PYSEC-2026-2617

## Summary

On Windows, a URI using backslash traversal (e.g. `\..\..\ secret.txt`) bypasses the directory traversal check in `Template.__init__` and the `posixpath`-based normalization in `TemplateLookup.get_template()`, allowing reads of files outside the configured template directory.

## Details

The root cause is a mismatch between `posixpath` (used for URI normalization in `get_template()`) and `os.path` (used for file access via `os.path.isfile()` and validation via `os.path.normpath()` in `Template.__init__`). On Windows, `os.path` is `ntpath`, which treats `\` as a path separator, while `posixpath` treats it as a literal character.

The vulnerability chain:

1. `get_template()` strips only leading `/` via `re.sub(r"^\/+", "", uri)` and normalizes with `posixpath` — backslash `\` is treated as a literal character, so `\..\ secret.txt` passes through with `..` undetected. 2. `Template.__init__()` validation uses `os.path.normpath()` — on Windows this resolves `\..\ secret.txt` to `\secret.txt`, which does not start with `..`, so the `startswith("..")` check passes. 3. `os.path.isfile()` on Windows interprets `\` as a path separator, resolving the `..` traversal and finding files outside the template directory.

### Affected code

- `mako/lookup.py`: `TemplateLookup.get_template()` uses `posixpath.normpath`/`posixpath.join` for path construction but `os.path.isfile()` for existence check - `mako/template.py`: `Template.__init__()` URI validation uses `os.path.normpath()` which on Windows resolves backslash traversal to a form that passes the `startswith("..")` guard

## Impact

If an application passes user-controlled template names or include paths to `TemplateLookup.get_template()`, an attacker on Windows may be able to load and disclose readable files outside the configured template directory. 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.

## Remediation

The fix should normalize backslashes to forward slashes early in the URI processing pipeline, before any path operations, to ensure consistent behavior across platforms.