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GHSA-434x-w66g-qw3r
No affected components available
Details
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.
This vulnerability is also known as RUSTSEC-2026-0007.
The vulnerability requires local access to the device to be exploited. It is easy for an attacker to exploit this vulnerability. An attacker does not need any special privileges or access rights. No user interaction is needed for the attacker to exploit this vulnerability.
Exploitation attempts have been detected. Elevated vigilance and prompt remediation are advised.
The exploit probability is very low. The vulnerability is unlikely to be exploited in the next 30 days.
We did not find any exploit available. Neither in GitHub repositories nor in the Exploit-Database.
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