Summary
The fix for GHSA-f5x2-vj4h-vg4c / CVE-2026-25754 introduced in commit 40e1c71 is incomplete and can be bypassed through nested prototype pollution payloads.
The original patch replaced the internal FormFields storage object with Object.create(null), preventing direct payloads such as __proto__.polluted. However, payloads containing a non-dangerous segment before __proto__ or constructor.prototype, such as user.__proto__.polluted, still lead to Object.prototype pollution.
This issue is exploitable remotely through a single unauthenticated multipart/form-data request using the default configuration.
Affected versions
>= 10.1.3 < 10.1.5>= 11.0.0-next.9 < 11.0.3
Details
The regression tests added by the original fix only covered direct payloads such as:
__proto__.pollutedconstructor.prototype.polluted
These payloads are blocked because the root object no longer inherits from Object.prototype.
However, lodash _.set() (via @poppinss/utils) still creates intermediate objects using plain {} values. Once a normal segment is encountered, subsequent __proto__ or constructor.prototype segments regain access to Object.prototype.
Impact
An unauthenticated attacker can remotely pollute Object.prototype on any route accepting multipart/form-data requests behind BodyParserMiddleware.
Because the pollution is process-wide, the impact may include authorization bypasses, unexpected behavior in downstream libraries, or prototype pollution gadget chains leading to remote code execution.
Patches
Fixes targeting v6 and v7 have been published below.
Users should upgrade to a version that includes the following fix:
References
References
EchoSkorJjj Reporter
Summary
The fix for GHSA-f5x2-vj4h-vg4c / CVE-2026-25754 introduced in commit
40e1c71is incomplete and can be bypassed through nested prototype pollution payloads.The original patch replaced the internal
FormFieldsstorage object withObject.create(null), preventing direct payloads such as__proto__.polluted. However, payloads containing a non-dangerous segment before__proto__orconstructor.prototype, such asuser.__proto__.polluted, still lead toObject.prototypepollution.This issue is exploitable remotely through a single unauthenticated
multipart/form-datarequest using the default configuration.Affected versions
>= 10.1.3 < 10.1.5>= 11.0.0-next.9 < 11.0.3Details
The regression tests added by the original fix only covered direct payloads such as:
__proto__.pollutedconstructor.prototype.pollutedThese payloads are blocked because the root object no longer inherits from
Object.prototype.However, lodash
_.set()(via@poppinss/utils) still creates intermediate objects using plain{}values. Once a normal segment is encountered, subsequent__proto__orconstructor.prototypesegments regain access toObject.prototype.Impact
An unauthenticated attacker can remotely pollute
Object.prototypeon any route accepting multipart/form-data requests behindBodyParserMiddleware.Because the pollution is process-wide, the impact may include authorization bypasses, unexpected behavior in downstream libraries, or prototype pollution gadget chains leading to remote code execution.
Patches
Fixes targeting v6 and v7 have been published below.
Users should upgrade to a version that includes the following fix:
References
References