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- CVE-2026-39860 represents a critical security flaw residing within the core mechanism
- that the Nix package manager utilizes to register outputs of fixed-output derivations.
- Specifically, during the build registration phase, the Nix daemon creates a temporary
- working copy of the build output before moving it to its permanent destination.
- Due to insufficient validation checks during this symlink following process, low-privileged
- local users who are allowed to submit builds to the daemon can manipulate symbolic links.
- By strategically placing crafted symlinks within the build directory tree, an attacker
- can trick the privileged daemon process into following pointers outside sandbox boundaries.
- Because the Nix daemon executes with root privileges in multi-user installations,
- this behavior leads directly to arbitrary file overwrites across the underlying system.
- An attacker can leverage these unrestricted file overwrites to modify sensitive system files,
- overwrite configuration binaries, or plant unauthorized executable payloads.
- When the operating system or administrative services access these overwritten files,
- it triggers a seamless local privilege escalation directly to the root user.
- The vulnerability specifically impacts sandboxed Linux configurations across deployments
- while leaving sandboxed macOS configurations unaffected due to platform differences.
- It also serves as an incomplete patch regression or closely related security variant
- stemming from prior fixes implemented previously for CVE-2024-27297.
- Cooperating fixed-output derivations could previously pass file descriptors
- and communicate across distinct sandboxes, widening the overall attack surface.
- Mitigating this security flaw requires updating the Nix daemon package immediately
- to the latest secure versions released by the official maintainers.
- System administrators managing multi-tenant build nodes or shared build servers
- are at the highest risk if untrusted local users have active build access.
- Monitoring build submission permissions and restricting allowed users in configuration
- can temporarily reduce exposure until patches are fully deployed across infrastructure.
- The vulnerability highlights the inherent complexities of securely managing immutable stores
- and daemon-mediated file registration tasks under multi-user operating systems.
- Proper validation of path traversal and symlink resolution remains crucial for isolation.
- Auditing daemon communication channels prevents similar sandbox breakout scenarios.
DailyCVE Form:
Platform: Nix Package Manager
Version: Versions Since 2.21
Vulnerability: Sandbox Escape
Severity: Critical
date: April 7 2026
Prediction: Patched April 2026
What Undercode Say
nix --version nix-env -iA nix -f '<nixpkgs>' systemctl status nix-daemon
import os
def check_symlink_safety(path, store_root):
real_path = os.path.realpath(path)
if not real_path.startswith(store_root):
raise ValueError("Unsafe symlink escape detected!")
return True
How Exploit: (Educational Purposes!)
mkdir -p /tmp/exploit_build
ln -s /etc/passwd /tmp/exploit_build/target_link
nix-build -E 'derivation { name = "exploit"; builder = "/bin/sh"; system = "x86_64-linux"; }'
Protection: from this CVE
To protect systems against CVE-2026-39860, administrators must immediately update the Nix package manager to fixed versions (2.34.5, 2.33.4, 2.32.7, 2.31.4, 2.30.4, 2.29.3, 2.28.6, or higher). Additionally, restrict build permissions in `nix.conf` by tightening the `allowed-users` setting to prevent untrusted local users from submitting arbitrary builds to the daemon.
Impact:
Successful exploitation of this vulnerability allows any unprivileged local user with build access to achieve arbitrary file overwrites as the root user. This leads directly to total system compromise, local privilege escalation, unauthorized modification of critical system binaries, and complete bypass of build sandbox isolation boundaries.
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Sources:
Reported By: github.com
Extra Source Hub:
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