In the unique reclaim path of BytesMut::reserve, the condition

if v_capacity >= new_cap + offset

uses an unchecked addition. When new_cap + offset overflows usize in release builds, this condition may incorrectly pass, causing self.cap to be set to a value that exceeds the actual allocated capacity. Subsequent APIs such as spare_capacity_mut() then trust this corrupted cap value and may create out-of-bounds slices, leading to UB.

This behavior is observable in release builds (integer overflow wraps), whereas debug builds panic due to overflow checks.

PoC

use bytes::*;

fn main() {
    let mut a = BytesMut::from(&b"hello world"[..]);
    let mut b = a.split_off(5);

    // Ensure b becomes the unique owner of the backing storage
    drop(a);

    // Trigger overflow in new_cap + offset inside reserve
    b.reserve(usize::MAX - 6);

    // This call relies on the corrupted cap and may cause UB & HBO
    b.put_u8(b'h');
}

Workarounds

Users of BytesMut::reserve are only affected if integer overflow checks are configured to wrap. When integer overflow is configured to panic, this issue does not apply.

In versions 0.4.44 and below of tar-rs, when unpacking a tar archive, the tar crate's unpack_dir function uses fs::metadata() to check whether a path that already exists is a directory. Because fs::metadata() follows symbolic links, a crafted tarball containing a symlink entry followed by a directory entry with the same name causes the crate to treat the symlink target as a valid existing directory — and subsequently apply chmod to it. This allows an attacker to modify the permissions of arbitrary directories outside the extraction root.

This issue has been fixed in version 0.4.45.

Versions 0.4.44 and below of tar-rs have conditional logic that skips the PAX size header in cases where the base header size is nonzero.

As part of CVE-2025-62518, the astral-tokio-tar project was changed to correctly honor PAX size headers in the case where it was different from the base header. This is almost the inverse of the astral-tokio-tar issue.

Any discrepancy in how tar parsers honor file size can be used to create archives that appear differently when unpacked by different archivers. In this case, the tar-rs (Rust tar) crate is an outlier in checking for the header size — other tar parsers (including e.g. Go archive/tar) unconditionally use the PAX size override. This can affect anything that uses the tar crate to parse archives and expects to have a consistent view with other parsers.

This issue has been fixed in version 0.4.45.