Run static analysis (SAST) on the codebase looking for the unsafe pattern in the data flow.
CWE-127 Variant Draft
Buffer Under-read
A buffer under-read occurs when a program attempts to read data from a memory location positioned before the start of an allocated buffer.
Definition
What is CWE-127?
A buffer under-read occurs when a program attempts to read data from a memory location positioned before the start of an allocated buffer.
This vulnerability typically happens due to incorrect pointer arithmetic, where a pointer is decremented past the buffer's beginning, or when a negative index is used in an array access. These operations cause the program to read from unintended memory regions that were not allocated for the buffer's use.
Reading from memory preceding the buffer can expose sensitive information, such as remnants of other data structures, passwords, or encryption keys, leading to information disclosure. It can also cause the application to crash if it accesses protected or invalid memory addresses, resulting in a denial of service.
Real-world impact
Real-world CVEs caused by CWE-127
-
HTML conversion package has a buffer under-read, allowing a crash
How attackers exploit it
Step-by-step attacker path
- 1
Identify a code path that handles untrusted input without validation.
- 2
Craft a payload that exercises the unsafe behavior — injection, traversal, overflow, or logic abuse.
- 3
Deliver the payload through a normal request and observe the application's reaction.
- 4
Iterate until the response leaks data, executes attacker code, or escalates privileges.
Vulnerable code example
Vulnerable C
In the following code, the method retrieves a value from an array at a specific array index location that is given as an input parameter to the method
int getValueFromArray(int *array, int len, int index) {
int value;
```
// check that the array index is less than the maximum*
*// length of the array*
if (index < len) {
```
```
// get the value at the specified index of the array*
value = array[index];}
*// if array index is invalid then output error message*
*// and return value indicating error*
else {
```
printf("Value is: %d\n", array[index]);
value = -1;
}
return value;
} Secure code example
Secure C
However, this method only verifies that the given array index is less than the maximum length of the array but does not check for the minimum value (CWE-839). This will allow a negative value to be accepted as the input array index, which will result in reading data before the beginning of the buffer (CWE-127) and may allow access to sensitive memory. The input array index should be checked to verify that is within the maximum and minimum range required for the array (CWE-129). In this example the if statement should be modified to include a minimum range check, as shown below.
...
```
// check that the array index is within the correct*
*// range of values for the array*
if (index >= 0 && index < len) {
... 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-127
- 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-127
Run dynamic application security testing against the live endpoint.
Watch runtime logs for unusual exception traces, malformed input, or authorization bypass attempts.
Code review: flag any new code that handles input from this surface without using the validated framework helpers.
CWE-127
Don't catalog this weakness. Prove it's reachable.
Plexicus turns CWE catalogs into evidence: every CWE-pattern is matched against your real code graph, reach is proven on a sandbox clone, and verified findings ship as reviewed PRs.
Frequently asked questions What is CWE-127?
How serious is CWE-127?
What languages or platforms are affected by CWE-127?
How can I prevent CWE-127?
How does Plexicus detect and fix CWE-127?
Where can I learn more about CWE-127?
Frequently asked questions
What is CWE-127?
A buffer under-read occurs when a program attempts to read data from a memory location positioned before the start of an allocated buffer.
How serious is CWE-127?
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-127?
MITRE lists the following affected platforms: C, C++.
How can I prevent CWE-127?
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-127?
Plexicus's SAST engine matches the data-flow signature for CWE-127 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-127?
MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/127.html. You can also reference OWASP and NIST documentation for adjacent guidance.
Related weaknesses
Weaknesses related to CWE-127
CWE-786 Parent
Access of Memory Location Before Start of Buffer
This vulnerability occurs when software attempts to read from or write to a memory location positioned before the official start of a…
CWE-124 Sibling
Buffer Underwrite ('Buffer Underflow')
A buffer underwrite, also known as buffer underflow, happens when a program writes data to a memory location before the official start of…
Resources
Further reading
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