SandboxJS sandbox escape via Function.caller leading to arbitrary host JavaScript execution

CVE-2026-43898 is a critical sandbox escape vulnerability in the @nyariv/sandboxjs JavaScript sandboxing library affecting versions prior to 0.9.6 (including 0.9.5 and earlier). Sandbox-defined functions improperly expose sensitive function properties such as Function.caller, which allows sandboxed code to recover the internal host-side runtime callback LispType.Call. Because this callback can then be invoked with attacker-controlled fake context and obj/parameter values, an attacker can abuse the runtime’s trust assumptions to extract blocked host statics, recover the real host Function constructor, and escape the sandbox boundary. Once the genuine host Function constructor is obtained, the attacker can execute arbitrary JavaScript in the host environment. Supporting context indicates the vulnerable logic is in SandboxJS property access handling, specifically in prop.ts within addOps, where access to caller, callee, and arguments was not properly blocked.