Double Free in Fastapi with Basic Auth

A Double Free class of memory safety vulnerability can manifest in FastAPI applications that use Basic Authentication in an unsafe manner, particularly when request state or external libraries mishandle credential parsing and deallocation. In the context of an unauthenticated black-box scan, middleBrick tests for behaviors that may indicate insecure handling of authentication headers and memory, including unexpected crashes, information leaks, or authorization bypasses under malformed input.

When FastAPI processes a request with Basic Auth, the framework typically relies on the underlying ASGI server and HTTP libraries to decode the Authorization header. If the application code parses the header manually (e.g., splitting a base64-encoded string) and does not validate input length or encoding, it may pass invalid pointers or zero-length buffers to downstream components. In native extensions or C-based dependencies (such as cryptography bindings), this can trigger a double-free condition: a chunk of memory is freed once during cleanup and then freed again when the request context is torn down, leading to undefined behavior, crashes, or potential code execution.

Consider a scenario where a developer implements custom Basic Auth parsing to extract a username and password without using a validated library. If the header is malformed (e.g., missing padding, excessively long, or containing null bytes), the parsing logic might allocate temporary buffers incorrectly. When FastAPI’s request lifecycle completes, both the application code and the server attempt to release the same memory, creating a double-free condition. middleBrick’s checks for unsafe consumption and input validation would flag such endpoints, as malformed credentials could lead to authentication bypass or server instability.

Moreover, if the application integrates third-party authentication libraries that internally manage memory without strict bounds checking, the combination of Basic Auth’s base64-encoded credentials and improper error handling can exacerbate the risk. For example, an endpoint that decodes credentials and passes them to a native cryptographic function might expose a double-free path if the decoded payload is unexpectedly truncated or oversized. This aligns with OWASP API Security Top 10 categories such as Broken Object Level Authorization and Security Misconfiguration, which middleBrick evaluates through parallel checks including Input Validation and Authentication.

During a scan, middleBrick tests unauthenticated attack surfaces by sending malformed or missing Authorization headers, observing how the service responds. Indicators of potential double-free conditions include 500-level errors, process restarts, or inconsistent behavior when identical requests are sent with slight variations in header encoding. While middleBrick detects and reports these anomalies, developers must review the underlying code and dependencies to confirm whether memory management flaws exist.

Secure handling of Basic Authentication in FastAPI requires using well-maintained libraries and avoiding manual parsing of credentials. The following examples demonstrate safe patterns that mitigate risks such as double-free conditions by relying on framework-managed validation and robust dependencies.

Safe Basic Auth with HTTPBearer and a validation library

Use fastapi.security.HTTPBearer or fastapi.security.HTTPBasic combined with a schema validation library like Pydantic to ensure credentials are properly structured before processing.

from fastapi import FastAPI, Depends, HTTPException, status
from fastapi.security import HTTPBasic, HTTPBasicCredentials
import secrets

app = FastAPI()
security = HTTPBasic()

def get_current_user(credentials: HTTPBasicCredentials = Depends(security)):
    # Rely on the framework to decode safely; avoid manual base64 handling
    correct_username = secrets.compare_digest(credentials.username, "admin")
    correct_password = secrets.compare_digest(credentials.password, "S3cur3P@ss!")
    if not (correct_username and correct_password):
        raise HTTPException(
            status_code=status.HTTP_401_UNAUTHORIZED,
            detail="Invalid credentials",
            headers={"WWW-Authenticate": "Basic"},
        )
    return credentials.username

@app.get("/secure-endpoint")
async def read_secure_item(username: str = Depends(get_current_user)):
    return {"message": f"Hello, {username}"}

Mitigation via dependency injection and secure credential storage

Store credentials using secure mechanisms (e.g., environment variables or secret managers) and validate them within dependencies. This reduces the risk of malformed input causing memory-handling issues in underlying libraries.

import os
from fastapi import Depends
from fastapi.security import HTTPBasic

security = HTTPBasic()

# Credentials fetched from a secure source at startup
VALID_USERNAME = os.getenv("API_USER", "admin")
VALID_PASSWORD = os.getenv("API_PASS")

def verify_basic_auth(credentials: HTTPBasicCredentials = Depends(security)):
    if credentials.username != VALID_USERNAME or credentials.password != VALID_PASSWORD:
        raise HTTPException(status_code=401, detail="Unauthorized")
    return credentials.username

@app.get("/items/")
async def list_items(username: str = Depends(verify_basic_auth)):
    return {"data": ["item1", "item2"]}

Avoid manual parsing and prefer managed libraries

Do not decode the Authorization header manually. Let HTTP libraries handle base64 decoding and string handling to prevent memory corruption issues that could contribute to double-free conditions.

# Avoid this pattern:
# import base64
# token = request.headers.get("authorization")
# if token and token.startswith("Basic "):
#     decoded = base64.b64decode(token[6:]).decode("utf-8")
#     username, password = decoded.split(":", 1)

# Instead, use framework-provided security schemes as shown above.

Additional hardening

• Ensure all dependencies, especially those interfacing with native code (e.g., cryptography), are up to date.
• Use constant-time comparison functions (e.g., secrets.compare_digest) to mitigate timing attacks.
• Apply input length and format validation at the API gateway or through Pydantic models to reject malformed credentials early.