# Nix and Lix patch three flaws that can combine into root escalation and a macOS sandbox escape A coordinated October 13 security advisory warns that Nix and Lix installations evaluating untrusted builds can face heap corruption, unprivileged store-repair privilege escalation and a macOS sandbox-policy escape; multiple maintained branches need explicit security updates. Nix and Lix have shipped security fixes for three distinct issues, including a store-repair route to root. Operators of shared builders and CI should verify their actual installed versions and channel rollout. - Status: Active - Published: 2026-10-14T19:44:51+13:00 - Updated: 2026-10-14T19:44:51+13:00 - Categories: Web Development, Cloud & Infrastructure, Deployment & DevOps, Developer Tools - Tags: CI/CD, Lix, Nix, package management, security - Canonical HTML: https://beyondthe.news/dossiers/nix-lix-october-2026-repair-privilege-escalation-seatbelt-overflow ## What changed On October 13, 2026, NixOS security coordinators and Lix maintainers disclosed three related vulnerabilities affecting Nix and Lix. One is an integer overflow in allocation of Nix evaluation lists that can produce an out-of-bounds heap write. Another lets unprivileged daemon clients invoke non-atomic store repair operations that can replace a legitimate store path before verification, opening a local privilege-escalation route. On macOS, a bug in Seatbelt sandbox-policy generation can produce an unconstrained allow rule when a missing optional path crosses a policy-size boundary. The coordinated advisory says these flaws can be combined to achieve root privilege escalation. CVE identifiers for the new October issues were not assigned at disclosure; they are distinct from earlier 2026 Nix/Lix CVEs. ## Why it matters Nix and Lix are used to make development and CI builds reproducible, including systems that evaluate third-party derivations and run multi-user build daemons. In those environments a local build user or untrusted expression may cross the boundary between isolated builds and the privileged store daemon. A package-manager update therefore changes the trustworthiness of the entire build pipeline, not just one developer workstation. Channel lag matters: a fix merged upstream does not prove the installed NixOS channel has delivered it. ## Three vulnerabilities, different preconditions The list-allocation flaw concerns evaluating untrusted expressions; the store-repair flaw matters where unprivileged users can talk to a multi-user daemon, including Linux and macOS. The Seatbelt bug affects macOS configurations with sandbox=true or sandbox=relaxed, not macOS machines with sandboxing disabled. The advisory says Guix is not affected. The Lix team did not establish a further exploit chain for the heap write by itself, while the coordinated advisory describes combined privilege-escalation risk. ## Patch the exact release line, not merely the package name The Nix advisory lists fixed versions 2.35.3, 2.34.9, 2.31.6, 2.30.6 and 2.28.8; 2.29.x, 2.32.x and 2.33.x are unsupported. Lix fixes are in 2.94.3 and 2.95.4; branches 2.93 and older are unsupported. Lix 2.96 also shipped on October 13, with further daemon and remote-store protocol changes. Operators must compare their installed binary and package/channel against the applicable fixed version. ## Repair permissions and sandbox behaviour change The fix prohibits unprivileged repair operations, so existing automated repair workflows may now need privileged intervention. The macOS fix prevents an unqualified Seatbelt allow clause from opening access beyond the intended paths. Lix 2.96 also deprecates ssh:// store URIs ahead of their removal in 2.97 in favour of ssh-ng://; this is a separate compatibility issue and should not be confused with the security patch. ## NixOS channel rollouts are not instantaneous The Lix disclosure points operators to Nixpkgs security backport pull requests for 26.05 and unstable and warns that Hydra builds and channel promotion can lag the source patch. Shared builders should check the installed version, rebuild or apply the security overlay where appropriate, and avoid evaluating untrusted expressions on exposed build hosts until remediated. There is no verified public evidence here of widespread exploitation. ## Key details - Disclosure and coordinated security advisory: October 13, 2026. - Three shared issue classes: heap out-of-bounds evaluation write, unprivileged repair privilege escalation, macOS Seatbelt sandbox escape. - Nix fixed lines: 2.35.3, 2.34.9, 2.31.6, 2.30.6, 2.28.8. - Lix fixed lines: 2.94.3 and 2.95.4; older unmaintained branches require migration. - Nix-specific installer ownership and daemon-client authorisation issues were disclosed separately and do not all affect Lix. - October issue CVE numbers were pending at publication; do not reuse May CVE-2026-44028. - No verified widespread in-the-wild exploitation or affected-host count. ## Builder takeaways - Inventory actual Nix/Lix daemon versions on developer workstations, CI runners and shared builders, not just the source package pin. - Prioritise machines that evaluate untrusted derivations or allow unprivileged daemon access. - For NixOS, verify whether your subscribed channel has built and delivered the backport; upstream merge alone is insufficient. - Expect formerly unprivileged repair automation to fail safely until an administrator handles the operation. - On macOS, review whether sandbox=true or sandbox=relaxed is enabled; treat the sandbox escape as conditional, not universal. ## What to watch - Assignment of official CVE identifiers and severity assessments for the October issues. - Backport delivery in NixOS 26.05 and unstable channels and unsupported branch migration. - Follow-up exploitation analysis or independently confirmed incidents. - Compatibility changes after disabling unprivileged repairs and future Lix 2.97 ssh URI removal. ## Uncertainties - The standalone exploitability of the heap overflow is not established by Lix's disclosure. - Exposure depends on daemon permissions, untrusted-build evaluation and macOS sandbox settings. - Patch availability in downstream channels depends on build and rollout timing. ## Sources - [Security advisory for Nix and Lix — October 2026](https://discourse.nixos.org/t/security-advisory-for-nix-and-lix-october-2026/80547) — NixOS Security · primary · 2026-10-13T00:00:00+13:00. Coordinated affected/fixed versions, conditions and separate Nix-specific issues. - [Three security issues in Lix](https://lix.systems/blog/2026-10-13-lix-cves/) — Lix project · primary · 2026-10-13T00:00:00+13:00. Technical root causes, mitigation, disclosure chronology and channel update guidance. - [Announcing Lix 2.96 Bingsu](https://lix.systems/blog/2026-10-13-lix-2.96-release/) — Lix project · primary · 2026-10-13T00:00:00+13:00. Concurrent version release and separate ssh:// deprecation.