CVE-2026-39860 [Critical] | Security Vulnerability in Nix

Nix is a package manager for Linux and other Unix systems. A bug in the fix for CVE-2024-27297 allowed for arbitrary overwrites of files writable by the Nix process orchestrating the builds (typically the Nix daemon running as root in multi-user installations) by following symlinks during fixed-output derivation output registration. This affects sandboxed Linux builds - sandboxed macOS builds are unaffected. The location of the temporary output used for the output copy was located inside the build chroot. A symlink, pointing to an arbitrary location in the filesystem, could be created by the derivation builder at that path. During output registration, the Nix process (running in the host mount namespace) would follow that symlink and overwrite the destination with the derivation's output contents. In multi-user installations, this allows all users able to submit builds to the Nix daemon (allowed-users - defaulting to all users) to gain root privileges by modifying sensitive files. This vulnerability is fixed in 2.34.5, 2.33.4, 2.32.7, 2.31.4, 2.30.4, 2.29.3, and 2.28.6.

#	Date	Old EPSS score	New EPSS score	Delta (New - Old)
1	2026-04-09	—	0.02%	—

Date

Old EPSS score

New EPSS score

Delta (New - Old)

2026-04-09

—

0.02%

—

Base score	Version	Severity	Vector	Exploitability	Impact	Score source
9.0	3.1	CRITICAL	`CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:N` Click to expand Attack vector (AV:L) They already need access on the box, or another person has to do something wrong; it’s not a remote drive-by. Attack complexity (AC:L) Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup. Privileges required (PR:N) No account or special rights needed—anonymous or random user is enough. User interaction (UI:N) Nobody has to click “OK” or open a trap file; it can work without a victim helping. Scope (S:C) Breaking this can reach past the original component and bite other resources—bigger blast radius. Confidentiality (C:H) Serious risk that confidential data gets exposed in a big way. Integrity (I:H) They could widely tamper with or forge data—trust in the data is badly hurt. Availability (A:N) Service keeps running; no real outage angle.	2.5	5.8	[email protected]
8.4	3.1	HIGH	`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:N` Click to expand Attack vector (AV:L) They already need access on the box, or another person has to do something wrong; it’s not a remote drive-by. Attack complexity (AC:L) Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup. Privileges required (PR:L) A normal user session is enough; they don’t have to be admin. User interaction (UI:N) Nobody has to click “OK” or open a trap file; it can work without a victim helping. Scope (S:C) Breaking this can reach past the original component and bite other resources—bigger blast radius. Confidentiality (C:H) Serious risk that confidential data gets exposed in a big way. Integrity (I:H) They could widely tamper with or forge data—trust in the data is badly hurt. Availability (A:N) Service keeps running; no real outage angle.	2.0	5.8	[email protected]

Base score

Version

Severity

Vector

Exploitability

Impact

Score source

9.0

3.1

CRITICAL

CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:N Click to expand

Attack vector (AV:L): They already need access on the box, or another person has to do something wrong; it’s not a remote drive-by.
Attack complexity (AC:L): Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:N): No account or special rights needed—anonymous or random user is enough.
User interaction (UI:N): Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:C): Breaking this can reach past the original component and bite other resources—bigger blast radius.
Confidentiality (C:H): Serious risk that confidential data gets exposed in a big way.
Integrity (I:H): They could widely tamper with or forge data—trust in the data is badly hurt.
Availability (A:N): Service keeps running; no real outage angle.

2.5

5.8

[email protected]

8.4

3.1

HIGH

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:N Click to expand

Attack vector (AV:L): They already need access on the box, or another person has to do something wrong; it’s not a remote drive-by.
Attack complexity (AC:L): Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:L): A normal user session is enough; they don’t have to be admin.
User interaction (UI:N): Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:C): Breaking this can reach past the original component and bite other resources—bigger blast radius.
Confidentiality (C:H): Serious risk that confidential data gets exposed in a big way.
Integrity (I:H): They could widely tamper with or forge data—trust in the data is badly hurt.
Availability (A:N): Service keeps running; no real outage angle.

2.0

5.8

[email protected]

OS Trackers for CVE-2026-39860

vendor	priority	summary	link
`debian`	not yet assigned	CVE-2026-39860 not yet assigned priority: Debian including 1 source packages (nix), 5 status rows across 5 suites (bookworm, bullseye, forky, sid, trixie): open 3, resolved 2.	https://security-tracker.debian.org/tracker/CVE-2026-39860
`ubuntu`	medium	CVE-2026-39860 medium priority: Ubuntu including 1 source packages (nix), 4 status rows across 4 suites (jammy, noble, questing, upstream): needs-triage 4.	https://ubuntu.com/security/CVE-2026-39860

Affected software / configurations for CVE-2026-39860

Vendor	Product	Version	Raw CPE
nixos	nix	>= 2.18.2, <= 2.18.9	cpe:2.3:a:nixos:nix::::::::
nixos	nix	>= 2.19.4, <= 2.19.7	cpe:2.3:a:nixos:nix::::::::
nixos	nix	>= 2.20.5, <= 2.20.9	cpe:2.3:a:nixos:nix::::::::
nixos	nix	>= 2.21.0, < 2.28.6	cpe:2.3:a:nixos:nix::::::::
nixos	nix	>= 2.29.0, < 2.29.3	cpe:2.3:a:nixos:nix::::::::
nixos	nix	>= 2.30.0, < 2.30.4	cpe:2.3:a:nixos:nix::::::::
nixos	nix	>= 2.31.0, < 2.31.4	cpe:2.3:a:nixos:nix::::::::
nixos	nix	>= 2.32.0, < 2.32.7	cpe:2.3:a:nixos:nix::::::::
nixos	nix	>= 2.33.0, < 2.33.4	cpe:2.3:a:nixos:nix::::::::
nixos	nix	>= 2.34.0, < 2.34.5	cpe:2.3:a:nixos:nix::::::::

References for CVE-2026-39860

URL	Tags
https://github.com/NixOS/nix/commit/244f3eee0bbc7f11e9b383a15ed7368e2c4becc9	Patch
https://github.com/NixOS/nix/commit/4bc5a3510fa3735798f9ed3a2a30a3ea7b32343a	Patch
https://github.com/NixOS/nix/commit/7794354a982449927ee7401cdeb573ddd16c4688	Patch
https://github.com/NixOS/nix/commit/a3163b9eabb952b4aa96e376dea95ebcca97b31a	Patch
https://github.com/NixOS/nix/pull/10178	Issue Tracking Patch
https://github.com/NixOS/nix/security/advisories/GHSA-g3g9-5vj6-r3gj	Mitigation Patch Vendor Advisory

URL

CVE-2026-39860 | Nix sandbox escape: file write via symlink at FOD `.tmp` copy destination

Exploit prediction scoring system (EPSS) score for CVE-2026-39860

Common vulnerability scoring system (CVSS) metrics for CVE-2026-39860

Weakness enumeration for CVE-2026-39860

OS Trackers for CVE-2026-39860

Affected software / configurations for CVE-2026-39860

References for CVE-2026-39860