Agent Leaks User Data Across Sessions — Shared In-Memory State
Symptom
- User B sees User A’s name or conversation history in responses
- Agent says “As you mentioned earlier, your name is Alice” to a user named Bob
- Global dict or list accumulates data from multiple users without isolation
- Module-level variable modified in one request is visible in all subsequent requests
- Tool call uses previous user’s context — sends email to wrong person
- Privacy incident: one user’s query returns another user’s data
Root Cause
Python module-level variables, class variables, and any state not scoped to a request or session are shared across all concurrent users in the same process. A FastAPI handler that reads from user_data["name"] without a session key mixes all users into the same dict. asyncio doesn’t isolate coroutines — they share the same memory space. Multi-threaded servers share global state unless explicitly synchronized and namespaced per session.
Fix
Option 1: Scope all state to session ID — never use global dicts
from dataclasses import dataclass, field
from typing import Any
import uuid
import time
@dataclass
class SessionState:
"""All state for a single user session — never shared with other sessions"""
session_id: str
user_id: str | None = None
created_at: float = field(default_factory=time.time)
last_active: float = field(default_factory=time.time)
# Session-scoped data — isolated per user
conversation_history: list[dict] = field(default_factory=list)
user_preferences: dict = field(default_factory=dict)
task_state: dict = field(default_factory=dict)
context: dict = field(default_factory=dict)
def touch(self):
self.last_active = time.time()
class SessionStore:
"""
Thread-safe session storage — each session completely isolated.
Sessions are identified by session_id, never shared across users.
"""
def __init__(self, ttl_seconds: int = 3600):
self._sessions: dict[str, SessionState] = {}
self._ttl = ttl_seconds
self._lock = __import__("threading").Lock()
def create(self, user_id: str | None = None) -> SessionState:
session_id = str(uuid.uuid4())
session = SessionState(session_id=session_id, user_id=user_id)
with self._lock:
self._sessions[session_id] = session
return session
def get(self, session_id: str) -> SessionState | None:
with self._lock:
session = self._sessions.get(session_id)
if session:
session.touch()
return session
def get_or_create(self, session_id: str, user_id: str | None = None) -> SessionState:
session = self.get(session_id)
if session is None:
session = SessionState(session_id=session_id, user_id=user_id)
with self._lock:
self._sessions[session_id] = session
return session
def delete(self, session_id: str):
with self._lock:
self._sessions.pop(session_id, None)
def prune_expired(self):
"""Remove sessions that have been idle beyond TTL"""
now = time.time()
with self._lock:
expired = [
sid for sid, s in self._sessions.items()
if now - s.last_active > self._ttl
]
for sid in expired:
del self._sessions[sid]
if expired:
print(f"Pruned {len(expired)} expired sessions")
# One global store — all session data isolated by session_id:
sessions = SessionStore(ttl_seconds=3600)
# WRONG — global state shared across all users:
# conversation_history = [] # ALL users share this!
# user_name = "" # Last writer wins!
# RIGHT — session-scoped state:
def add_message(session_id: str, role: str, content: str):
session = sessions.get(session_id)
if session is None:
raise ValueError(f"Session {session_id} not found")
session.conversation_history.append({"role": role, "content": content})
Option 2: FastAPI dependency injection — session per request
from fastapi import FastAPI, Header, HTTPException, Depends
from fastapi.security import HTTPBearer
from typing import Annotated
import jwt
app = FastAPI()
sessions = SessionStore()
async def get_session(
x_session_id: Annotated[str | None, Header()] = None,
authorization: Annotated[str | None, Header()] = None,
) -> SessionState:
"""
FastAPI dependency — injects the correct session for each request.
Every request gets its own isolated SessionState.
No global state accessible to handlers.
"""
if x_session_id:
session = sessions.get(x_session_id)
if session is None:
raise HTTPException(status_code=404, detail="Session not found or expired")
return session
# Create new session from auth token
if authorization:
try:
token = authorization.replace("Bearer ", "")
payload = jwt.decode(token, SECRET_KEY, algorithms=["HS256"])
user_id = payload["sub"]
session = sessions.create(user_id=user_id)
return session
except jwt.InvalidTokenError:
raise HTTPException(status_code=401, detail="Invalid token")
# Anonymous session
return sessions.create()
@app.post("/chat")
async def chat(
request: dict,
session: Annotated[SessionState, Depends(get_session)]
) -> dict:
"""
Handler receives isolated session — no global state possible.
User A's handler and User B's handler have completely separate SessionState objects.
