RUSTSEC-2026-0190: Unsoundness in `Error::downcast_mut()`

> Unsoundness in `Error::downcast_mut()`

| Details | |
| ------------------- | ---------------------------------------------- |
| Status | unsound |
| Package | `anyhow` |
| Version | `1.0.100` |
| URL | [https://github.com/dtolnay/anyhow/issues/451](https://github.com/dtolnay/anyhow/issues/451) |
| Date | 2026-06-25 |

Affected versions of this crate violate borrow rules, resulting in undefined behavior, when the user adds context to an error via `Error::context` and then later calls `Error::downcast_mut` on the returned `Error`.

The flaw was corrected in commit `6e8c000` by revising how the mutable reference is constructed, avoiding inclusion of a shared reference in the resulting borrow chain.

## Example

```rust
use anyhow::Error;
use std::fmt;

#[derive(Debug)]
struct ErrorContext(&'static str);

impl fmt::Display for ErrorContext {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
fmt::Display::fmt(&self.0, f)
}
}

fn main() {
let mut error = Error::msg("inner error").context(ErrorContext("old context"));
let context: &mut ErrorContext = error.downcast_mut().unwrap();
context.0 = "new context";
println!("{:?}", error);
}
```

## Miri output

```
error: Undefined Behavior: trying to retag from <1538> for Unique permission at alloc602[0x38], but that tag only grants SharedReadOnly permission for this location
--> src/ptr.rs:170:18
|
170 | unsafe { &mut *self.ptr.as_ptr() }
| ^^^^^^^^^^^^^^^^^^^^^^^ this error occurs as part of retag at alloc602[0x38..0x48]
|
= help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental
= help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information
help: <1538> was created by a SharedReadOnly retag at offsets [0x38..0x48]
--> src/ptr.rs:89:18
|
89 | ptr: NonNull::from(ptr),
| ^^^^^^^^^^^^^^^^^^
= note: stack backtrace:
0: anyhow::ptr::Mut::<'_, ErrorContext>::deref_mut
at src/ptr.rs:170:18: 170:41
1: anyhow::error::<impl anyhow::Error>::downcast_mut::<ErrorContext>
at src/error.rs:560:18: 560:46
2: main
at examples/downcast_mut.rs:15:38: 15:58
```

See [advisory page](https://rustsec.org/advisories/RUSTSEC-2026-0190.html) for additional details.

The report concerns anyhow's Error::downcast_mut and names src/ptr.rs, src/error.rs, and examples/downcast_mut.rs; begin by checking whether this repository contains the affected code. Compare its current state with the advisory and commit 6e8c000. The issue does not define a remediation or completion condition for this repository, so confirm the maintainer's requested action before changing anything.

Written by the indexing model from the issue text.