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

What to do

1. See which version you have: cargo tree -i anyhow (it also shows which of your dependencies pulls it in).
2. Update within your version requirement: cargo update -p anyhow. If the fix is a newer major or minor than your requirement allows, raise it in Cargo.toml (for example anyhow = "1.0.103") or update the dependency that pulls it in.
3. Check again with cargo audit, or paste your Cargo.lock into our Cargo.toml checker.

Details from the advisory

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

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

References

• Package: crates.io/crates/anyhow
• Advisory: rustsec.org/advisories/RUSTSEC-2026-0190.html
• Report: github.com/dtolnay/anyhow/issues/451

From the RustSec Advisory Database (CC0-1.0) via the OSV export, downloaded Oct 24, 2026. Severity score computed from the published CVSS vector.