CVE-2026-34208: SandboxJS: Sandbox integrity escape

Should you act?

Critical and remotely reachable

No known exploitation yet, but it is network-reachable without credentials. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

• Not in CISA KEV, but CISA’s SSVC assessment records a public proof-of-concept, so attack code exists.
• EPSS 0.6%, percentile 48: FIRST’s model estimates a 0.6% chance of exploitation activity in the next 30 days, higher than 48% of all scored CVEs (low). EPSS is a probability, not a verdict.
• Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
• Only applies if your dependency tree includes @nyariv/sandboxjs (npm) < 0.8.36, fixed in 0.8.36. Check your lockfile, including transitive dependencies.

What it is

SandboxJS is a JavaScript sandboxing library. Prior to 0.8.36, SandboxJS blocks direct assignment to global objects (for example Math.random = ...), but this protection can be bypassed through an exposed callable constructor path: this.constructor.call(target, attackerObject). Because this.constructor resolves to the internal SandboxGlobal function and Function.prototype.call is allowed, attacker code can write arbitrary properties into host global objects and persist those mutations across sandbox instances in the same process. This vulnerability is fixed in 0.8.36.

From the GitHub advisory

Summary SandboxJS blocks direct assignment to global objects (for example Math.random = ...), but this protection can be bypassed through an exposed callable constructor path: this.constructor.call(target, attackerObject). Because this.constructor resolves to the internal SandboxGlobal function and Function.prototype.call is allowed, attacker code can write arbitrary properties into host global objects and persist those mutations across sandbox instances in the same process.

Details The intended safety model relies on write-time checks in assignment operations. In assignCheck, writes are denied when the destination is marked global (obj.isGlobal), which correctly blocks straightforward payloads like Math.random = () => 1.

Reference: src/executor.ts#L215-L218

The bypass works because the dangerous write is not performed by an assignment opcode. Instead, attacker code reaches a host callable that performs writes internally. The constructor used for sandbox global objects is SandboxGlobal, implemented as a function that copies all keys from a provided object into this.

Reference: src/utils.ts#L84-L88

Weakness type: CWE-693, CWE-915.

Product: @nyariv/sandboxjs (npm) · Assigned by: GitHub_M · Published: 2026-04-06

Affected and fixed versions

Severity scores

• Attack vector: network (remotely reachable)
• Attack complexity: low
• Privileges required: none (no login)
• User interaction: none
• Scope: changed (can affect other components)

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

• CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
• CISA SSVC assessment: exploitation poc, automatable yes, technical impact total.
• FIRST EPSS (model of 2026-10-21): 0.60% probability of exploitation activity in the next 30 days; percentile 48.

Timeline

• 2026-04-03 — GitHub-reviewed advisory GHSA-2gg9-6p7w-6cpj published (critical)
• 2026-04-06 — CVE record last updated
• 2026-04-06 — CVE record published by GitHub_M

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Vulnerability records

1. GitHub Advisory GHSA-2gg9-6p7w-6cpj github.com
2. CVE.org record CVE-2026-34208 cve.org
3. NVD entry CVE-2026-34208 nvd.nist.gov

Other references from the CVE record and advisory

1. github.com github.com

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