Summary
bytes has integer overflow in BytesMut::reserve
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.
Impact
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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Frequently Asked Questions
- What is CVE-2026-25541? CVE-2026-25541 is a medium-severity security vulnerability in bytes (rust), affecting versions >= 1.2.1, < 1.11.1. It is fixed in 1.11.1.
- Which versions of bytes are affected by CVE-2026-25541? bytes (rust) versions >= 1.2.1, < 1.11.1 is affected.
- Is there a fix for CVE-2026-25541? Yes. CVE-2026-25541 is fixed in 1.11.1. Upgrade to this version or later.
- Is CVE-2026-25541 exploitable, and should I be worried? Whether CVE-2026-25541 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-25541 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-25541? Upgrade
bytesto 1.11.1 or later.