It happens when code copies data using the source buffer size instead of the destination buffer size, or assumes both are equal. If the destination is smaller, the copy can overflow it and corrupt memory. Defenders should base copy limits on the destination, use safe copy patterns, and verify lengths at every boundary.
Why this weakness happens
This flaw appears when a program uses the source buffer length as the copy limit instead of the destination buffer length, or assumes the two buffers are the same size. The immediate failure is not the copy itself, but the bad size assumption. Once the destination is smaller, a routine copy can write past the end of allocated memory and corrupt adjacent data.
The root cause is usually poor boundary discipline: the code trusted the size of the input object, reused a length from a different object, or let one length variable stand in for another. That mistake can be hidden in helper functions, wrappers, or refactors where a “known good” size is carried forward without rechecking which buffer it actually describes.
What the overflow path looks like in practice
In practice, the dangerous pattern is easy to miss because the code may look formally correct at a glance. The source may be validated, the copy may be bounded, and the developer may even be using a length field, but the bound belongs to the wrong buffer. The copy succeeds until the destination is exhausted, then memory beyond the buffer becomes part of the write path.
That creates classic memory-safety failure modes: overwritten neighboring objects, corrupted control data, unexpected crashes, or, in the worst case, a condition an attacker can shape into code execution. The precise outcome depends on allocator behavior, object layout, and whether the overflow reaches security-relevant state before the process fails.
For root-cause framing, the CWE Top 25 Guide is useful because it places this weakness in the broader family of buffer and validation errors and helps distinguish it from related length and bounds mistakes.
How to prevent the length mistake from recurring
The safest pattern is to make the destination buffer the source of truth for any write limit, then treat the copy as a capacity problem rather than a data-size problem. Length checks should be tied to the actual write target, not to the incoming payload, and helper functions should preserve that distinction instead of abstracting it away.
Good practice also means validating lengths at every boundary where data changes form or ownership. If a value is parsed, transformed, concatenated, or handed to another function, the code should re-establish the destination capacity before writing. When available, use APIs or wrappers that require the destination size explicitly and fail closed on truncation or overflow risk.
Where the codebase already contains similar weaknesses, review adjacent functions for the same pattern rather than fixing only the visible call site. This class of bug often appears in clusters because one mistaken size assumption gets copied into multiple paths.
Risk and Threat Considerations
This weakness is high risk because it turns a simple length mismatch into memory corruption. Attackers look for it in input handling, file parsing, protocol decoding, and any path where externally influenced data is copied into a fixed buffer.
Failure mechanism: The program writes more bytes than the destination can hold because the copy limit is derived from the source buffer or another unrelated length. That mismatch can overwrite adjacent memory, destabilise the process, or alter execution state in a way that supports exploitation.
Impact: The most common outcome is a crash, but the security impact can escalate to data corruption, denial of service, or arbitrary code execution depending on where the overwrite lands and what memory protections are in place.
Standards & Framework Alignment
This section maps relevant standards and security frameworks to the operational risks and controls described in this guidance.
MITRE ATT&CK addresses the attack and risk surface, while OWASP ASVS, NIST SP 800-53 Rev 5 and CIS Controls v8 set the governance and control requirements practitioners need to meet.
| Framework | Control / Reference | Relevance |
|---|---|---|
| MITRE ATT&CK | T1055 — Process Injection | Memory corruption can become an attack path that supports execution and persistence outcomes. |
| Recommendation — Correlate crashes and overwrites with attacker technique patterns during triage. | ||
| OWASP ASVS | V2 — Validation and Business Logic | Bounds validation and correct length handling are core to preventing unsafe writes in application code. |
| Recommendation — Verify input-to-destination length checks at every write boundary. | ||
| NIST SP 800-53 Rev 5 | SI-10 — Information Input Validation | Correct size validation is required to stop malformed or oversized data from reaching unsafe writes. |
| Recommendation — Enforce destination-aware validation before data is copied or transformed. | ||
| CIS Controls v8 | CIS-16 — Application Software Security | Secure coding practices and review of unsafe memory operations address this weakness category. |
| Recommendation — Prioritise secure code review and testing for unsafe buffer operations. | ||
Practitioner Guidance
What to verify: Confirm that every copy, concatenation, and serialization path uses the destination capacity, not the incoming length, as the controlling bound. Review both direct calls and wrapper functions, because the bug is often introduced one layer away from the actual write.
Common mistake: Developers often trust a length field because it came from a parsed structure or a previously validated object. That is unsafe when the destination changes size, is reused, or is smaller than the source after transformation.
Practitioner takeaway: Treat buffer size mismatches as a memory-safety defect first and a coding style issue second, because once the write bound is wrong, the exploitability depends on layout and timing, not on intent.
Related resources from NHI Mgmt Group
- Guide: CWE Top 25 Guide
- How do developers prevent buffer access using the size of the source buffer?
- Buffer Access Using Size of Source Buffer
- What is the difference between using a verified browser extension and installing a free access tool from an untrusted source?
- When should organisations avoid using AI for access review decisions?
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Reviewed and updated by the NHIMG editorial team on October 18, 2026.
NHI Mgmt Group — the #1 independent authority on Non-Human Identity, IAM, and Agentic AI security. nhimg.org