VulnerabilityPublished 23 Oct 2026Updated 29 Oct 2026

CVE-2026-113721: virtualenv · Generated files can follow existing destination links

Affects toxSource GCVE Database

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Authoritative record:CVE-2026-113721· Medium 4.5

Not known to be exploited · CVSS 4.5 Medium : upgrade to 4.67.0 when convenient.

Data as of 30 Oct 2026, 07:30 UTC

CVSS 3.1 base score

4.5Medium

How severe the flaw is on the 0–10 CVSS scale.

Thin evidence

One scorer or none

CVSS scores by source
CNA (GitHub_M)	CVSS 3.1	4.5

Patch: Available – fixed in 4.67.0

CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:L

Attack Vector: Local
Attack Complexity: High
Privileges Required: Low
User Interaction: None
Scope: Unchanged
Confidentiality: Low
Integrity: Low
Availability: Low

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Local: The vulnerable component is not bound to the network stack and the attacker’s path is via read/write/execute capabilities. Either: the attacker exploits the vulnerability by accessing the target system locally (e.g., keyboard, console), or remotely (e.g., SSH); or the attacker relies on User Interaction by another person to perform actions required to exploit the vulnerability (e.g., tricking a legitimate user into opening a malicious document).
Attack Complexity: High: A successful attack depends on conditions beyond the attacker's control. That is, a successful attack cannot be accomplished at will, but requires the attacker to invest in some measurable amount of effort in preparation or execution against the vulnerable component before a successful attack can be expected. For example, a successful attack may require an attacker to: gather knowledge about the environment in which the vulnerable target/component exists; prepare the target environment to improve exploit reliability; or inject themselves into the logical network path between the target and the resource requested by the victim in order to read and/or modify network communications (e.g., a man in the middle attack).
Privileges Required: Low: The attacker is authorized with (i.e., requires) privileges that provide basic user capabilities that could normally affect only settings and files owned by a user. Alternatively, an attacker with Low privileges may have the ability to cause an impact only to non-sensitive resources.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Unchanged: An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
Confidentiality: Low: There is some loss of confidentiality. Access to some restricted information is obtained, but the attacker does not have control over what information is obtained, or the amount or kind of loss is limited. The information disclosure does not cause a direct, serious loss to the impacted component.
Integrity: Low: Modification of data is possible, but the attacker does not have control over the consequence of a modification, or the amount of modification is limited. The data modification does not have a direct, serious impact on the impacted component.
Availability: Low: Performance is reduced or there are interruptions in resource availability. Even if repeated exploitation of the vulnerability is possible, the attacker does not have the ability to completely deny service to legitimate users. The resources in the impacted component are either partially available all of the time, or fully available only some of the time, but overall there is no direct, serious consequence to the impacted component.

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 20 Oct 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 29 Oct 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponized: A Metasploit exploit module or Exploit-DB entry exists (Metasploit scanner modules do not count), or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade tox to 4.67.0.

Affected software

Product	Affected	Fixed in
Homebrewtox`pkg:brew/tox`Other threats	Affected >=3.0.0 <4.67.0	Fixed in 4.67.0

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

Description

## Summary Generated activation files and pyvenv.cfg can follow existing links and write outside the requested environment. The template writer tests exists() before unlinking, which misses dangling symlinks; the configuration writer opens the existing path directly. Anthropic's OSS Scanner reported this as `ANT-2026-XBEOCBS1` in `virtualenv-20261015-005`, received on 15 October 2026. ## Affected area Final file entries written during environment creation and seeding. ## Attacker-controlled input Another account must be able to prepare file links in the destination before creation. The invoking user must have write permission at the linked target. The destination parent must remain controlled during creation; concurrent parent-directory replacement is a separate concern. ## Affected code The relevant 21.14.8 source is in [template writer](https://github.com/pypa/virtualenv/blob/a795ab116356afde11039cf2763d77e8a06da738/src/virtualenv/activation/via_template.py), [configuration writer](https://github.com/pypa/virtualenv/blob/a795ab116356afde11039cf2763d77e8a06da738/src/virtualenv/create/pyenv_cfg.py). ## Mechanism `ViaTemplateActivator._generate()` removes an existing output only when `Path.exists()` is true. A dangling symlink therefore survives, and `write_bytes()` creates its target outside the environment. Other generated files have related final-entry behavior. `PyEnvCfg.write()` opens `pyvenv.cfg` without first replacing an existing link; a symlink or hard link there can overwrite an existing target. These paths require a prepared destination tree and the invoking user's permission to write the linked target. ## Reproduction and evidence The scanner's project tree contains `venv/bin/activate -> ../../../.bash_aliases` and an executable `venv/bin/basename`. Before creation, a new interactive bash does not create the marker. A plain `virtualenv venv` writes the activation script through the dangling link, creating `~/.bash_aliases` as a regular file. Debian's stock `.bashrc` sources that file; the next interactive bash adds `venv/bin` to `PATH` and the planted `basename` writes the marker. The reproducer checks that the link remains, `~/.bash_aliases` appears, and the second shell executes the planted file. A separate manual check links `pyvenv.cfg` to `~/.bashrc` and observes the latter replaced with generated configuration text. The link lets creation write outside the requested environment. The attacker chooses the target path but the generated bytes come from virtualenv. The executable in the example was already in the prepared tree; this finding is not arbitrary code injection into an otherwise trusted destination. The scanner traces the template behavior to #1454, first released in 20.0.0b1. [PR #3431](https://github.com/pypa/virtualenv/pull/3431) replaces pre-existing output links and uses exclusive creation for generated text. It does not make a destination safe if another account can replace parent directories during creation. ## Attack sequence ```mermaid sequenceDiagram participant A as Prepared project tree participant V as virtualenv writer participant H as User home participant B as Next interactive bash A->>V: venv/bin/activate links to missing ~/.bash_aliases V->>V: Path.exists() follows link and returns false V->>H: write activation bytes through link H-->>B: ~/.bash_aliases now exists B->>H: .bashrc sources ~/.bash_aliases B->>A: PATH reaches planted venv/bin/basename A-->>B: marker created Note over V,H: pyvenv.cfg can also overwrite an existing linked target ``` ## Reproducer files The following files are from the scanner report for `ANT-2026-XBEOCBS1`. They were tested against commit `a857a7823c73` in the scanner image. The `repro` entrypoint expects companion files under `/tmp/repro.d`; cases that change user IDs need a disposable root-capable container. The last output line is `BUG` when the reported behavior occurs. These files are included as evidence and were not rerun for this advisory update. **`repro`** ```text #!/usr/bin/env python3 import os import shutil import subprocess import sys import tempfile # set up: a fresh home with Debian's stock ~/.bashrc, which sources ~/.bash_aliases when that file exists home = tempfile.mkdtemp(prefix="home-") shutil.copy("/etc/skel/.bashrc", f"{home}/.bashrc") bash_aliases = f"{home}/.bash_aliases" marker = f"{home}/attacker-ran" open_terminal = ["bash", "-i", "-c", "true"] terminal_env = {"HOME": home, "PATH": "/usr/bin:/bin", "TERM": "dumb"} # the attacker's repository, cloned by the user to ~/proj checkout = f"{home}/proj" subprocess.run([sys.executable, "/tmp/repro.d/make_hostile_checkout.py", checkout], check=True) activate = f"{checkout}/venv/bin/activate" print("shipped: venv/bin/activate ->", os.readlink(activate), flush=True) # a terminal opened before virtualenv runs does not touch the checkout (stderr: bash -i without a tty warns) subprocess.run(open_terminal, e

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