Use After Free in Fastapi with Basic Auth

Use After Free occurs when memory that has been freed is still referenced and subsequently accessed. In a Fastapi application that uses HTTP Basic Authentication, this can arise when request-scoped objects—such as a parsed credentials structure or an authentication state container—are deallocated while a downstream handler or background task still holds a reference to them.

Consider a route that authenticates with Basic Auth, constructs a mutable user context, and then schedules an asynchronous task without ensuring the context remains valid:

from fastapi import Depends, FastAPI, Header, BackgroundTasks
import base64

app = FastAPI()

async def background_task(user_data: dict):
    # Simulated async operation that may outlive the request
    await asyncio.sleep(2)
    print(f"Processing for: {user_data.get('username')}")

@app.get('/items')
def get_items(authorization: str = Header(None), background_tasks: BackgroundTasks = BackgroundTasks()):
    if not authorization or not authorization.startswith('Basic '):
        raise HTTPException(status_code=401, detail='Missing basic auth')
    encoded = authorization.split(' ')[1]
    decoded = base64.b64decode(encoded).decode('utf-8')
    username, password = decoded.split(':', 1)
    user_data = {'username': username, 'role': 'user'}
    background_tasks.add_task(background_task, user_data)
    # user_data may be freed at end of request, but background_task still references it
    return {'status': 'accepted'}

In this pattern, user_data is passed into a background task. If Fastapi’s request context is reclaimed before the background task executes, the task may read or write to memory that has been repurposed—manifesting as corrupted data, crashes, or information disclosure. The risk is higher when the handler spawns multiple concurrent tasks or when the runtime’s garbage collection behavior interacts with object lifetimes across threads.

The same category of issue can appear with stateful authentication helpers that cache decoded credentials in a request-scoped container and later attempt to reuse that container after it has been logically released. Because Fastapi does not inherently extend the lifetime of objects beyond the request cycle, developers must explicitly manage ownership and lifetimes when cross-request or asynchronous references are used.

Another scenario involves middleware that temporarily mutates authentication state and releases it prematurely. For example, a middleware layer might decode Basic Auth, attach a user object to the request state, and then clear the intermediate buffer to free memory. If a downstream dependency retains a pointer to that buffer beyond the middleware’s scope, it may operate on freed memory, leading to erratic behavior or security-sensitive data exposure.

To mitigate Use After Free in Fastapi with Basic Auth, ensure that any data used by asynchronous or background operations is explicitly copied and does not depend on transient request-scoped buffers. Prefer structured dependency injection with explicit lifetimes, and avoid passing raw references to objects that may be deallocated.

Safe pattern using deep copies and explicit lifetimes:

import asyncio
import base64
from copy import deepcopy
from fastapi import FastAPI, Header, BackgroundTasks, HTTPException

app = FastAPI()

async def background_task(user_snapshot: dict):
    await asyncio.sleep(2)
    print(f"Background processing for: {user_snapshot.get('username')}")

@app.get('/items')
def get_items(
    authorization: str = Header(None),
    background_tasks: BackgroundTasks = BackgroundTasks()
):
    if not authorization or not authorization.startswith('Basic '):
        raise HTTPException(status_code=401, detail='Missing basic auth')
    encoded = authorization.split(' ')[1]
    decoded = base64.b64decode(encoded).decode('utf-8')
    username, password = decoded.split(':', 1)
    user_data = {'username': username, 'role': 'user'}
    # Create an independent snapshot to avoid use-after-free
    user_snapshot = deepcopy(user_data)
    background_tasks.add_task(background_task, user_snapshot)
    return {'status': 'accepted'}

For synchronous background work or when using FastAPI dependencies, structure dependencies with explicit return types and scopes. Use a dependency that returns a validated credential object and ensure downstream consumers do not hold references beyond the dependency’s lifecycle:

from fastapi import Depends, FastAPI, Header, HTTPException
import base64
from typing import Dict

app = FastAPI()

def get_current_user(authorization: str = Header(None)) -> Dict[str, str]:
    if not authorization or not authorization.startswith('Basic '):
        raise HTTPException(status_code=401, detail='Missing basic auth')
    encoded = authorization.split(' ')[1]
    decoded = base64.b64decode(encoded).decode('utf-8')
    username, password = decoded.split(':', 1)
    # Return a plain dict snapshot rather than a reference to a mutable request-bound object
    return {'username': username, 'role': 'user'}

@app.get('/profile')
def read_profile(user: Dict[str, str] = Depends(get_current_user)):
    # user is a concrete dict, independent of any request buffer
    return {'user': user}

Additionally, review any use of global or cached authentication state. If credentials are cached, ensure that cached entries are not overwritten while still in use elsewhere. In Fastapi, combine dependency scopes carefully: use Depends with appropriate scopes (e.g., scope="request") and avoid singletons that retain stale references across requests.

These practices align well with broader API security checks available in scans that test for Authentication issues and BOLA/IDOR patterns. By validating that your authentication handling does not inadvertently expose freed memory, you reduce the likelihood of instability and information leakage in production deployments.