copy-fail-tiny-elf-CVE-2026-31431

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copy-fail-tiny-elf-CVE-2026-31431

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Minimal no-libc Linux x86_64 ELF PoC build for Copy Fail (CVE-2026-31431)

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# Copy Fail Tiny ELF - CVE-2026-31431

Minimal 436-bytes exploit for Copy Fail, thanks to the power of golfing

## Magic

This ELF ultra golfs the exploit by being totally unmapped, rwx, and by using itself conditionally as the ELF payload to use to poison the cache. More cursed ELF internals golfs coming soon

Have a better idea? Compete at https://copy.golf/

=== BEGIN SLOPPED PORTION ===

## References

- Technical writeup: https://copy.fail/
- Upstream PoC repository: https://github.com/theori-io/copy-fail-CVE-2026-31431
- NVD entry: https://nvd.nist.gov/vuln/detail/CVE-2026-31431

## Requirements

- Linux x86_64
- gcc
- ld
- strip

On Debian/Ubuntu:

```bash
sudo apt install build-essential binutils
```

## Nix Environment

The repository includes a pinned `shell.nix` and `.envrc`.

With `nix-shell`:

```bash
nix-shell
```

With `direnv`:

```bash
direnv allow
```

This provides the reproducible toolchain used here:

```text
gcc 15.2.1
GNU ld / strip from llvmPackages_19.bintools
python3
```

## Build

```bash
make
```

If you are using Nix or `direnv`, `make` will use the toolchain from the shell automatically.

## Quick VM Test

You can also try the exploit inside a quick `Ubuntu Server 24.04 LTS` VM

```bash
cd test-vm
./quick-test.sh
```

## Check size

```bash
make size
```

## Clean

```bash
make clean
```

## Tested toolchain

```text
gcc version 13.3.0 (Ubuntu 13.3.0-6ubuntu2~24.04.1)
GNU ld (GNU Binutils for Ubuntu) 2.42
GNU strip (GNU Binutils for Ubuntu) 2.42
```

Reproducible smaller build:

```text
gcc 15.2.1
GNU ld 2.46
GNU strip with --strip-section-headers
```

Final size varies with GCC, binutils, and the strip implementation.

copy-fail-tiny-elf-CVE-2026-31431

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