Common Weakness Enumeration

A Community-Developed List of Software Weakness Types

CWE/SANS Top 25 Most Dangerous Software Errors
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CWE-126: Buffer Over-read

Weakness ID: 126
Abstraction: Variant
Structure: Simple
Status: Draft
Presentation Filter:
+ Description
The software reads from a buffer using buffer access mechanisms such as indexes or pointers that reference memory locations after the targeted buffer.
+ Extended Description
This typically occurs when the pointer or its index is incremented to a position beyond the bounds of the buffer or when pointer arithmetic results in a position outside of the valid memory location to name a few. This may result in exposure of sensitive information or possibly a crash.
+ Relationships

The table(s) below 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)
ChildOfBaseBase125Out-of-bounds Read
ChildOfBaseBase788Access of Memory Location After End of Buffer
CanFollowBaseBase170Improper Null Termination
+ Relevant to the view "Development Concepts" (CWE-699)
ChildOfBaseBase125Out-of-bounds Read
ChildOfBaseBase788Access of Memory Location After End of 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 software life cycle at which introduction may occur, while the Note provides a typical scenario related to introduction during the given phase.

+ Applicable Platforms
The listings below show 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.


C (Undetermined Prevalence)

C++ (Undetermined Prevalence)

+ Common Consequences

The table below 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.


Technical Impact: Read Memory

+ Demonstrative Examples

Example 1

In the following C/C++ example the method processMessageFromSocket() will get a message from a socket, placed into a buffer, and will parse the contents of the buffer into a structure that contains the message length and the message body. A for loop is used to copy the message body into a local character string which will be passed to another method for processing.

(bad code)
Example Language:
int processMessageFromSocket(int socket) {
int success;

char buffer[BUFFER_SIZE];
char message[MESSAGE_SIZE];
// get message from socket and store into buffer
//Ignoring possibliity that buffer > BUFFER_SIZE

if (getMessage(socket, buffer, BUFFER_SIZE) > 0) {
// place contents of the buffer into message structure

ExMessage *msg = recastBuffer(buffer);
// copy message body into string for processing

int index;
for (index = 0; index < msg->msgLength; index++) {
message[index] = msg->msgBody[index];

message[index] = '\0';
// process message

success = processMessage(message);

return success;


However, the message length variable from the structure is used as the condition for ending the for loop without validating that the message length variable accurately reflects the length of message body. This can result in a buffer over read by reading from memory beyond the bounds of the buffer if the message length variable indicates a length that is longer than the size of a message body (CWE-130).

Example 2

The following C/C++ example demonstrates a buffer over-read due to a missing NULL terminator. The main method of a pattern matching utility that looks for a specific pattern within a specific file uses the string strncopy() method to copy the command line user input file name and pattern to the Filename and Pattern character arrays respectively.

(bad code)
Example Language:
int main(int argc, char **argv)
char Filename[256];
char Pattern[32];

/* Validate number of parameters and ensure valid content */

/* copy filename parameter to variable, may cause off-by-one overflow */
strncpy(Filename, argv[1], sizeof(Filename));

/* copy pattern parameter to variable, may cause off-by-one overflow */
strncpy(Pattern, argv[2], sizeof(Pattern));

printf("Searching file: %s for the pattern: %s\n", Filename, Pattern);
Scan_File(Filename, Pattern);


However, the code do not take into account that strncpy() will not add a NULL terminator when the source buffer is equal in length of longer than that provide size attribute. Therefore if a user enters a filename or pattern that are the same size as (or larger than) their respective character arrays, a NULL terminator will not be added (CWE-170) which leads to the printf() read beyond the expected end of the Filename and Pattern buffers.

To fix this problem, be sure to subtract 1 from the sizeof() call to allow room for the null byte to be added.

(good code)
Example Language:
/* copy filename parameter to variable, no off-by-one overflow */
strncpy(Filename, argv[2], sizeof(Filename)-1);

/* copy pattern parameter to variable, no off-by-one overflow */
strncpy(Pattern, argv[3], sizeof(Pattern)-1);
+ Observed Examples
Chain: "Heartbleed" bug receives an inconsistent length parameter (CWE-130) enabling an out-of-bounds read (CWE-126), returning memory that could include private cryptographic keys and other sensitive data.
Chain: product does not handle when an input string is not NULL terminated, leading to buffer over-read or heap-based buffer overflow.
+ Weakness Ordinalities
(where the weakness exists independent of 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.
MemberOfCategoryCategory970SFP Secondary Cluster: Faulty Buffer Access
+ Notes


These problems may be resultant from missing sentinel values (CWE-463) or trusting a user-influenced input length variable.
+ Taxonomy Mappings
Mapped Taxonomy NameNode IDFitMapped Node Name
PLOVERBuffer over-read
Software Fault PatternsSFP8Faulty Buffer Access
+ Content History
Submission DateSubmitterOrganization
Modification DateModifierOrganization
2008-09-08CWE Content TeamMITRE
updated Applicable_Platforms, Relationships, Taxonomy_Mappings, Weakness_Ordinalities
2009-10-29CWE Content TeamMITRE
updated Description, Relationship_Notes, Relationships
2011-03-29CWE Content TeamMITRE
updated Demonstrative_Examples
2011-06-01CWE Content TeamMITRE
updated Common_Consequences
2012-05-11CWE Content TeamMITRE
updated Demonstrative_Examples, Relationships
2014-06-23CWE Content TeamMITRE
updated Observed_Examples
2014-07-30CWE Content TeamMITRE
updated Relationships, Taxonomy_Mappings
2017-11-08CWE Content TeamMITRE
updated Causal_Nature, Demonstrative_Examples

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Page Last Updated: January 18, 2018