Sandbox Escape in Fastapi with Dynamodb

A sandbox escape in the context of FastAPI and DynamoDB occurs when user-controlled input influences how DynamoDB requests are constructed in a way that bypasses intended access restrictions or exposes unintended operations. This typically maps to Broken Object Level Authorization (BOLA) and Improper Authorization (IDOR) checks, but the specific interaction with DynamoDB can amplify risk.

FastAPI does not enforce authorization by default; it relies on application logic to validate that a requesting identity can access a given resource. When that logic is coupled with DynamoDB, unsafe construction of request parameters—such as using user-supplied IDs directly in key expressions—can allow an attacker to substitute their own identifiers into queries or scans. For example, if an endpoint uses user_id from a JWT to build a KeyConditionExpression but also accepts a target resource_id from the request without verifying ownership, an attacker can change resource_id to access another user’s data.

DynamoDB’s low-level API and query semantics provide flexibility, but they do not enforce constraints on the application’s authorization model. If the application uses a scan or query that lacks a required filter on ownership or tenant context, an attacker may leverage crafted input to retrieve or modify items outside their scope. This becomes a sandbox escape when the attacker can pivot from their own data set to another user’s or higher-privilege data set, effectively breaking tenant or permission boundaries.

Additionally, malformed or unexpected input can affect pagination or conditional expressions in ways that expose more data than intended. For example, an attacker may supply a limit or ExclusiveStartKey-like value that changes the scan’s progression or causes inclusion of items that should have been excluded. While DynamoDB itself does not introduce the escape, the API layer’s failure to enforce strict access control and input validation allows the vulnerability to manifest.

Real-world mappings include checks from the OWASP API Top 10 (2023), such as #1 Broken Object Level Authorization, and relevant patterns in PCI-DSS and SOC2 controls around access to cardholder or sensitive data. In unauthenticated scanning, middleBrick’s BOLA/IDOR and Property Authorization checks can surface endpoints where the combination of FastAPI route parameters and DynamoDB key construction lacks sufficient ownership verification.

To illustrate, consider an endpoint that retrieves a user’s profile by ID. If the route accepts both a path parameter and a query parameter that influence the DynamoDB key without validating that the target ID belongs to the authenticated subject, the boundary between user data blurs. middleBrick’s runtime checks compare spec definitions against observed behavior; if the spec claims per-user isolation but runtime calls show cross-user item access, a finding is surfaced with severity and remediation guidance.

Remediation centers on enforcing strict ownership checks, parameterizing queries, and validating all inputs before they reach DynamoDB. Below are concrete patterns for FastAPI that reduce the risk of sandbox escape when using DynamoDB.

1. Enforce ownership with path and contextual identifiers

Always derive key expressions from the authenticated subject and avoid using raw client-supplied IDs as the sole determinant of access.

from fastapi import Depends, HTTPException, status
from pydantic import BaseModel
import boto3
from botocore.exceptions import ClientError

# Assume authentication dependency provides user context
def get_current_user():
    # In practice, validate token and return user identity
    return {"user_id": "usr_abc123"}

class Item(BaseModel):
    user_id: str
    item_id: str
    data: str

# Use a dependency to inject user identity
from fastapi.security import HTTPBearer
security = HTTPBearer()

def get_user_from_token(token: str):
    # Validate and decode token; return user identity
    return {"user_id": "usr_abc123"}

# Example endpoint: retrieve item scoped to authenticated user
from fastapi import APIRouter, Depends
router = APIRouter()

dynamodb = boto3.resource('dynamodb', region_name='us-east-1')
table_name = 'UserItems'

@router.get('/items/{item_id}')
def read_item(
    item_id: str,
    token: str = Depends(security),
    user: dict = Depends(get_current_user)
):
    # Critical: use authenticated user_id, not item_id alone, to scope the query
    table = dynamodb.Table(table_name)
    try:
        response = table.get_item(
            Key={
                'user_id': user['user_id'],  # Enforce ownership
                'item_id': item_id
            }
        )
        item = response.get('Item')
        if not item:
            raise HTTPException(status_code=404, detail='Item not found or access denied')
        return item
    except ClientError as e:
        raise HTTPException(status_code=500, detail='Database error')

2. Validate and parameterize queries to avoid injection and over-fetching

Do not concatenate user input into expression strings. Use expression attribute values and validate constraints such as length and type.

from fastapi import Query
import re

@router.get('/search')
def search_items(
    user_id: str = Depends(lambda: get_current_user()['user_id']),
    prefix: str = Query(..., min_length=1, max_length=100),
    limit: int = Query(10, ge=1, le=50)
):
    table = dynamodb.Table(table_name)
    # Validate prefix to avoid unexpected behavior
    if not re.match(r'^[A-Za-z0-9_-]+$', prefix):
        raise HTTPException(status_code=400, detail='Invalid search prefix')
    response = table.query(
        KeyConditionExpression='user_id = :uid AND begins_with(item_id, :prefix)',
        ExpressionAttributeValues={
            ':uid': user_id,
            :prefix': prefix
        },
        Limit=limit
    )
    return response.get('Items', [])

3. Avoid scans; prefer queries with required filters

Scans are expensive and can expose data if pagination or filtering is misused. Prefer query operations that include the partition key and a sort key condition that includes ownership.

@router.get('/items')
def list_user_items(
    user_id: str = Depends(lambda: get_current_user()['user_id']),
    limit: int = Query(20, le=100)
):
    table = dynamodb.Table(table_name)
    response = table.query(
        KeyConditionExpression='user_id = :uid',
        ExpressionAttributeValues={':uid': user_id},
        Limit=limit
    )
    return response.get('Items', [])

4. Use middleware for consistent tenant/ownership resolution

Centralize ownership resolution to reduce mistakes across endpoints. For example, map a tenant ID from the token to a DynamoDB attribute and enforce it in all data access.

These patterns align with findings that middleBrick surfaces for BOLA/IDOR and Property Authorization by ensuring that runtime behavior matches documented access boundaries. For teams on the Pro plan, continuous monitoring can detect regressions where such checks are accidentally omitted.