Use After Free in Chi with Basic Auth

A Use After Free (UAF) in the context of Chi with HTTP Basic Authentication occurs when memory that was previously allocated to an object (e.g., a parsed credentials structure or a request context) is deallocated while references to it remain active. If an attacker can influence the timing or lifecycle of allocations and deallocations—often via concurrent requests or malformed input—they may cause the runtime to reuse freed memory. When Basic Authentication is handled naively in Chi, this can happen if credential containers are released prematurely, for example when middleware frees a context object before downstream handlers finish using it, or when authentication parsing reuses buffers across requests without proper isolation.

In Chi, Basic Authentication is commonly implemented by reading the Authorization header, base64-decoding the credentials, and storing the username and password in request context values for downstream handlers or middleware. If the implementation uses short-lived stack-allocated objects or improperly manages heap-allocated structs that are freed at the end of a request, an attacker might trigger conditions where a subsequent request reuses the same memory. For example, a handler could retain a pointer to a credentials struct after the request context is cleared, and if Chi’s request lifecycle frees that struct at the end of the request, later use of the pointer results in UAF. This can lead to information disclosure (reading stale credentials), corruption of authentication state, or potentially to controlled code execution depending on how the runtime reuses the freed memory.

The interaction with Basic Auth amplifies risk because credentials are often retained in memory longer than necessary. If Chi routes or middleware cache or log parsed credentials without ensuring secure memory handling, freed memory may still contain sensitive data. An attacker who can force allocations and deallocations—such as by sending many requests with malformed or missing Authorization headers—might observe timing differences or use side channels to infer whether UAF occurred. While Chi itself does not manage memory explicitly, the way handlers and middleware manage pointers and context values can create conditions where UAF becomes observable. Proper mitigation involves ensuring that credential data is copied into long-lived structures when necessary, avoiding retaining references to request-scoped objects beyond their lifecycle, and zeroizing sensitive buffers as soon as they are no longer required.

To prevent Use After Free in Chi when using Basic Authentication, ensure that credential data is safely copied and not tied to request-scoped memory that may be freed prematurely. Avoid storing pointers to transient request buffers; instead, extract values and store them in owned types with clear lifetimes. Below are concrete Chi middleware examples that demonstrate safe handling of Basic Auth credentials.

Safe Basic Auth parsing with owned data

import base64, strutils, chronicles
from http import Headers, Request

# Owned type to hold credentials beyond the request lifecycle
type
  Credentials* = object
    username*: string
    password*: string

# Middleware that parses Basic Auth and stores owned data in request context
proc basicAuthMiddleware*(req: Request, next: RequestHandler): Future[Response] {.async.} =
  let authHeader = req.headers.get("Authorization", "")
  if authHeader.startsWith("Basic "):
    let encoded = authHeader[6..^1]
    let decoded = decode(encoded)  # Assume decode returns a string "username:password"
    let parts = decoded.split(":", 1)
    if parts.len == 2:
      let creds = Credentials(username: parts[0], password: parts[1])
      # Store an owned copy in request context; avoid referencing transient data
      req.context["credentials"] = creds
  await next(req)

# Downstream handler that safely reads credentials from context
proc protectedHandler*(req: Request): Future[Response] {.async.} =
  let credsMaybe = req.context.get("credentials", Option[Credentials]())
  if credsMaybe.isSome:
    let creds = credsMaybe.get
    # Use creds.username and creds.password safely; they are owned strings
    # Do not access any request-scoped buffers here
    return newJsonResponse({"user": creds.username})
  return newHttpError(401, "Unauthorized")

Zeroizing sensitive data after use

import zeroize

proc clearCredentials*(creds: var Credentials) =
  # Zeroize memory before dropping credentials to prevent leaks
  zeroize(creds.username)
  zeroize(creds.password)

# Example usage at the end of request processing
proc requestCleanup*(req: Request) =
  if req.context.hasKey("credentials"):
    var creds = req.context["credentials"].Credentials
    clearCredentials(creds)
    req.context.del("credentials")

Comparison of safe vs unsafe patterns

By adopting owned data structures and explicitly clearing sensitive fields, you eliminate the conditions under which Use After Free can manifest in Chi with Basic Authentication. These patterns align with secure memory handling best practices and reduce the attack surface exposed by parsed credentials.