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Mako: Path traversal via double-slash URI prefix in TemplateLookup — mako

Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')

Summary

TemplateLookup.get_template() is vulnerable to path traversal when a URI starts with // (e.g., //../../../secret.txt). The root cause is an inconsistency between two slash-stripping implementations:

  • Template.__init__ strips one leading / using if/slice
  • TemplateLookup.get_template() strips all leading / using re.sub(r"^\/+", "")

When a URI like //../../../../etc/passwd is passed:

  1. get_template() strips all /../../../../etc/passwd → file found via posixpath.join(dir_, u)
  2. Template.__init__ strips one //../../../../etc/passwdnormpath/etc/passwd
  3. /etc/passwd.startswith(..) → Falsecheck bypassed

Impact

Arbitrary file read: any file readable by the process can be returned as rendered template content when an application passes untrusted input directly to TemplateLookup.get_template().

Note: this is exploitable at the library API level. HTTP-based exploitation is mitigated by Python's BaseHTTPRequestHandler which normalizes double-slash prefixes since CPython gh-87389. Applications using other HTTP servers that do not normalize paths may be affected.

Fix

Changed Template.__init__ to use lstrip("/") instead of stripping only a single leading slash, so both code paths handle leading slashes consistently.

  • Published: Apr 16, 2026
  • Updated: Apr 17, 2026
  • GHSA: GHSA-v92g-xgxw-vvmm
  • Severity: Medium
  • Exploit:
  • CISA KEV:

No technical information available.

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