GHSA-7g7r-gx96-252g

### Summary

`AppImage` targets built by `app-builder-lib` could use an empty path component when setting the `LD_LIBRARY_PATH` environment variable at runtime. This causes the current working directory to be added to the dynamic linker search path, which may allow an attacker to execute arbitrary code by placing a malicious shared library in the directory from which the `AppImage` is launched.

This vulnerability is the same class as [`CVE-2024-41817`](https://github.com/ImageMagick/ImageMagick/security/advisories/GHSA-8rxc-922v-phg8).

### Details

The vulnerability existed in two independent code paths within `app-builder-lib` (toolset `1.0.0`) and through upstream dependency `app-builder-bin` (toolset `0.0.0`).

#### Path 1 — Modern static runtime (`AppRun` generated by TypeScript)

The `AppRun` script generated by `app-builder-lib` contained this line:

```bash export LD_LIBRARY_PATH="${APPDIR}/usr/lib:${LD_LIBRARY_PATH}" ```

When `LD_LIBRARY_PATH` is not set in the environment at launch time, this evaluates to:

``` /path/to/app.AppDir/usr/lib: ```

The trailing `:` is treated by the dynamic linker as an empty path component, which resolves to the current working directory. If an attacker can place a malicious shared library (e.g., `libfoo.so`) in the directory from which the `AppImage` is executed, that library will be loaded in place of the legitimate one, resulting in arbitrary code execution.

The same issue affected `PATH`, `XDG_DATA_DIRS`, and `GSETTINGS_SCHEMA_DIR` in the same script.

```bash export LD_LIBRARY_PATH="${APPDIR}/usr/lib${LD_LIBRARY_PATH:+:${LD_LIBRARY_PATH}}" ```

#### Path 2 — Legacy FUSE2 toolset (`app-builder-bin`)

`AppImage` targets built using the legacy FUSE2 toolset (`toolsets.appimage = "0.0.0"`) delegated `AppRun` script generation to the `app-builder-bin` Go binary, which contained the same vulnerable template:

https://github.com/develar/app-builder/blob/7004925f95d8f034fc88d7e782c9aa7583debb8e/pkg/package-format/appimage/templates/AppRun.sh#L27

### Impact

An attacker with the ability to write files to the directory from which a vulnerable `AppImage` is executed can cause arbitrary shared libraries to be loaded into the application process, resulting in arbitrary code execution with the privileges of the user running the `AppImage`.

### Affected Versions

This was fully resolved in **`app-builder-lib@26.15.0`** (commit [`01b8ba979`](https://github.com/electron-userland/electron-builder/commit/01b8ba979), PR [#9829](https://github.com/electron-userland/electron-builder/pull/9829)) when `app-builder-bin` was removed from the dependency tree entirely and all AppImage construction was migrated to the TypeScript implementation.

### Workarounds

Set `LD_LIBRARY_PATH` to a non-empty value before launching the `AppImage`, so that the concatenation does not produce an empty path component. Alternatively, avoid running `AppImage` files from world-writable directories such as `/tmp`.