djust: A template binding inherits a context safety grant it never earned (XSS)

A context safety grant was inherited by a template binding that never earned it, so rebinding a name the view had marked safe left the mark attached to the new, attacker-controlled value.

djust's context safety channel is keyed by name, not by value. Every bind copied the value and left the grant in place:

view marks   p = mark_safe('<b>trusted</b>')
template     {% with p=user_input %}{{ p }}{% endwith %}

  djust   '<img src=x onerror=alert(1)>'          <- executes
  django  '&lt;img src=x onerror=alert(1)&gt;'

Affected binds

Eight distinct shapes were live. All are fixed in 1.1.2:

| shape | example | |---|---| | {% with %} rebinding a marked name | {% with p=hostile %}{{ p }}{% endwith %} | | {% with %} descendant of a rebound name | {% with p=hostile %}{{ p.a }}{% endwith %} | | {% with %} binding over a marked name | {% with q=hostile %}{{ q }}{% endwith %} | | {% for %} loop variable | {% for p in hostiles %}{{ p }}{% endfor %} | | {% for %} descendant | {% for p in hostiles %}{{ p.a }}{% endfor %} | | {% for %} tuple unpacking | {% for k, v in rows %}{{ v }}{% endfor %} | | {% include … with %} | {% include "c.html" with q=hostile %} | | assign tags | {% … as x %} |

No filter chain and no |safe anywhere in the template is required. The precondition is that the application marked *some* value safe under a name that a template later rebinds — an ordinary pattern, since a view commonly marks trusted markup safe and templates commonly reuse short names such as p, item or row.

Impact

Stored or reflected XSS in any djust application that combines mark_safe (or any framework path that grants a context key safety) with a template that rebinds that name. Exploitation requires no unusual template construct.

Patches

Fixed in 1.1.2, and in 1.2.0 (main).

The rule is that a bind replaces the grant rather than inheriting one. It is stated that way deliberately: the originally reported defects pointed the *opposite* direction — a grant being lost across some binds, causing over-escaping — and fixing only the reported direction would have left this leak in place. Both directions are the same rule.

Not fixed in 1.1.2, and tracked for 1.2.0: two over-escaping cases where a grant fails to travel with a value that legitimately carries one ({% with body=post.text|linebreaks %}, and mark_safe reaching a template through dict.values / dict.items). Both escape *more* than Django rather than less, so neither is a leak.

Workarounds

Before upgrading, avoid reusing a context name for both mark_safe'd content and untrusted input, and avoid rebinding such a name in {% with %}, {% for %}, {% include … with %} or an assign tag.

Credit

Found while preparing the 1.1.1 security release: the reproducer published in GHSA-9395-2g46-rj3f was run against the built 1.1.1 artifact before that advisory was published, and proved the class was still live. Publishing the original text would have reported this class as patched when it was not.