10

CVSS Score

3.1

-

CVSS Score

Basic Information

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CVE-2026-25586

CVE-2026-25586: SandboxJS has a Sandbox Escape via Prototype Whitelist Bypass and Host Prototype Pollution

SandboxJS is a JavaScript sandboxing library. Prior to 0.8.29, a sandbox escape is possible by shadowing hasOwnProperty on a sandbox object, which disables prototype whitelist enforcement in the property-access path. This permits direct access to proto and other blocked prototype properties, enabling host Object.prototype pollution and persistent cross-sandbox impact. This vulnerability is fixed in 0.8.29.

(GitHub Advisory)

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CVE-2026-25586

CVE-2026-25586:

10

CVSS Score

3.1

-

CVSS Score

Basic Information

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Technical Details

Package Name	Ecosystem	Vulnerable Versions	First Patched Version
@nyariv/sandboxjs	npm	<= 0.8.28	0.8.29

Technical Details

Vulnerability Intelligence
Miggo AI

Root Cause Analysis

The vulnerability is a sandbox escape caused by improper handling of property access and assignment checks. The root cause is the direct use of the hasOwnProperty method on objects that can be controlled by code running inside the sandbox. An attacker can create an object with a custom hasOwnProperty method that always returns a predictable value (e.g., true or false). This allows bypassing security checks that rely on hasOwnProperty to distinguish between an object's own properties and inherited properties from its prototype.

The analysis of the patch 67cb186c41c78c51464f70405504e8ef0a6e43c3 reveals several vulnerable functions where this pattern occurs:

propAccessHandler (anonymous function in src/executor.ts): This function determines whether to enforce prototype access restrictions. By shadowing hasOwnProperty to return true, an attacker can make the prototypeAccess flag false, effectively disabling the prototype whitelist and gaining access to __proto__, which leads to prototype pollution.
assignCheck (in src/executor.ts): This function checks if an assignment is attempting to override a prototype method. This check is also bypassed by shadowing hasOwnProperty, allowing the attacker to modify the prototype of host objects.
Scope.get and Scope.declare (in src/utils.ts): These methods, responsible for variable lookup and declaration, also used unsafe hasOwnProperty calls on context-related objects. This could be exploited to interfere with variable management within the sandbox.

The fix applied in the patch is to replace all direct calls to obj.hasOwnProperty(prop) with a safe utility function hasOwnProperty(obj, prop) which is defined as Object.prototype.hasOwnProperty.call(obj, prop). This ensures that the original, un-tampered hasOwnProperty method is always used, preventing the sandbox escape.

Vulnerable functions

assignCheck

src/executor.ts

The `assignCheck` function, which is intended to prevent prototype property overrides, uses `obj.context.hasOwnProperty(obj.prop)`. An attacker can shadow the `hasOwnProperty` method on the `obj.context` object within the sandbox. By providing a malicious `hasOwnProperty` implementation that returns `false`, they can bypass this check and modify properties on the host's `Object.prototype`, leading to prototype pollution.

propAccessHandler (anonymous)

src/executor.ts

An anonymous function that handles property access (`LispType.Prop`) uses `a.hasOwnProperty(b)` to determine if prototype access checks are needed. An attacker can shadow `hasOwnProperty` on the object `a` to always return `true`. This causes `prototypeAccess` to become `false`, disabling the prototype whitelist enforcement. This allows the attacker to access and modify forbidden properties like `__proto__`, leading to prototype pollution and sandbox escape.

Scope.get

src/utils.ts

The `Scope.get` method used `this.globals.hasOwnProperty(key)` to check for the existence of a global variable. The `this.globals` object could be controlled by sandboxed code. By shadowing `hasOwnProperty` on this object, an attacker could manipulate variable resolution, potentially leading to incorrect behavior or allowing access to unauthorized data.

Scope.declare

src/utils.ts

The `Scope.declare` method used `this[type].hasOwnProperty(key)` and `this.globals.hasOwnProperty(key)` to check if a variable has already been declared. Since these objects could be part of the sandbox, an attacker could shadow the `hasOwnProperty` method to bypass these checks. This would allow re-declaring variables, potentially leading to logic errors or vulnerabilities.

Vulnerability Intelligence
Miggo AI

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WAF Protection Rules

WAF Rule

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Reasoning

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