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Write-what-where Condition

Draft Base
Structure: Simple
Description

A write-what-where condition occurs when an attacker can control both the data written and the exact memory location where it's written, often due to a severe memory corruption flaw like a buffer overflow.

Extended Description

This vulnerability is one of the most dangerous memory corruption issues. It gives an attacker near-total control over a program's execution by allowing them to overwrite critical data or code pointers in memory. Think of it as an attacker having a precise remote control to edit the program's own instruction manual while it's running, which can directly lead to arbitrary code execution. For developers, this highlights the critical importance of secure memory management. Preventing this condition requires rigorous bounds checking, using safe functions that limit write lengths, and employing modern security features like address space layout randomization (ASLR) and stack canaries. It's often the final, exploitable result of simpler bugs like buffer overflows, making those initial flaws far more severe.
Common Consequences 3
Scope: IntegrityConfidentialityAvailabilityAccess Control

Impact: Modify MemoryExecute Unauthorized Code or CommandsGain Privileges or Assume IdentityDoS: Crash, Exit, or RestartBypass Protection Mechanism

Clearly, write-what-where conditions can be used to write data to areas of memory outside the scope of a policy. Also, they almost invariably can be used to execute arbitrary code, which is usually outside the scope of a program's implicit security policy. If the attacker can overwrite a pointer's worth of memory (usually 32 or 64 bits), they can redirect a function pointer to their own malicious code. Even when the attacker can only modify a single byte arbitrary code execution can be possible. Sometimes this is because the same problem can be exploited repeatedly to the same effect. Other times it is because the attacker can overwrite security-critical application-specific data -- such as a flag indicating whether the user is an administrator.

Scope: IntegrityAvailability

Impact: DoS: Crash, Exit, or RestartModify Memory

Many memory accesses can lead to program termination, such as when writing to addresses that are invalid for the current process.

Scope: Access ControlOther

Impact: Bypass Protection MechanismOther

When the consequence is arbitrary code execution, this can often be used to subvert any other security service.
Potential Mitigations 2
Phase: Architecture and Design

Strategy: Language Selection

Use a language that provides appropriate memory abstractions.
Phase: Operation
Use OS-level preventative functionality integrated after the fact. Not a complete solution.
Demonstrative Examples 1
The classic example of a write-what-where condition occurs when the accounting information for memory allocations is overwritten in a particular fashion. Here is an example of potentially vulnerable code:

Code Example:

Bad
C
c
Vulnerability in this case is dependent on memory layout. The call to strcpy() can be used to write past the end of buf1, and, with a typical layout, can overwrite the accounting information that the system keeps for buf2 when it is allocated. Note that if the allocation header for buf2 can be overwritten, buf2 itself can be overwritten as well.
The allocation header will generally keep a linked list of memory "chunks". Particularly, there may be a "previous" chunk and a "next" chunk. Here, the previous chunk for buf2 will probably be buf1, and the next chunk may be null. When the free() occurs, most memory allocators will rewrite the linked list using data from buf2. Particularly, the "next" chunk for buf1 will be updated and the "previous" chunk for any subsequent chunk will be updated. The attacker can insert a memory address for the "next" chunk and a value to write into that memory address for the "previous" chunk.
This could be used to overwrite a function pointer that gets dereferenced later, replacing it with a memory address that the attacker has legitimate access to, where they have placed malicious code, resulting in arbitrary code execution.
Observed Examples 2
CVE-2019-19911Chain: Python library does not limit the resources used to process images that specify a very large number of bands (Improper Validation of Specified Quantity in Input), leading to excessive memory consumption (Memory Allocation with Excessive Size Value) or an integer overflow (Integer Overflow or Wraparound).
CVE-2022-0545Chain: 3D renderer has an integer overflow (Integer Overflow or Wraparound) leading to write-what-where condition (Write-what-where Condition) using a crafted image.
References 2
24 Deadly Sins of Software Security
Michael Howard, David LeBlanc, and John Viega
McGraw-Hill
2010
ID: REF-44
The CLASP Application Security Process
Secure Software, Inc.
2005
https://cwe.mitre.org/documents/sources/TheCLASPApplicationSecurityProcess.pdf(2024-11-17)
ID: REF-18
Likelihood of Exploit

High

Applicable Platforms
Languages:
C : UndeterminedC++ : Undetermined
Modes of Introduction
Implementation
Functional Areas
  1. Memory Management
Affected Resources
  1. Memory
Related Weaknesses
ChildOf: Out-of-bounds Write (CWE-787)
ChildOf: Out-of-bounds Write (CWE-787)
ChildOf: Improper Restriction of Operations within the Bounds of a Memory Buffer (CWE-119)
Taxonomy Mapping
  • CLASP
  • CERT C Secure Coding
  • CERT C Secure Coding
  • CERT C Secure Coding
  • CERT C Secure Coding
  • Software Fault Patterns