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Demystifying / Abusing Flash Cookies

ID: 82e3450b-a00b-532d-9f17-318bb6861cf7

STIX ID: report--82e3450b-a00b-532d-9f17-318bb6861cf7

Feed Name: Zscaler Security Research Blog

Threat Score
50/100

Date Published: 2025-04-02

Date Updated: 2026-05-01

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This report describes Flash Local SharedObjects (LSOs or "Flash cookies"), documenting their larger storage capacity, persistent nature, storage locations on Windows/Mac/Linux, and how attackers can read/write LSOs via uploaded SWF content. It includes ActionScript examples and explains the privacy and authentication risks (tracking and cookie hijacking) posed when malicious Flash content is allowed to run on a site.

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