High severityNVD Advisory· Published Mar 20, 2026· Updated Mar 24, 2026

flatted: Prototype Pollution via parse()

CVE-2026-33228

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.

AI Insight

LLM-synthesized narrative grounded in this CVE's description and references.

Flatted parse() prior to 3.4.2 allows prototype pollution via '__proto__' key, leaking Array.prototype.

Root

Cause

The parse() function in flatted prior to version 3.4.2 uses attacker-controlled string values from the parsed JSON as direct array index keys without validating that they are numeric [1]. 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 [3].

Exploitation

An attacker can supply a crafted flatted string to parse() such that the returned object holds a live reference to Array.prototype. No authentication or special network position is required; the attacker only needs to deliver the malicious payload to an application that calls parse() on untrusted data [3].

Impact

Any code that subsequently writes to that property will pollute the global prototype chain. This can lead to denial of service, property injection, or other unpredictable behavior depending on the affected application [1][3]. The vulnerability is classified under CWE-1321 (Prototype Pollution).

Mitigation

This issue has been patched in flatted version 3.4.2 [4]. Users should upgrade immediately. No known workarounds are available. The vulnerability is not currently listed in CISA KEV.

References

AI Insight generated on May 18, 2026. Synthesized from this CVE's description and the cited reference URLs; citations are validated against the source bundle.

Affected packages

Versions sourced from the GitHub Security Advisory.

Package	Affected versions	Patched versions
flattednpm	< 3.4.2	3.4.2

Affected products

WebReflection/flattedv5
Range: < 3.4.2

Patches

885ddcc33cf9

fix CWE-1321

https://github.com/WebReflection/flattedwebreflectionMar 17, 2026via ghsa

commit

6 files changed · +6 −6

cjs/index.js+1 −1 modified

@@ -27,7 +27,7 @@ const resolver = (input, lazy, parsed, $) => output => {
     const k = ke[y];
     const value = output[k];
     if (value instanceof Primitive) {
-      const tmp = input[value];
+      const tmp = input[+value];
       if (typeof tmp === object && !parsed.has(tmp)) {
         parsed.add(tmp);
         output[k] = ignore;

