CVE-2026-43898: SandboxJS has a sandbox escape via Function.caller leakage of internal call op

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 50: FIRST’s model estimates a 0.6% chance of exploitation activity in the next 30 days, higher than 50% 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) <= 0.9.5, fixed in 0.9.6. Check your lockfile, including transitive dependencies.

What it is

SandboxJS is a JavaScript sandboxing library. Prior to 0.9.6, sandbox-defined functions expose Function.caller, allowing sandboxed code to recover the internal LispType.Call runtime callback. That callback can then be invoked with attacker-controlled fake context and obj values to extract blocked host statics, recover the real host Function constructor, and execute arbitrary host JavaScript. This vulnerability is fixed in 0.9.6.

From the GitHub advisory

Summary Sandbox-defined functions expose Function.caller, allowing sandboxed code to recover the internal LispType.Call runtime callback. That callback can then be invoked with attacker-controlled fake context and obj values to extract blocked host statics, recover the real host Function constructor, and execute arbitrary host JavaScript. Details

The vulnerability is in the property access logic registered via addOps in prop.ts. Sandboxed code could access the caller, callee, and arguments properties on functions. In the CommonJS build, this allowed sandboxed code to read Function.caller and leak a privileged internal LispType.Call callback.

In executorUtils.ts createFunction() constructs normal host JS functions, and because these are ordinary host functions, sandbox code can observe:

That leaks the host-side callback that invoked the sandbox function. This leaked callback is the internal LispType.Call op, which is registered in call.ts. The escape was possible because the LispType.Call handler accepts a params object from the attacker and uses its fields without authenticating that they came from the executor. if you looked at those branches call.ts:47, call.ts:70, call.ts:149. This means the attacker controls obj.context, obj.prop, obj.get, context.evals.get and a. This can lead to direct invocation of an internal primitive with forged operands

PoC

Weakness type: CWE-94.

Product: @nyariv/sandboxjs (npm) · Assigned by: GitHub_M · Published: 2026-05-28

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): 0.63% probability of exploitation activity in the next 30 days; percentile 50.

Timeline

• 2026-05-11 — GitHub-reviewed advisory GHSA-g8f2-4f4f-5jqw published (critical)
• 2026-05-28 — CVE record last updated
• 2026-05-28 — 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-g8f2-4f4f-5jqw github.com
2. CVE.org record CVE-2026-43898 cve.org
3. NVD entry CVE-2026-43898 nvd.nist.gov

Other references from the CVE record and advisory

1. github.com github.com
2. github.com github.com

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