Sandbox Security Flaw Exposes Data Exfiltration Risk Through DNS Queries

Security researchers have identified a concerning pattern in sandbox runtime implementations that allows unexpected data exfiltration through DNS resolution mechanisms. This discovery emerged from a GitHub issue filed against Anthropic's experimental sandbox-runtime project, highlighting how a seemingly innocuous setting - allowLocalBinding: true - can create unintended security consequences.

The core vulnerability occurs when sandbox configurations permit local port binding while restricting network access through domain allowlists. As demonstrated in issue #88, an attacker can craft DNS queries containing sensitive data (like SSH keys) in subdomains of malicious domains. Normally, the sandbox should block requests to unapproved domains like evil.com. However, when attackers configure NS records for subdomains (such as a.evil.com), major DNS resolvers automatically redirect queries for your-ssh-key.a.evil.com to the attacker-controlled nameservers. This bypasses the sandbox's domain restrictions entirely.

Evidence from the sandbox-runtime test case shows how the DNS query successfully returns an IP address from the malicious server despite not being on the allowed domains list. The dig command execution within the sandbox demonstrates complete exfiltration capability, with the attacker receiving both the subdomain name (containing stolen data) and originating IP information. This technique effectively transforms DNS resolution into a covert channel, undermining the sandbox's security guarantees.

Counter-perspectives suggest that local binding remains valuable for legitimate development purposes. Enabling allowLocalBinding facilitates testing network services within constrained environments and supports workflows requiring internal communication. Some argue that completely disabling this feature would hinder practical sandbox utility. Alternative mitigation proposals include implementing stricter DNS filtering at the resolver level, adding payload inspection for DNS queries, or creating allowlist exceptions only for trusted recursive resolvers.

The vulnerability underscores broader challenges in sandbox security design. Many runtime environments treat DNS as a trusted resolution service rather than a potential attack vector. This incident serves as a reminder that security controls must account for protocol-level behaviors and delegation chains. Developers using sandboxing technologies should audit their DNS handling configurations, especially when processing untrusted input that could be embedded in domain queries. As sandboxing becomes increasingly critical for AI safety and code isolation, such edge cases highlight the ongoing tension between functionality and security in constrained execution environments.

Anthropic's experimental sandbox-runtime project continues to evolve, with this issue prompting discussions about more granular network control policies. The incident demonstrates how open collaboration through platforms like GitHub enables rapid identification and remediation of subtle security flaws that might otherwise go unnoticed in proprietary systems.