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netfoil seccomp flag not applied and domain allowlist bypass (GHSA-vjgj-42f6-7997 / GHSA-84g5-x8j3-7235)

Signal: GitHub Security Advisories published 2026-04-29. netfoil fixed a defense-in-depth sandboxing failure and an off-by-one domain-filter bypass.

What it is

GHSA-vjgj-42f6-7997 reports that the optional --filter-system-calls seccomp sandbox flag was not applied even when specified. GHSA-84g5-x8j3-7235 reports an off-by-one suffix-trie bug where an allowlist entry such as example.com could be bypassed with a different first character such as fxample.com.

Affected package: Go github.com/tinfoil-factory/netfoil < 0.2.1. Fixed version: 0.2.1.

References: https://github.com/advisories/GHSA-vjgj-42f6-7997, https://github.com/advisories/GHSA-84g5-x8j3-7235

Triage

  1. Inventory netfoil deployments using DNS/domain filtering or relying on --filter-system-calls.
  2. Review allowlist policy for high-value domains and whether bypassable lookalikes could reach sensitive destinations.
  3. Confirm whether systemd, container, or kernel-level sandboxing exists outside the netfoil optional flag.

Mitigation

  • Upgrade netfoil to 0.2.1 or later.
  • Treat DNS filtering as one layer only; pair it with egress firewalling and destination IP controls.
  • Enforce seccomp/systemd sandboxing outside the application where possible.

Detection ideas

  • Review DNS and connection logs for one-character variants of allowlisted domains.
  • Check process startup logs/configs where --filter-system-calls was expected to harden the binary.

Durable lesson

Allowlists and optional sandboxes need negative tests. A control that silently fails closed only in documentation is not a control in production.