es.js+1 −1 modified

@@ -1 +1 @@
-self.Flatted=function(t){"use strict";const{parse:e,stringify:n}=JSON,{keys:r}=Object,o=String,s="string",c={},l="object",f=(t,e)=>e,i=t=>t instanceof o?o(t):t,a=(t,e)=>typeof e===s?new o(e):e,u=(t,e,n)=>{const r=o(e.push(n)-1);return t.set(n,r),r},p=(t,n)=>{const s=e(t,a).map(i),u=n||f;let p=s[0];if(typeof p===l&&p){const t=[],e=((t,e,n,s)=>f=>{for(let i=r(f),{length:a}=i,u=0;u<a;u++){const r=i[u],a=f[r];if(a instanceof o){const o=t[a];typeof o!==l||n.has(o)?f[r]=s.call(f,r,o):(n.add(o),f[r]=c,e.push({o:f,k:r,r:o}))}else f[r]!==c&&(f[r]=s.call(f,r,a))}return f})(s,t,new Set,u);p=e(p);let n=0;for(;n<t.length;){const{o:r,k:o,r:s}=t[n++];r[o]=u.call(r,o,e(s))}}return u.call({"":p},"",p)},g=(t,e,r)=>{const o=e&&typeof e===l?(t,n)=>""===t||-1<e.indexOf(t)?n:void 0:e||f,c=new Map,i=[],a=[];let p=+u(c,i,o.call({"":t},"",t)),g=!p;for(;p<i.length;)g=!0,a[p]=n(i[p++],h,r);return"["+a.join(",")+"]";function h(t,e){if(g)return g=!g,e;const n=o.call(this,t,e);switch(typeof n){case l:if(null===n)return n;case s:return c.get(n)||u(c,i,n)}return n}};return t.fromJSON=t=>p(n(t)),t.parse=p,t.stringify=g,t.toJSON=t=>e(g(t)),t}({});
+self.Flatted=function(t){"use strict";const{parse:e,stringify:n}=JSON,{keys:r}=Object,o=String,s="string",c={},l="object",f=(t,e)=>e,i=t=>t instanceof o?o(t):t,a=(t,e)=>typeof e===s?new o(e):e,u=(t,e,n)=>{const r=o(e.push(n)-1);return t.set(n,r),r},p=(t,n)=>{const s=e(t,a).map(i),u=n||f;let p=s[0];if(typeof p===l&&p){const t=[],e=((t,e,n,s)=>f=>{for(let i=r(f),{length:a}=i,u=0;u<a;u++){const r=i[u],a=f[r];if(a instanceof o){const o=t[+a];typeof o!==l||n.has(o)?f[r]=s.call(f,r,o):(n.add(o),f[r]=c,e.push({o:f,k:r,r:o}))}else f[r]!==c&&(f[r]=s.call(f,r,a))}return f})(s,t,new Set,u);p=e(p);let n=0;for(;n<t.length;){const{o:r,k:o,r:s}=t[n++];r[o]=u.call(r,o,e(s))}}return u.call({"":p},"",p)},g=(t,e,r)=>{const o=e&&typeof e===l?(t,n)=>""===t||-1<e.indexOf(t)?n:void 0:e||f,c=new Map,i=[],a=[];let p=+u(c,i,o.call({"":t},"",t)),g=!p;for(;p<i.length;)g=!0,a[p]=n(i[p++],h,r);return"["+a.join(",")+"]";function h(t,e){if(g)return g=!g,e;const n=o.call(this,t,e);switch(typeof n){case l:if(null===n)return n;case s:return c.get(n)||u(c,i,n)}return n}};return t.fromJSON=t=>p(n(t)),t.parse=p,t.stringify=g,t.toJSON=t=>e(g(t)),t}({});

esm/index.js+1 −1 modified

@@ -26,7 +26,7 @@ const resolver = (input, lazy, parsed, $) => output => {
     const k = ke[y];
     const value = output[k];
     if (value instanceof Primitive) {
-      const tmp = input[value];
+      const tmp = input[+value];
       if (typeof tmp === object && !parsed.has(tmp)) {
         parsed.add(tmp);
         output[k] = ignore;

