Automated static analysis, commonly referred to as Static Application Security Testing (SAST), can find some instances of this weakness by analyzing source code (or binary/compiled code) without having to execute it. Typically, this is done by building a model of data flow and control flow, then searching for potentially-vulnerable patterns that connect "sources" (origins of input) with "sinks" (destinations where the data interacts with external components, a lower layer such as the OS, etc.)
CWE-126 Variant Draft
Buffer Over-read
This vulnerability occurs when a program reads data from a memory buffer using an index or pointer that points beyond the buffer's allocated boundary, accessing unintended memory locations.
Definition
What is CWE-126?
This vulnerability occurs when a program reads data from a memory buffer using an index or pointer that points beyond the buffer's allocated boundary, accessing unintended memory locations.
Buffer over-reads happen when software fails to properly validate that a read operation stays within the bounds of a buffer. This can lead to the exposure of sensitive information stored in adjacent memory, cause crashes, or create unexpected program behavior. It's a common pitfall when using low-level languages like C/C++ with functions that don't inherently check boundaries, or when manual bounds checking logic contains errors.
Detecting these flaws manually across a large codebase is challenging. While SAST tools can identify the risky patterns, Plexicus uses AI to not only find them but also generate specific, context-aware fixes—transforming a security finding into a ready-to-apply code suggestion. This automates the remediation step, helping developers secure their applications faster and more consistently.
Real-world impact
Real-world CVEs caused by CWE-126
-
Text editor has out-of-bounds read past end of line while indenting C code
-
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.
How attackers exploit it
Step-by-step attacker path
- 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.
- 2
However, the message length variable (msgLength) from the structure is used as the condition for ending the for loop without validating that msgLength accurately reflects the actual length of the message body (CWE-606). If msgLength indicates a length that is longer than the size of a message body (CWE-130), then this can result in a buffer over-read by reading past the end of the buffer (CWE-126).
- 3
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.
- 4
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.
- 5
To fix this problem, be sure to subtract 1 from the sizeof() call to allow room for the null byte to be added.
Vulnerable code example
Vulnerable C
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.
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;} Secure code example
Secure C
To fix this problem, be sure to subtract 1 from the sizeof() call to allow room for the null byte to be added.
```
/* copy filename parameter to variable, no off-by-one overflow */*
strncpy(Filename, argv[2], sizeof(Filename)-1);
Filename[255]='\0';
*/* copy pattern parameter to variable, no off-by-one overflow */*
strncpy(Pattern, argv[3], sizeof(Pattern)-1);Pattern[31]='\0'; What changed: the unsafe sink is replaced (or the input is validated/escaped) so the same payload no longer triggers the weakness.
Prevention checklist
How to prevent CWE-126
- Architecture Use safe-by-default frameworks and APIs that prevent the unsafe pattern from being expressible.
- Implementation Validate input at trust boundaries; use allowlists, not denylists.
- Implementation Apply the principle of least privilege to credentials, file paths, and runtime permissions.
- Testing Cover this weakness in CI: SAST rules + targeted unit tests for the data flow.
- Operation Monitor logs for the runtime signals listed in the next section.
Detection signals
How to detect CWE-126
Plexicus auto-detects CWE-126 and opens a fix PR in under 60 seconds.
Codex Remedium scans every commit, identifies this exact weakness, and ships a reviewer-ready pull request with the patch. No tickets. No hand-offs.
Frequently asked questions What is CWE-126?
How serious is CWE-126?
What languages or platforms are affected by CWE-126?
How can I prevent CWE-126?
How does Plexicus detect and fix CWE-126?
Where can I learn more about CWE-126?
Frequently asked questions
What is CWE-126?
This vulnerability occurs when a program reads data from a memory buffer using an index or pointer that points beyond the buffer's allocated boundary, accessing unintended memory locations.
How serious is CWE-126?
MITRE has not published a likelihood-of-exploit rating for this weakness. Treat it as medium-impact until your threat model proves otherwise.
What languages or platforms are affected by CWE-126?
MITRE lists the following affected platforms: C, C++.
How can I prevent CWE-126?
Use safe-by-default frameworks, validate untrusted input at trust boundaries, and apply the principle of least privilege. Cover the data-flow signature in CI with SAST.
How does Plexicus detect and fix CWE-126?
Plexicus's SAST engine matches the data-flow signature for CWE-126 on every commit. When a match is found, our Codex Remedium agent opens a fix PR with the corrected code, tests, and a one-line summary for the reviewer.
Where can I learn more about CWE-126?
MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/126.html. You can also reference OWASP and NIST documentation for adjacent guidance.
Related weaknesses
Weaknesses related to CWE-126
CWE-788 Parent
Access of Memory Location After End of Buffer
This vulnerability occurs when software attempts to read from or write to a memory buffer using an index or pointer that points past the…
CWE-121 Sibling
Stack-based Buffer Overflow
A stack-based buffer overflow occurs when a program writes more data to a buffer located on the call stack than it can hold, corrupting…
CWE-122 Sibling
Heap-based Buffer Overflow
A heap-based buffer overflow occurs when a program writes more data to a memory buffer allocated in the heap than it can hold, corrupting…
Resources
Further reading
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