CWE - CWE-123: Write-what-where Condition (4.11)

Weakness ID: 123

Abstraction: Base
Structure: Simple

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Description

Any condition where the attacker has the ability to write an arbitrary value to an arbitrary location, often as the result of a buffer overflow.

Relationships

This table shows the weaknesses and high level categories that are related to this weakness. These relationships are defined as ChildOf, ParentOf, MemberOf and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as PeerOf and CanAlsoBe are defined to show similar weaknesses that the user may want to explore.

Relevant to the view "Research Concepts" (CWE-1000)

Nature	Type	ID	Name
ChildOf	Base - a weakness that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource.	787	Out-of-bounds Write
PeerOf	Variant - a weakness that is linked to a certain type of product, typically involving a specific language or technology. More specific than a Base weakness. Variant level weaknesses typically describe issues in terms of 3 to 5 of the following dimensions: behavior, property, technology, language, and resource.	415	Double Free
CanFollow	Base - a weakness that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource.	120	Buffer Copy without Checking Size of Input ('Classic Buffer Overflow')
CanFollow	Base - a weakness that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource.	134	Use of Externally-Controlled Format String
CanFollow	Base - a weakness that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource.	364	Signal Handler Race Condition
CanFollow	Variant - a weakness that is linked to a certain type of product, typically involving a specific language or technology. More specific than a Base weakness. Variant level weaknesses typically describe issues in terms of 3 to 5 of the following dimensions: behavior, property, technology, language, and resource.	416	Use After Free
CanFollow	Variant - a weakness that is linked to a certain type of product, typically involving a specific language or technology. More specific than a Base weakness. Variant level weaknesses typically describe issues in terms of 3 to 5 of the following dimensions: behavior, property, technology, language, and resource.	479	Signal Handler Use of a Non-reentrant Function
CanFollow	Variant - a weakness that is linked to a certain type of product, typically involving a specific language or technology. More specific than a Base weakness. Variant level weaknesses typically describe issues in terms of 3 to 5 of the following dimensions: behavior, property, technology, language, and resource.	590	Free of Memory not on the Heap

Relevant to the view "CISQ Quality Measures (2020)" (CWE-1305)

Nature	Type	ID	Name
ChildOf	Class - a weakness that is described in a very abstract fashion, typically independent of any specific language or technology. More specific than a Pillar Weakness, but more general than a Base Weakness. Class level weaknesses typically describe issues in terms of 1 or 2 of the following dimensions: behavior, property, and resource.	119	Improper Restriction of Operations within the Bounds of a Memory Buffer

Relevant to the view "CISQ Data Protection Measures" (CWE-1340)

Nature	Type	ID	Name
ChildOf	Class - a weakness that is described in a very abstract fashion, typically independent of any specific language or technology. More specific than a Pillar Weakness, but more general than a Base Weakness. Class level weaknesses typically describe issues in terms of 1 or 2 of the following dimensions: behavior, property, and resource.	119	Improper Restriction of Operations within the Bounds of a Memory Buffer

Modes Of Introduction

The different Modes of Introduction provide information about how and when this weakness may be introduced. The Phase identifies a point in the life cycle at which introduction may occur, while the Note provides a typical scenario related to introduction during the given phase.

Phase	Note
Implementation

Applicable Platforms

This listing shows possible areas for which the given weakness could appear. These may be for specific named Languages, Operating Systems, Architectures, Paradigms, Technologies, or a class of such platforms. The platform is listed along with how frequently the given weakness appears for that instance.

Languages

C (Undetermined Prevalence)

C++ (Undetermined Prevalence)

Common Consequences

This table specifies different individual consequences associated with the weakness. The Scope identifies the application security area that is violated, while the Impact describes the negative technical impact that arises if an adversary succeeds in exploiting this weakness. The Likelihood provides information about how likely the specific consequence is expected to be seen relative to the other consequences in the list. For example, there may be high likelihood that a weakness will be exploited to achieve a certain impact, but a low likelihood that it will be exploited to achieve a different impact.

Scope	Impact	Likelihood
Integrity Confidentiality Availability Access Control	Technical Impact: Modify Memory; Execute Unauthorized Code or Commands; Gain Privileges or Assume Identity; DoS: Crash, Exit, or Restart; Bypass 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.
Integrity Availability	Technical Impact: DoS: Crash, Exit, or Restart; Modify Memory Many memory accesses can lead to program termination, such as when writing to addresses that are invalid for the current process.
Access Control Other	Technical Impact: Bypass Protection Mechanism; Other When the consequence is arbitrary code execution, this can often be used to subvert any other security service.

