GHSA-2jx3-ff3v-j7jj

> [!NOTE] > This finding was identified during an agentic unsafe Rust code review performed by Gemini AI, followed by human review and verification.

## The Issue

The crate exports a public safe API [`Rules::deserialize`](https://github.com/VirusTotal/yara-x/blob/5bd1f35db783679c90a3ea1a66bd15fe4e55bef1/lib/src/compiler/rules.rs#L187-L254) accepting any generic byte sequence `B: AsRef<[u8]>`. It restores compiled rule structures directly from raw bytes using `bincode::serde::decode_from_slice`.

This decoded `Rules` struct contains internal lookup tables, including `sub_patterns: Vec<(PatternId, SubPattern)>`, `atoms: Vec<SubPatternAtom>`, and `lit_pool: BStringPool`. Subsequent safe operations assume these internal tables satisfy strict structural invariants:

- `Rules::get_sub_pattern` executes `unsafe { self.sub_patterns.get_unchecked(sub_pattern_id.0 as usize) }`. If untrusted serialized bytes contain an atom referencing an out-of-bounds `SubPatternId`, calling `get_sub_pattern` during scanning triggers an out-of-bounds memory read (Undefined Behavior).

https://github.com/VirusTotal/yara-x/blob/5bd1f35db783679c90a3ea1a66bd15fe4e55bef1/lib/src/compiler/rules.rs#L355-L360

- `Metadata::next()` extracts string metadata via `unsafe { s.to_str_unchecked() }`. If serialized bytes corrupt `lit_pool` indices or structural data, `to_str_unchecked` constructs a `&str` pointing to invalid UTF-8 bytes (Undefined Behavior).

https://github.com/VirusTotal/yara-x/blob/5bd1f35db783679c90a3ea1a66bd15fe4e55bef1/lib/src/models.rs#L204-L210

Because passing malformed or untrusted data to `Rules::deserialize` induces Undefined Behavior in subsequent safe calls (`Scanner::new`, `Scanner::scan`) without any `unsafe` blocks in caller code, this API is unsound.

<details><summary>Minimal Reproduction (Miri / Native Crash)</summary>

Zip file with crashing_payload: [crashing_payload.zip](https://github.com/user-attachments/files/29173221/crashing_payload.zip)

We have a payload crashing_payload.bin where only a single byte in the structural metadata tail is mutated (changing a `SubPatternId` from `1` to `248` while keeping the WebAssembly bytecode completely untouched and valid).

Below is the self-contained verification script which compiles and runs against the official **unmodified** `yara-x v1.17.0` crate:

```rust use yara_x::{Rules, Scanner};

fn main() { // Embed the crashing payload generated by the fuzzer at compile time. let serialized = include_bytes!("crashing_payload.bin"); println!("Loaded embedded crashing payload, length: {}", serialized.len());

// Deserialize. On unmodified library, this succeeds because the WASM and headers // are pristine and structural corruption isn't validated. if let Ok(deserialized) = Rules::deserialize(serialized) { println!("Deserialization succeeded! Running scanner..."); let mut scanner = Scanner::new(&deserialized); // Run the standard scan, which will execute the WASM and trigger the out-of-bounds read! let _ = scanner.scan(b"lorem ipsum dolor sit amet"); println!("Scanner finished."); } else { println!("Deserialization failed!"); } } ```

### 1. Miri Trace

NOTE: This needs to be run with 1.17.0. I haven't tested this against other versions.

Unfortunately I was able to get a miri trace, but I'm not able to reproduce it right now because of lockfile changes. If you're trying this out be sure to use `MIRIFLAGS="-Zmiri-disable-stacked-borrows"`

### 2. Segfault / panics

When run natively (without Miri or any sanitizers) on a standard Linux platform, the process immediately segfaults:

```bash $ cargo run --bin verify Loaded embedded crashing payload, length: 11777 Deserialization succeeded! Running scanner... Segmentation fault (core dumped) ```

And with a newer compiler (which appears to have debug assertions in `get_unchecked)`

```bash Deserialization succeeded! Running scanner...

thread 'main' (997442) panicked at lib/src/compiler/rules.rs:404:36: unsafe precondition(s) violated: slice::get_unchecked requires that the index is within the slice

This indicates a bug in the program. This Undefined Behavior check is optional, and cannot be relied on for safety. note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace ```

</details>

<details><summary>Suggested Fix</summary>

To uphold Rust soundness guarantees, either mark `Rules::deserialize` as `pub unsafe fn deserialize` with a formal `/// # Safety` contract documenting that callers are responsible for verifying the authenticity and structural integrity of the input bytes (e.g. via cryptographic signatures), or replace all internal `get_unchecked` and `to_str_unchecked` calls on deserialized data structures with safe bounds checks (`.get()`) and UTF-8 validation (`std::str::from_utf8`).

</details>

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