GHSA-72w7-mf9g-733p: nono-py has proxy-only network fallback bypass on older Linux kernels
SummaryOn Linux kernels that do not support Landlock network rules, `nono_py.sandboxed_exec()` could run `CapabilitySet.proxy_only(proxy)` without supervising the seccomp-notify proxy-only fallback returned by the Rust core.In that configuration, a sandboxed child process could remove `HTTP_PROXY` / `HTTPS_PROXY` environment variables or use raw sockets and then open direct TCP connections that should have been denied by proxy-only policy.The issue affects proxy-only enforcement. It does not mean that all nono-py network blocking is ineffective. ECS validation showed `caps.block_network()` denied regular TCP and ECS metadata TCP on the tested Linux 6.1 host.ImpactThe intended `proxy_only()` security property is:child processes may connect only to the local nono proxy portthe proxy enforces host allowlists and metadata/link-local denialdirect TCP to any other target is deniedBefore the fix, on kernels without Landlock `AccessNet`, the Python binding applied the sandbox and then executed the child, but did not install and supervise the proxy-only seccomp-notify fallback. A child could therefore bypass the proxy layer in that old-kernel path.The highest-impact scenario is a sandboxed workload with access to cloud metadata discovery inputs, where direct TCP to a metadata endpoint could retrieve task or instance credentials after proxy environment variables are removed.Affected ConditionsThe issue requires all of the following:Linux runtime.Kernel without Landlock network support, such as Linux 6.1. Landlock network rules require Landlock ABI v4 / Linux 6.7 or newer.`nono_py.sandboxed_exec()` is used.The capability set uses `caps.proxy_only(proxy)`.The child process removes or ignores proxy environment variables, or uses raw sockets.macOS Seatbelt proxy-only enforcement is not affected by this Linux seccomp-notify fallback issue.Affected VersionsKnown affected builds include nono-py versions that expose and use `CapabilitySet.proxy_only()` through `sandboxed_exec()` before the supervised fallback fix in this working tree.Earlier versions that did not expose `CapabilitySet.proxy_only()` are not affected by this specific proxy-only enforcement bug, though they may have separate environment-inheritance risks if callers passed broad parent environment variables into sandboxed children.**CVSS Score Rationale**| Metric | Value | Rationale |
|---|---|---|
| **Attack Vector (AV)** | L — Local | Exploit is performed by a local process (unsetting env vars or opening raw sockets). Not remotely triggerable. |
| **Attack Complexity (AC)** | H — High | All of the following must be true: Linux runtime; kernel < 6.7 (no Landlock ABI v4); `sandboxed_exec()` used; capability set calls `proxy_only()`; child actively bypasses proxy env vars or uses raw sockets. |
| **Privileges Required (PR)** | L — Low | Attacker is already executing code inside the sandbox — some user-level privilege is required to get there. |
| **User Interaction (UI)** | N — None | No action from a user or operator is needed once the sandboxed child is running. |
| **Scope (S)** | C — Changed | The exploit crosses the sandbox security boundary, allowing the child to reach network resources outside the defined policy scope. |
| **Confidentiality (C)** | H — High | Highest-impact path: direct TCP to cloud metadata endpoint (169.254.169.254) yields IAM / task credentials. |
| **Integrity (I)** | L — Low | Attacker can make arbitrary outbound requests; no direct data modification from the bypass itself, but lateral credential use creates indirect risk. |
| **Availability (A)** | N — None | No denial-of-service impact described or implied. |
CVSS v3.1: 6.4