"""
user_message = request.get("message", "")
# All state goes through session — never global vars:
session.conversation_history.append({"role": "user", "content": user_message})
response = await call_llm(
history=session.conversation_history, # Session-specific history
user_preferences=session.user_preferences # Session-specific prefs
)
session.conversation_history.append({"role": "assistant", "content": response})
return {
"response": response,
"session_id": session.session_id
}
Option 3: asyncio context variables — per-coroutine state
import asyncio
from contextvars import ContextVar
from dataclasses import dataclass
# ContextVar is isolated per asyncio Task — perfect for coroutine-per-request patterns
current_session_id: ContextVar[str] = ContextVar("current_session_id")
current_user_id: ContextVar[str | None] = ContextVar("current_user_id", default=None)
@dataclass
class RequestContext:
session_id: str
user_id: str | None
request_id: str
started_at: float
current_request: ContextVar[RequestContext | None] = ContextVar("current_request", default=None)
async def handle_request(session_id: str, user_id: str, user_message: str) -> str:
"""
Set context variables at request start — they don't leak to other coroutines.
"""
import uuid
import time
ctx = RequestContext(
session_id=session_id,
user_id=user_id,
request_id=str(uuid.uuid4()),
started_at=time.time()
)
# Set context vars — only visible to this coroutine and its children
token_session = current_session_id.set(session_id)
token_user = current_user_id.set(user_id)
token_ctx = current_request.set(ctx)
try:
return await process_message(user_message)
finally:
# Reset — context vars don't carry over to next request on same event loop
current_session_id.reset(token_session)
current_user_id.reset(token_user)
current_request.reset(token_ctx)
async def process_message(message: str) -> str:
"""Can safely access context vars — they're isolated to this coroutine"""
session_id = current_session_id.get() # Always this user's session
user_id = current_user_id.get()
# No risk of seeing another user's session_id here
session = sessions.get(session_id)
return await call_llm(session.conversation_history, message)
Option 4: Audit existing code for global state leaks
import ast
import sys
from pathlib import Path
class GlobalStateAuditor(ast.NodeVisitor):
"""
AST visitor that identifies potential global state mutations.
Run this in CI to catch session isolation bugs before they reach production.
"""
def __init__(self, filename: str):
self.filename = filename
self.violations: list[dict] = []
self._global_names: set[str] = set()
def visit_Global(self, node: ast.Global):
"""Track variables declared global"""
for name in node.names:
self._global_names.add(name)
self.generic_visit(node)
def visit_Assign(self, node: ast.Assign):
"""Flag module-level assignments to mutable types"""
# Only care about module-level (not inside functions/classes)
for target in node.targets:
if isinstance(target, ast.Name):
# Check if value is a mutable type
if isinstance(node.value, (ast.Dict, ast.List, ast.Set)):
self.violations.append({
"type": "mutable_module_level",
"name": target.id,
"line": node.lineno,
"file": self.filename,
"issue": f"Module-level mutable {type(node.value).__name__} '{target.id}' "
f"will be shared across all sessions"
})
self.generic_visit(node)
def visit_AugAssign(self, node: ast.AugAssign):
"""Flag augmented assignments to potentially global vars"""
if isinstance(node.target, ast.Subscript):
# dict[key] = value or list.append() patterns
if isinstance(node.target.value, ast.Name):
if node.target.value.id in self._global_names:
self.violations.append({
"type": "global_mutation",
"name": node.target.value.id,
"line": node.lineno,
"file": self.filename,
"issue": f"Mutation of global variable '{node.target.value.id}' "
f"will affect all concurrent sessions"
})
self.generic_visit(node)
def audit_file_for_session_leaks(file_path: str) -> list[dict]:
source = Path(file_path).read_text()
tree = ast.parse(source, filename=file_path)
auditor = GlobalStateAuditor(file_path)
auditor.visit(tree)
return auditor.violations
# Run in CI:
violations = []
for py_file in Path("src").rglob("*.py"):
violations.extend(audit_file_for_session_leaks(str(py_file)))
if violations:
print("SESSION ISOLATION VIOLATIONS:")
for v in violations:
print(f" {v['file']}:{v['line']}: {v['issue']}")
sys.exit(1)
Option 5: Test for session data leakage
import asyncio
import pytest
from fastapi.testclient import TestClient
def test_no_data_leakage_between_sessions(client: TestClient):
"""
Verify user A's data cannot be seen by user B.
Run this test before every release.
