often used to describe the consequences of writing to memory outside the bounds of a buffer, or to memory that is otherwise invalid.Functional Areas
- Memory Management
Affected Resources
- Memory
Related Weaknesses
Taxonomy Mapping
- ISA/IEC 62443
- ISA/IEC 62443
- ISA/IEC 62443
- ISA/IEC 62443
- ISA/IEC 62443
- ISA/IEC 62443