• It goes online only to download a game's picture, when you press Download or turn on automatic picture downloads in More options (off by default). It then fetches one PNG over HTTPS from raw.githubusercontent.com/libretro-thumbnails, checks it is a real picture of a sensible size, and keeps it in %APPDATA%\CD Injector 3DS\pictures. It has no update check, no telemetry and no analytics. (All of this lives in cdinjector/download.py; the tests fail if any other part of the app gets networking code, or if that file names a second web address.)
• It reads only the files you choose: the game's .cue and .bin files, the BIOS, a picture, and an optional NSUI banner, icon, sound or font, or NSUI's own program file (only read, never run or changed) to make NSUI's banner.
• It writes only the CIA, temporary files (deleted when it finishes), one small settings file, %APPDATA%\CD Injector 3DS\settings.json, holding the paths and colors you chose, and the NSUI banners you make (in %APPDATA%\CD Injector 3DS\nsui banners).
• It runs its two helper programs (makerom and bannertool) directly with a list of arguments, never through a shell, and stops them if they hang. They run inside a private work folder and only ever get plain relative file names made by the app, never a name taken from your files.

Only the latest release gets fixes. Please check you're on it before reporting.

Every file you choose is checked before it is used, and a bad one gives an error message, not a crash:

• .cue sheets: size and track-count limits; only files that sit in the .cue's own folder are packed, so a crafted sheet can't pull another file on your PC into a CIA (shortcuts to files elsewhere are refused too).
• NSUI banners and icons: size limits, structure checks, and a cap on how far a compressed model may expand (a "decompression bomb" is refused). NSUI's program file is read through a memory map (it is never loaded whole or run); its resource table is read with bounds checks, and each part it names is size-limited and checked like a banner before bannertool sees it. A damaged banner is refused rather than put in a CIA, because a broken banner can stop the Home Menu showing the game.
• Pictures: a pixel limit against decompression bombs, and each picture is fully decoded before the game is copied.
• Banner sounds: format, length and size checks.
• Titles: control and invisible characters are removed, and Windows reserved names (CON, NUL, ...) can't be used as file names.
• The settings file: only plain text values are accepted.

tests/test_security.py tries all of this with hostile and randomly damaged files.

Each release comes with SHA256SUMS.txt. In PowerShell:

Get-FileHash .\CD-Injector-3DS-vX.Y.Z-windows.zip -Algorithm SHA256

The result must match the line in SHA256SUMS.txt. The program itself is not code-signed, so Windows SmartScreen may warn about it the first time; that is expected for an unsigned program.

Please report security problems privately, not in a public issue: use GitHub's "Report a vulnerability" button (the repository's Security tab). Include the steps and, if you can, a sample file that triggers it. Ordinary bugs can go in normal issues.

• python scripts/hygiene.py checks for invisible characters, look-alike letters and hidden file metadata. Run it before committing; --fix cleans what it finds.
• python -m unittest discover -s tests -t . runs every test. FUZZ_ITERATIONS=2000 makes the damaged-file test much longer. The banner fuzz tests need real NSUI banners, which can't be shipped: list yours in CDI_TEST_BANNERS (separated by ; on Windows).
• Never commit games, BIOS files, CIAs, or Nintendo-derived files (see .gitignore).
• Build in a virtual environment: python -m venv .venv, then .venv\Scripts\pip install -r requirements-build.txt and scripts\build_windows.ps1 -Python .venv\Scripts\python.exe. The versions in requirements*.txt are exact.
• Before every release, check that no pinned library has a published security advisory, for example: gh api "advisories?ecosystem=pip&affects=pillow@12.3.0" --jq length (0 means none). Do this for each pinned package. Pillow matters most: it opens the pictures and fonts users choose. Dependabot does the same once the repository is on GitHub.