"""
# User A: start session and share personal info
response_a = client.post("/chat", json={"message": "My name is Alice Smith"})
assert response_a.status_code == 200
session_a = response_a.headers.get("x-session-id") or response_a.json().get("session_id")
assert session_a
# User B: start a completely separate session
response_b = client.post("/chat", json={"message": "Hello, who am I?"})
assert response_b.status_code == 200
session_b = response_b.headers.get("x-session-id") or response_b.json().get("session_id")
assert session_b
# Sessions must be different
assert session_a != session_b, "Two separate requests got the same session ID"
# User B should NOT see Alice's name in any subsequent response
for _ in range(3):
response = client.post(
"/chat",
json={"message": "What is my name?"},
headers={"x-session-id": session_b}
)
body = response.json().get("response", "").lower()
assert "alice" not in body, (
f"Session B leaked Session A's data. Response: {body[:200]}"
)
assert "smith" not in body, (
f"Session B leaked Session A's surname. Response: {body[:200]}"
)
def test_concurrent_sessions_isolated():
"""Test isolation under concurrent load"""
import threading
import uuid
results = {}
errors = []
def user_session(user_name: str, user_id: str):
try:
with TestClient(app) as client:
# Establish identity
client.post("/chat", json={"message": f"My name is {user_name}"})
# Ask for recall
response = client.post("/chat", json={"message": "What is my name?"})
result_text = response.json().get("response", "").lower()
results[user_id] = result_text
except Exception as e:
errors.append(str(e))
users = [("Alice", "user_a"), ("Bob", "user_b"), ("Charlie", "user_c")]
threads = [threading.Thread(target=user_session, args=u) for u in users]
[t.start() for t in threads]
[t.join() for t in threads]
assert not errors, f"Session errors: {errors}"
# Each user should only see their own name
for name, user_id in users:
result = results.get(user_id, "")
assert name.lower() in result, f"{name} not found in own session response"
for other_name, other_id in users:
if other_id != user_id:
assert other_name.lower() not in result, (
f"Data leak: {name}'s session contains {other_name}'s name"
)
Option 6: Redis-backed session store for distributed isolation
import redis
import json
import time
import uuid
from typing import Any
class RedisSessionStore:
"""
Redis-backed session store — works across multiple agent instances.
Sessions isolated by session_id — no in-process global state.
"""
def __init__(self, redis_url: str = "redis://localhost:6379", ttl: int = 3600):
self.r = redis.Redis.from_url(redis_url, decode_responses=True)
self.ttl = ttl
self._prefix = "agent:session:"
def _key(self, session_id: str) -> str:
return f"{self._prefix}{session_id}"
def create(self, user_id: str | None = None) -> str:
session_id = str(uuid.uuid4())
data = {
"session_id": session_id,
"user_id": user_id,
"created_at": time.time(),
"conversation_history": [],
"context": {}
}
self.r.setex(self._key(session_id), self.ttl, json.dumps(data))
return session_id
def get(self, session_id: str) -> dict | None:
raw = self.r.get(self._key(session_id))
if raw is None:
return None
data = json.loads(raw)
# Refresh TTL on access
self.r.expire(self._key(session_id), self.ttl)
return data
def update(self, session_id: str, field: str, value: Any):
"""Update a specific field in the session"""
data = self.get(session_id)
if data is None:
raise KeyError(f"Session {session_id} not found")
data[field] = value
self.r.setex(self._key(session_id), self.ttl, json.dumps(data))
def append_message(self, session_id: str, role: str, content: str):
"""Append to conversation history atomically"""
with self.r.pipeline() as pipe:
while True:
try:
pipe.watch(self._key(session_id))
data = json.loads(pipe.get(self._key(session_id)) or "{}")
history = data.get("conversation_history", [])
history.append({"role": role, "content": content})
data["conversation_history"] = history
pipe.multi()
pipe.setex(self._key(session_id), self.ttl, json.dumps(data))
pipe.execute()
break
except redis.WatchError:
continue # Retry on concurrent modification
def delete(self, session_id: str):
self.r.delete(self._key(session_id))
Session Isolation Checklist
| Pattern | Risk | Fix |
|---|---|---|
Module-level dict for user data |
Critical | Scope to session object |
| Class variable (not instance) for state | Critical | Use instance variables |
global keyword in handler functions |
High | Pass session as parameter |
asyncio.Queue without per-user queues |
High | One queue per session |
| Cache with user data, no session key | High | Include session_id in cache key |
| Logging user data without session ID | Medium | Add session_id to all log records |
Expected Token Savings
Privacy incident → incident response → disclosure → rebuild trust: priceless to avoid Session isolation implemented correctly: 0 risk of cross-session data exposure
Environment
- Any multi-tenant agent serving more than one user; critical for all production agents — single-user dev setups are safe, but any concurrent user scenario requires explicit session isolation
- Source: direct experience; global state leaks are the most common privacy vulnerability in agent deployments that were originally designed for single-user use and later scaled to multiple users
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