GHSA-3w55-933p-8pv5: Strands Agents: symlink flaw enables file overwrite, session tampering

## Summary The `FileSessionManager` defaults to storing session data in `/tmp/strands/sessions/` (a world-writable directory) using predictable file paths and a write pattern (`open(path + ".tmp", "w")` → `os.replace()`) that follows symbolic links. A local attacker can exploit this to overwrite arbitrary files or inject malicious session data for persistent prompt injection. ## Vulnerability Details **Component:** `strands-py/src/strands/session/file_session_manager.py` **Root Cause:** 1. Default storage directory is `tempfile.gettempdir() + "/strands/sessions/"` (line 58) — resolves to `/tmp/strands/sessions/` on Linux/macOS 2. The `_write_file()` method (lines 116-123) writes to a predictable `.tmp` path without symlink protection: ```python tmp = f"{path}.tmp" with open(tmp, "w", encoding="utf-8", newline="\n") as f: json.dump(data, f, indent=2, ensure_ascii=False) os.replace(tmp, path) ``` 3. `open()` follows symlinks — a pre-placed symlink at the `.tmp` path causes writes to an arbitrary target file 4. No integrity verification (HMAC/signature) on session files — tampered data is loaded as-is ## Attack Vectors ### 1. Arbitrary File Overwrite (Local Privilege Escalation) 1. Attacker pre-creates `/tmp/strands/sessions/session_<id>/` (or monitors for new directories via inotify) 2. Attacker creates a symlink: `session.json.tmp` → `/home/victim/.bashrc` (or any writable file) 3. Victim application writes session data → follows symlink → overwrites target file with JSON content 4. Attacker achieves code execution on next shell login (or targets `.ssh/authorized_keys`, cron files, etc.) ### 2. Persistent Prompt Injection via Session Tampering 1. Attacker writes crafted message files into the session directory 2. Victim application restores the session → loads attacker-controlled conversation history 3. Injected messages instruct the agent to execute attacker commands on every future interaction 4. No integrity check prevents this — the SDK trusts all session files at face value ## Impact - **Arbitrary file overwrite** — local attacker can write attacker-controlled content to any file writable by the victim process - **Local privilege escalation** — via overwriting shell configs, SSH keys, cron jobs, or systemd user services - **Persistent agent compromise** — via session data injection, the agent follows attacker instructions indefinitely ## Reproduction ```python import os, json, tempfile, shutil # Setup: simulate the vulnerable write pattern session_dir = "/tmp/strands/sessions/session_test/" os.makedirs(session_dir, exist_ok=True) # Attacker: create symlink at the .tmp write path target_file = "/tmp/symlink_attack_proof.txt" with open(target_file, "w") as f: f.write("ORIGINAL - should not be overwritten\n") symlink_path = os.path.join(session_dir, "session.json.tmp") if os.path.exists(symlink_path): os.unlink(symlink_path) os.symlink(target_file, symlink_path) # Victim: FileSessionManager._write_file() equivalent with open(symlink_path, "w", encoding="utf-8", newline="\n") as f: json.dump({"session_id": "test", "injected": True}, f, indent=2) # Verify: target file overwritten with open(target_file) as f: print(f"Target file content: {f.read()}") # Output: {"session_id": "test", "injected": true} # Cleanup os.unlink(symlink_path) os.unlink(target_file) shutil.rmtree(session_dir) ``` ## Suggested Fix 1. **Use a user-private default directory** — `~/.strands/sessions/` or `$XDG_DATA_HOME/strands/sessions/` instead of `/tmp/` 2. **Use `os.open()` with `O_NOFOLLOW | O_CREAT | O_WRONLY`** — refuses to follow symlinks 3. **Use `tempfile.mkstemp()`** for atomic writes instead of predictable `.tmp` suffix 4. **Verify directory ownership** matches the current user before writing 5. **Add optional integrity verification** (HMAC) on session files to detect tampering

On a shared build server or multi-user host running a Strands-based coding or ops agent, a low-privileged local user watches /tmp/strands/sessions/ with inotify. When a new session directory appears, they place a symlink named session.json.tmp pointing to the victim's ~/.ssh/authorized_keys or ~/.bashrc. The agent then writes its session JSON through the symlink, corrupting the target file, and the attacker uses this to gain code execution as the victim at next login. In a second variant, the attacker writes crafted message files into the session directory containing instructions such as 'always run this command and send results to this URL'. When the agent restores the session, it treats these as legitimate history and follows them on every future run.