# Rust 1.98.1 fixes a compiler miscompilation that can emit undefined behavior The Rust team has issued 1.98.1 after finding that rustc 1.98.0 can generate a trait-object vtable with a null pointer where a function pointer belongs, producing undefined behavior in compiled programs. This is a compiler-correctness fix rather than a routine patch. Code built with Rust 1.98.0 can be wrong even when the source is valid, so teams that adopted that stable release should update and rebuild affected artifacts. - Status: Active - Published: 2026-09-04T10:15:03+12:00 - Updated: 2026-09-04T10:15:03+12:00 - Categories: Web Development, Developer Tools - Tags: compiler bug, Rust, rustc, security, undefined behavior - Canonical HTML: https://beyondthe.news/dossiers/rust-1-98-1-vtable-miscompilation-undefined-behavior ## What changed Rust 1.98.1 was released on September 3 to correct a miscompilation introduced in Rust 1.98.0. In some circumstances, rustc generated a trait-object vtable containing a null pointer where a function pointer should have been emitted. The resulting compiled code has undefined behavior: the Rust team says this can present as a null-pointer segfault, but arbitrary effects are possible because the program has already crossed into undefined behavior. The recommended remediation for stable users is to update to 1.98.1 and rebuild relevant code. ## Why it matters Compiler miscompilations are different from ordinary library bugs because the defect can be baked into binaries even when application source code is correct. A team can fix its toolchain and still keep deploying an artifact compiled with the bad version unless it rebuilds. Rust’s safety guarantees also do not protect against incorrect machine code emitted by the compiler itself, so builders that moved quickly to 1.98.0 should treat the point release as a toolchain-correctness update rather than waiting for the next normal release. ## The bug is in emitted vtable code Rust 1.98.0 could incorrectly place a null pointer in a trait-object vtable slot that should contain a callable function pointer. If execution reaches that slot, the binary has undefined behavior; a crash is one possible outcome but not the only one. ## Updating the compiler is only half the remediation Installing Rust 1.98.1 prevents new builds from using the affected compiler behavior, but binaries already produced by 1.98.0 do not change automatically. Teams should identify and rebuild artifacts compiled during the affected window, especially long-lived services, release binaries and cached build outputs. ## The release is intentionally narrow The Rust team published 1.98.1 as a point release focused on this miscompilation rather than bundling a larger feature set. That makes the operational decision simpler: stable users can update without waiting for a normal release cadence just to receive the compiler fix. ## Key details - Rust 1.98.1 was released September 3, 2026. - It fixes a miscompilation present in Rust 1.98.0. - The defect affects trait-object vtable generation in some circumstances. - rustc could emit a null pointer where a function pointer should be. - The resulting binary has undefined behavior and can segfault; the Rust team notes arbitrary effects are possible under UB. - Users on the stable toolchain can update with `rustup update stable`. - Previously compiled artifacts need to be rebuilt to remove code emitted by the affected compiler. ## Builder takeaways - Update stable Rust toolchains from 1.98.0 to 1.98.1 rather than waiting for the next scheduled release. - Rebuild release artifacts that were compiled with 1.98.0; changing the toolchain does not repair an existing binary. - If you pin Rust versions in CI, containers or `rust-toolchain` files, check those pins explicitly instead of assuming `rustup update stable` covers every build environment. - Invalidate relevant compiler/build caches where necessary so a rebuild does not reuse affected outputs. - Treat unexpected trait-object crashes in freshly built 1.98.0 code as potentially compiler-related while validating against 1.98.1. ## What to watch - A more precise public description of the circumstances that trigger the miscompilation. - Whether downstream distributions or build systems identify packages that need rebuilding. - Any additional compiler-correctness fixes discovered during the 1.98.x cycle. - Regression reports from projects that rebuild with 1.98.1. ## Uncertainties - The release announcement does not quantify how commonly real-world code triggers the affected vtable-generation path. - Undefined behavior allows outcomes beyond the simple null-pointer crash, so impact depends on the compiled program and execution path. - The announcement describes a compiler correctness defect; it does not assign a CVE or characterize it as remotely exploitable. ## Sources - [Announcing Rust 1.98.1](https://blog.rust-lang.org/2026/09/03/Rust-1.98.1/) — Rust Project · primary · 2026-09-03T00:00:00+12:00. Primary release announcement describing the vtable miscompilation, undefined behavior and update path. - [1.98.1 pre-release testing](https://blog.rust-lang.org/inside-rust/2026/09/02/1.98.1-prerelease/) — Inside Rust · primary · 2026-09-02T00:00:00+12:00. Primary prerelease notice confirming the expedited 1.98.1 release process.