smallbitvec is a growable bit-vector for Rust, optimized for size. From 1.0.1 to 2.6.0, an integer overflow in the internal capacity calculation of smallbitvec can lead to an undersized heap allocation, resulting in a heap buffer overflow through safe APIs only. This allows memory corruption without requiring unsafe code from the caller. This vulnerability is fixed in 2.6.1.

Affected: crates.io: smallbitvec

### Summary An integer overflow in the internal capacity calculation of `smallbitvec` can lead to an undersized heap allocation, resulting in a heap buffer overflow through safe APIs only. This allows memory corruption without requiring `unsafe` code from the caller.

### Details The issue originates from unchecked arithmetic in the internal helper function responsible for computing the required buffer size:

``` (cap + bits_per_storage() - 1) / bits_per_storage() ```

When `cap` is close to `usize::MAX`, the addition:

``` cap + bits_per_storage() - 1 ```

can overflow in release builds and wrap around due to Rust’s default wrapping semantics for integer overflow in optimized builds.

As a result: - `buffer_len(cap)` may return a value significantly smaller than required. - The backing storage is allocated with insufficient size. - Internal metadata (logical length/capacity) reflects a much larger size than the actual allocation.

Subsequent safe API calls (e.g., `set`, `push`, `reserve`) rely on this corrupted metadata and perform index computations that assume sufficient backing storage. These operations eventually reach unsafe internal code paths (e.g., pointer arithmetic and unchecked indexing), leading to out-of-bounds memory access.

Summary of the issue: integer overflow → undersized allocation → inconsistent metadata (len/cap vs actual buffer) → unsafe internal access using corrupted metadata → heap buffer overflow (UB) ### PoC #### PoC 1: Out-of-bounds write via `f…

Affected: Ubuntu:22.04:LTS: rust-smallbitvec, Ubuntu:24.04:LTS: rust-smallbitvec, Ubuntu:25.10: rust-smallbitvec, Ubuntu:26.04:LTS: rust-smallbitvec

smallbitvec is a growable bit-vector for Rust, optimized for size. From 1.0.1 to 2.6.0, an integer overflow in the internal capacity calculation of smallbitvec can lead to an undersized heap allocation, resulting in a heap buffer overflow through safe APIs only. This allows memory corruption without requiring unsafe code from the caller. This vulnerability is fixed in 2.6.1.