CVE-2026-33228 flatted: Prototype Pollution via parse()
Prototype pollution in flatted allows attackers to corrupt global objects, potentially leading to code execution or privilege escalation.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-33228?
CVE-2026-33228 is a high-severity vulnerability in WebReflection flatted, published on 20 Mar 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
flatted is a circular JSON parser. Prior to version 3.4.2, the parse() function in flatted can use attacker-controlled string values from the parsed JSON as direct array index keys, without validating that they are numeric. Since the internal input buffer is a JavaScript Array, accessing it with the key "__proto__" returns Array.prototype via the inherited getter. This object is then treated as a legitimate parsed value and assigned as a property of the output object, effectively leaking a live reference to Array.prototype to the consumer. Any code that subsequently writes to that property will pollute the global prototype. This issue has been patched in version 3.4.2.
Which versions are affected by CVE-2026-33228?
2 open source packages ship an affected version, including flatted. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
flatted npm | All versions | 3.4.2 |
flatted npm | All versions | 3.4.2 |
Products include Webreflection Flatted.
How do I fix CVE-2026-33228?
Upgrade flatted to 3.4.2 or later.
Is CVE-2026-33228 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-33228 exists.
Other vulnerabilities in WebReflection flatted
Sources
Also known as AZL-80309, CLEANSTART-2026-PL93650, CLEANSTART-2026-SB96942, CLSA-2026-1791278813, DEBIAN-CVE-2026-33228, GHSA-rf6f-7fwh-wjgh and 8 more.