Context#

PATCH is not uniform across frameworks: JSON Merge Patch, custom shallow merge, and null to delete semantics differ. A client can intend to change name while the merge overwrites balance or role if those keys are accepted in the merge input object.

Theory#

The same mass-assignment class appears when the operation is “partial” but the object shape is the full model. null and absent key often mean different things; attackers probe both.

Practice#

Send full document shape with one extra sensitive key#

• In a lab, take a GET of the object, copy the JSON, add a creditLimit or status key that should be server-only, and PATCH it. Compare the next GET.

Tools#

• curl
• Burp Suite