Likelihood Of Exploit

High

Demonstrative Examples

Example 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:

(bad code)

Example Language: C

#define BUFSIZE 256
int main(int argc, char **argv) {

char *buf1 = (char *) malloc(BUFSIZE);
char *buf2 = (char *) malloc(BUFSIZE);
strcpy(buf1, argv[1]);
free(buf2);

}

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.

Potential Mitigations

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.

Weakness Ordinalities

Ordinality	Description
Resultant	(where the weakness is typically related to the presence of some other weaknesses)

Memberships

This MemberOf Relationships table shows additional CWE Categories and Views that reference this weakness as a member. This information is often useful in understanding where a weakness fits within the context of external information sources.

Nature	Type	ID	Name
MemberOf	Category - a CWE entry that contains a set of other entries that share a common characteristic.	970	SFP Secondary Cluster: Faulty Buffer Access
MemberOf	Category - a CWE entry that contains a set of other entries that share a common characteristic.	1160	SEI CERT C Coding Standard - Guidelines 06. Arrays (ARR)
MemberOf	Category - a CWE entry that contains a set of other entries that share a common characteristic.	1161	SEI CERT C Coding Standard - Guidelines 07. Characters and Strings (STR)
MemberOf	Category - a CWE entry that contains a set of other entries that share a common characteristic.	1399	Comprehensive Categorization: Memory Safety

Taxonomy Mappings

Mapped Taxonomy Name	Node ID	Fit	Mapped Node Name
CLASP			Write-what-where condition
CERT C Secure Coding	ARR30-C	Imprecise	Do not form or use out-of-bounds pointers or array subscripts
CERT C Secure Coding	ARR38-C	Imprecise	Guarantee that library functions do not form invalid pointers
CERT C Secure Coding	STR31-C	Imprecise	Guarantee that storage for strings has sufficient space for character data and the null terminator
CERT C Secure Coding	STR32-C	Imprecise	Do not pass a non-null-terminated character sequence to a library function that expects a string
Software Fault Patterns	SFP8		Faulty Buffer Access

References

[REF-44] Michael Howard, David LeBlanc and John Viega. "24 Deadly Sins of Software Security". "Sin 5: Buffer Overruns." Page 89. McGraw-Hill. 2010.

[REF-18] Secure Software, Inc.. "The CLASP Application Security Process". 2005. <https://cwe.mitre.org/documents/sources/TheCLASPApplicationSecurityProcess.pdf>.

Content History

Submissions
Submission Date	Submitter	Organization
2006-07-19	CLASP
2006-07-19
Modifications
Modification Date	Modifier	Organization
2008-07-01	Eric Dalci	Cigital
2008-07-01	updated Time_of_Introduction
2008-09-08	CWE Content Team	MITRE
2008-09-08	updated Applicable_Platforms, Common_Consequences, Relationships, Other_Notes, Taxonomy_Mappings, Weakness_Ordinalities
2008-11-24	CWE Content Team	MITRE
2008-11-24	updated Common_Consequences, Other_Notes
2009-01-12	CWE Content Team	MITRE
2009-01-12	updated Common_Consequences
2009-05-27	CWE Content Team	MITRE
2009-05-27	updated Relationships
2010-12-13	CWE Content Team	MITRE
2010-12-13	updated Relationships
2011-06-01	CWE Content Team	MITRE
2011-06-01	updated Common_Consequences
2012-05-11	CWE Content Team	MITRE
2012-05-11	updated Common_Consequences, References, Relationships
2012-10-30	CWE Content Team	MITRE
2012-10-30	updated Demonstrative_Examples
2013-02-21	CWE Content Team	MITRE
2013-02-21	updated Potential_Mitigations
2014-07-30	CWE Content Team	MITRE
2014-07-30	updated Relationships, Taxonomy_Mappings
2015-12-07	CWE Content Team	MITRE
2015-12-07	updated Relationships
2017-11-08	CWE Content Team	MITRE
2017-11-08	updated Causal_Nature, Common_Consequences, Demonstrative_Examples, Taxonomy_Mappings
2019-01-03	CWE Content Team	MITRE
2019-01-03	updated Relationships
2019-06-20	CWE Content Team	MITRE
2019-06-20	updated Relationships
2019-09-19	CWE Content Team	MITRE
2019-09-19	updated Relationships
2020-02-24	CWE Content Team	MITRE
2020-02-24	updated Relationships, Taxonomy_Mappings
2020-08-20	CWE Content Team	MITRE
2020-08-20	updated Relationships
2020-12-10	CWE Content Team	MITRE
2020-12-10	updated Relationships
2021-03-15	CWE Content Team	MITRE
2021-03-15	updated References
2022-10-13	CWE Content Team	MITRE
2022-10-13	updated Relationships, Taxonomy_Mappings
2023-04-27	CWE Content Team	MITRE
2023-04-27	updated Relationships


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Common Weakness Enumeration

CWE-123: Write-what-where Condition

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