esm.js+1 −1 modified

@@ -1 +1 @@
-const{parse:t,stringify:e}=JSON,{keys:n}=Object,o=String,r="string",s={},c="object",l=(t,e)=>e,f=t=>t instanceof o?o(t):t,i=(t,e)=>typeof e===r?new o(e):e,a=(t,e,n)=>{const r=o(e.push(n)-1);return t.set(n,r),r},u=(e,r)=>{const a=t(e,i).map(f),u=r||l;let p=a[0];if(typeof p===c&&p){const t=[],e=((t,e,r,l)=>f=>{for(let i=n(f),{length:a}=i,u=0;u<a;u++){const n=i[u],a=f[n];if(a instanceof o){const o=t[a];typeof o!==c||r.has(o)?f[n]=l.call(f,n,o):(r.add(o),f[n]=s,e.push({o:f,k:n,r:o}))}else f[n]!==s&&(f[n]=l.call(f,n,a))}return f})(a,t,new Set,u);p=e(p);let r=0;for(;r<t.length;){const{o:n,k:o,r:s}=t[r++];n[o]=u.call(n,o,e(s))}}return u.call({"":p},"",p)},p=(t,n,o)=>{const s=n&&typeof n===c?(t,e)=>""===t||-1<n.indexOf(t)?e:void 0:n||l,f=new Map,i=[],u=[];let p=+a(f,i,s.call({"":t},"",t)),h=!p;for(;p<i.length;)h=!0,u[p]=e(i[p++],g,o);return"["+u.join(",")+"]";function g(t,e){if(h)return h=!h,e;const n=s.call(this,t,e);switch(typeof n){case c:if(null===n)return n;case r:return f.get(n)||a(f,i,n)}return n}},h=e=>t(p(e)),g=t=>u(e(t));export{g as fromJSON,u as parse,p as stringify,h as toJSON};
+const{parse:t,stringify:e}=JSON,{keys:n}=Object,o=String,r="string",s={},c="object",l=(t,e)=>e,f=t=>t instanceof o?o(t):t,i=(t,e)=>typeof e===r?new o(e):e,a=(t,e,n)=>{const r=o(e.push(n)-1);return t.set(n,r),r},u=(e,r)=>{const a=t(e,i).map(f),u=r||l;let p=a[0];if(typeof p===c&&p){const t=[],e=((t,e,r,l)=>f=>{for(let i=n(f),{length:a}=i,u=0;u<a;u++){const n=i[u],a=f[n];if(a instanceof o){const o=t[+a];typeof o!==c||r.has(o)?f[n]=l.call(f,n,o):(r.add(o),f[n]=s,e.push({o:f,k:n,r:o}))}else f[n]!==s&&(f[n]=l.call(f,n,a))}return f})(a,t,new Set,u);p=e(p);let r=0;for(;r<t.length;){const{o:n,k:o,r:s}=t[r++];n[o]=u.call(n,o,e(s))}}return u.call({"":p},"",p)},p=(t,n,o)=>{const s=n&&typeof n===c?(t,e)=>""===t||-1<n.indexOf(t)?e:void 0:n||l,f=new Map,i=[],u=[];let p=+a(f,i,s.call({"":t},"",t)),h=!p;for(;p<i.length;)h=!0,u[p]=e(i[p++],g,o);return"["+u.join(",")+"]";function g(t,e){if(h)return h=!h,e;const n=s.call(this,t,e);switch(typeof n){case c:if(null===n)return n;case r:return f.get(n)||a(f,i,n)}return n}},h=e=>t(p(e)),g=t=>u(e(t));export{g as fromJSON,u as parse,p as stringify,h as toJSON};

index.js+1 −1 modified

@@ -38,7 +38,7 @@ self.Flatted = (function (exports) {
         var k = ke[y];
         var value = output[k];
         if (value instanceof Primitive) {
-          var tmp = input[value];
+          var tmp = input[+value];
           if (_typeof(tmp) === object && !parsed.has(tmp)) {
             parsed.add(tmp);
             output[k] = ignore;

