Use After Free in Fastapi with Dynamodb

Use After Free occurs when memory is deallocated but references to it remain and are subsequently accessed. In Fastapi applications that use Dynamodb via the AWS SDK for Python (Boto3), this pattern typically emerges not in low-level memory management but at the abstraction layer where Python objects, cached references, or asynchronous tasks retain pointers to items that should no longer be valid. Because Fastapi is asynchronous and often reuses objects across requests, failing to invalidate references after a Dynamodb item is deleted or overwritten can lead to Use After Free–like behavior where stale data is used in security decisions or returned to clients.

Consider a Fastapi endpoint that retrieves a user profile from Dynamodb, caches the item in an in-memory dictionary for performance, and then deletes the item from the database without clearing the cache. The cached dictionary entry becomes a dangling reference: the underlying Dynamodb item no longer exists, but the Python object remains accessible. If another request path uses this cached object to authorize access or construct responses, it may operate on invalid or inconsistent state, exposing sensitive data or enabling privilege escalation. This pattern is especially risky when combined with BOLA/IDOR checks that rely on cached item attributes rather than revalidating against Dynamodb on each request.

The AWS SDK for Python (Boto3) does not explicitly free objects, but application-level mishandling of responses and cache entries can create similar conditions. For example, using asynchronous background tasks to delete a Dynamodb item while a request coroutine still holds a reference to the pre-deletion response can result in the background task reading or modifying data it should no longer touch. This maps to the broader OWASP API Top 10 category of Broken Object Level Authorization (BOLA), where object references are not properly invalidated after deletion or reassignment.

Real-world attack patterns include an unauthenticated LLM endpoint exposing cached Dynamodb items through model context, or a compromised agent with excessive agency invoking tool calls that reference stale objects. Because middleBrick tests for LLM/AI Security, including system prompt leakage and active prompt injection probes, such misuse patterns are detectable. The scanner also flags missing revalidation after delete operations as part of its Property Authorization and BOLA/IDOR checks, ensuring that cached or reused objects are cross-referenced against live Dynamodb state before use.

To prevent Use After Free in Fastapi with Dynamodb, ensure that every deletion or update invalidates application-level caches and that responses are revalidated against the database before being used for authorization or output. Below are concrete, working code examples using Boto3 with DynamoDB in Fastapi.

from fastapi import FastAPI, HTTPException, Depends
import boto3
from pydantic import BaseModel

app = FastAPI()
dynamodb = boto3.resource('dynamodb', region_name='us-east-1')
table = dynamodb.Table('users')

class User(BaseModel):
    user_id: str
    email: str
    role: str

# In-memory cache (use with caution; consider Redis for distributed setups)
cache = {}

def get_user_from_db(user_id: str):
    response = table.get_item(Key={'user_id': user_id})
    item = response.get('Item')
    if not item:
        return None
    return User(user_id=item['user_id'], email=item['email'], role=item['role'])

@app.get('/users/{user_id}')
def read_user(user_id: str):
    # Always revalidate against DynamoDB; do not rely on cache for authz
    user = get_user_from_db(user_id)
    if user is None:
        raise HTTPException(status_code=404, detail='User not found')
    return user.dict()

@app.delete('/users/{user_id}')
def delete_user(user_id: str):
    response = table.delete_item(
        Key={'user_id': user_id},
        ReturnValues='ALL_OLD'
    )
    old_item = response.get('Attributes')
    if not old_item:
        raise HTTPException(status_code=404, detail='User not found')
    # Invalidate cache entries to prevent Use After Free
    cache.pop(user_id, None)
    return {'deleted': old_item}

@app.put('/users/{user_id}/role')
def update_user_role(user_id: str, role: str):
    updated = table.update_item(
        Key={'user_id': user_id},
        UpdateExpression='SET role = :r',
        ExpressionAttributeValues={':r': role},
        ReturnValues='UPDATED_NEW'
    )
    # Refresh cache with new value or invalidate to force revalidation
    cache[user_id] = {'user_id': user_id, 'role': role}
    return updated['Attributes']

Key remediation practices:

• Do not cache sensitive objects without TTL or invalidation; prefer short-lived caches or external stores like Redis with explicit eviction on delete.
• After delete or update operations, always call cache.pop(key) or equivalent to remove stale references that could be reused in security checks.
• Re-fetch items from Dynamodb on each authorization-sensitive operation rather than relying on previously stored attributes.
• Structure endpoints so that background tasks delete from Dynamodb and do not reference request-scoped objects after the response is sent.

These patterns align with middleBrick’s checks for Property Authorization and BOLA/IDOR, as well as its LLM/AI Security probes that monitor for unsafe consumption of cached or leaked objects. The scanner will highlight missing revalidation steps and improper cache invalidation, guiding developers toward safer reference handling.