Run static analysis (SAST) on the codebase looking for the unsafe pattern in the data flow.
CWE-123 Base Draft
Write-what-where Condition
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…
Definition
What is CWE-123?
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.
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.
Real-world impact
Real-world CVEs caused by CWE-123
-
Chain: Python library does not limit the resources used to process images that specify a very large number of bands (CWE-1284), leading to excessive memory consumption (CWE-789) or an integer overflow (CWE-190).
-
Chain: 3D renderer has an integer overflow (CWE-190) leading to write-what-where condition (CWE-123) using a crafted image.
How attackers exploit it
Step-by-step attacker path
- 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:
- 2
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.
- 3
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.
- 4
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.
Vulnerable code example
Vulnerable C
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:
#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);
} Secure code example
Secure pseudo
// Validate, sanitize, or use a safe API before reaching the sink.
function handleRequest(input) {
const safe = validateAndEscape(input);
return executeWithGuards(safe);
} 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-123
- Architecture and Design Use a language that provides appropriate memory abstractions.
- Operation Use OS-level preventative functionality integrated after the fact. Not a complete solution.
Detection signals
How to detect CWE-123
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.
Plexicus auto-detects CWE-123 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-123?
How serious is CWE-123?
What languages or platforms are affected by CWE-123?
How can I prevent CWE-123?
How does Plexicus detect and fix CWE-123?
Where can I learn more about CWE-123?
Frequently asked questions
What is CWE-123?
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.
How serious is CWE-123?
MITRE rates the likelihood of exploit as High — this weakness is actively exploited in the wild and should be prioritized for remediation.
What languages or platforms are affected by CWE-123?
MITRE lists the following affected platforms: C, C++.
How can I prevent CWE-123?
Use a language that provides appropriate memory abstractions. Use OS-level preventative functionality integrated after the fact. Not a complete solution.
How does Plexicus detect and fix CWE-123?
Plexicus's SAST engine matches the data-flow signature for CWE-123 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-123?
MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/123.html. You can also reference OWASP and NIST documentation for adjacent guidance.
Related weaknesses
Weaknesses related to CWE-123
CWE-787 Parent
Out-of-bounds Write
This vulnerability occurs when software incorrectly writes data outside the boundaries of its allocated memory buffer, either beyond the…
CWE-120 Sibling
Buffer Copy without Checking Size of Input ('Classic Buffer Overflow')
This vulnerability occurs when a program copies data from one memory location to another without first verifying that the source data will…
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…
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…
Ready when you are
Don't Let Security
Weigh You Down.
Stop choosing between AI velocity and security debt. Plexicus is the only platform that runs Vibe Coding Security and ASPM in parallel — one workflow, every codebase.