min.js+1 −1 modified

@@ -1 +1 @@
-self.Flatted=function(n){"use strict";function t(n){return t="function"==typeof Symbol&&"symbol"==typeof Symbol.iterator?function(n){return typeof n}:function(n){return n&&"function"==typeof Symbol&&n.constructor===Symbol&&n!==Symbol.prototype?"symbol":typeof n},t(n)}var r=JSON.parse,e=JSON.stringify,o=Object.keys,u=String,f="string",i={},c="object",a=function(n,t){return t},l=function(n){return n instanceof u?u(n):n},s=function(n,r){return t(r)===f?new u(r):r},y=function(n,t,r){var e=u(t.push(r)-1);return n.set(r,e),e},p=function(n,e){var f=r(n,s).map(l),y=e||a,p=f[0];if(t(p)===c&&p){var v=[],S=function(n,r,e,f){return function(a){for(var l=o(a),s=l.length,y=0;y<s;y++){var p=l[y],v=a[p];if(v instanceof u){var S=n[v];t(S)!==c||e.has(S)?a[p]=f.call(a,p,S):(e.add(S),a[p]=i,r.push({o:a,k:p,r:S}))}else a[p]!==i&&(a[p]=f.call(a,p,v))}return a}}(f,v,new Set,y);p=S(p);for(var b=0;b<v.length;){var m=v[b++],g=m.o,h=m.k,O=m.r;g[h]=y.call(g,h,S(O))}}return y.call({"":p},"",p)},v=function(n,r,o){for(var u=r&&t(r)===c?function(n,t){return""===n||-1<r.indexOf(n)?t:void 0}:r||a,i=new Map,l=[],s=[],p=+y(i,l,u.call({"":n},"",n)),v=!p;p<l.length;)v=!0,s[p]=e(l[p++],S,o);return"["+s.join(",")+"]";function S(n,r){if(v)return v=!v,r;var e=u.call(this,n,r);switch(t(e)){case c:if(null===e)return e;case f:return i.get(e)||y(i,l,e)}return e}};return n.fromJSON=function(n){return p(e(n))},n.parse=p,n.stringify=v,n.toJSON=function(n){return r(v(n))},n}({});
\ No newline at end of file
+self.Flatted=function(n){"use strict";function t(n){return t="function"==typeof Symbol&&"symbol"==typeof Symbol.iterator?function(n){return typeof n}:function(n){return n&&"function"==typeof Symbol&&n.constructor===Symbol&&n!==Symbol.prototype?"symbol":typeof n},t(n)}var r=JSON.parse,e=JSON.stringify,o=Object.keys,u=String,f="string",i={},c="object",a=function(n,t){return t},l=function(n){return n instanceof u?u(n):n},s=function(n,r){return t(r)===f?new u(r):r},y=function(n,t,r){var e=u(t.push(r)-1);return n.set(r,e),e},p=function(n,e){var f=r(n,s).map(l),y=e||a,p=f[0];if(t(p)===c&&p){var v=[],S=function(n,r,e,f){return function(a){for(var l=o(a),s=l.length,y=0;y<s;y++){var p=l[y],v=a[p];if(v instanceof u){var S=n[+v];t(S)!==c||e.has(S)?a[p]=f.call(a,p,S):(e.add(S),a[p]=i,r.push({o:a,k:p,r:S}))}else a[p]!==i&&(a[p]=f.call(a,p,v))}return a}}(f,v,new Set,y);p=S(p);for(var b=0;b<v.length;){var m=v[b++],g=m.o,h=m.k,O=m.r;g[h]=y.call(g,h,S(O))}}return y.call({"":p},"",p)},v=function(n,r,o){for(var u=r&&t(r)===c?function(n,t){return""===n||-1<r.indexOf(n)?t:void 0}:r||a,i=new Map,l=[],s=[],p=+y(i,l,u.call({"":n},"",n)),v=!p;p<l.length;)v=!0,s[p]=e(l[p++],S,o);return"["+s.join(",")+"]";function S(n,r){if(v)return v=!v,r;var e=u.call(this,n,r);switch(t(e)){case c:if(null===e)return e;case f:return i.get(e)||y(i,l,e)}return e}};return n.fromJSON=function(n){return p(e(n))},n.parse=p,n.stringify=v,n.toJSON=function(n){return r(v(n))},n}({});
\ No newline at end of file

Vulnerability mechanics

Generated on May 9, 2026. Inputs: CWE entries + fix-commit diffs from this CVE's patches. Citations validated against bundle.

References

github.com/advisories/GHSA-rf6f-7fwh-wjghghsaADVISORY
nvd.nist.gov/vuln/detail/CVE-2026-33228ghsaADVISORY
github.com/WebReflection/flatted/commit/885ddcc33cf9657caf38c57c7be45ae1c5272802ghsax_refsource_MISCWEB
github.com/WebReflection/flatted/releases/tag/v3.4.2ghsax_refsource_MISCWEB
github.com/WebReflection/flatted/security/advisories/GHSA-rf6f-7fwh-wjghghsax_refsource_CONFIRMWEB

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cvss	0.455
epss	0.000
exploit	0.000
kev	0.000
patch	-0.070
ransomware	0.000