djust: Six template-layer defects emit attacker-controlled markup unescaped (XSS)

Five independent defects in djust's template auto-escaping cause attacker-controlled input to be rendered as live markup where Django escapes it. All four are present in shipped 1.1.0 and are fixed in 1.1.1. They share one shape: **a filter or grant that escapes nothing itself and relies on the render-time auto-escape, which something downstream then removes.** They are grouped into a single advisory because the mitigation is identical — upgrade to 1.1.1 — and because no single one of them is meaningfully actionable in isolation. ## 1. `linenumbers` never escaped its input (#2291) ``` {{ p|linenumbers|safe }} with p = '<img src=x onerror=alert(1)>' djust '1. <img src=x onerror=alert(1)>' <- executes django '1. &lt;img src=x onerror=alert(1)&gt;' ``` The filter deferred all escaping to render time; a trailing `|safe` suppressed exactly that. The exposure is wider than the `|safe` form: **any downstream filter that reads the output as markup is affected**, including `{{ p|linenumbers|truncatechars_html:"5" }}`, which contains no `|safe` at all. ## 2. `escape` was a no-op (#2281) ``` {{ p|escape|safe }} djust '<img src=x onerror=alert(1)>' <- executes django '&amp;lt;img src=x onerror=alert(1)&amp;gt;' ``` Django's `escape` is eager (`conditional_escape`, returning `SafeString`). djust's returned its input unchanged and let the render site escape it — indistinguishable for `{{ p|escape }}` alone, wrong for every chain. The security cell is `{{ p|escape|safe }}`: an idiom that reads as "escape it, then it is safe to emit" — which is what Django's semantics make true — was a bare `|safe` on attacker input. A sweep of every length-2 and length-3 chain containing `escape` found **104** live-markup cells. ## 3. `unordered_list` / `safeseq` handed a string back under a safe grant (#2274) Both carry an unconditional "emit without escaping" grant, earned because they escape every item they emit. Given a **string** rather than a sequence they emitted nothing and returned the input verbatim under that same grant, making `{{ hostile|safeseq }}` an exact synonym for `|safe` with no `mark_safe` anywhere in the template. ## 4. A safety grant outlived the value it was granted for (#2300) **No filter chain and no `|safe` anywhere; a bare `{{ p }}` is the whole reproducer.** Safe context keys accumulated on the view and were never revoked, so a key marked safe once stayed safe for the lifetime of the view — which spans **every event on a WebSocket connection**: ```python render 1: p = mark_safe('<b>trusted</b>') -> '<b>trusted</b>' correct render 2: p = '<img src=x onerror=alert(1)>' -> executes ``` A view that renders trusted markup into a variable and later renders user input into the same variable emits it live. ## 5. A custom tag handler's return was emitted raw — including djust's own `{% render_slot %}` (#2379) **Reachable with no `|safe`, no `mark_safe`, and no application code: using component slots is enough.** Django's `SimpleNode.render` runs `conditional_escape` over a `simple_tag`'s return unless it carries `__html__`. djust inserted the return verbatim, so a handler as ordinary as `return f"Hello {name}"` emitted attacker markup live. Of the 221 handlers djust registers, one echoes a context value unescaped — `render_slot`, the framework's own function-component/slot tag: ``` {% render_slot p %} p = '<img src=x onerror=alert(1)>' djust '<img src=x onerror=alert(1)>' <- executes django '&lt;img src=x onerror=alert(1)&gt;' ``` Together with defect 3 this is one of the two classes reachable without the application writing anything unusual. ## 6. `linebreaks` / `linebreaksbr` — and `|safe` was the only spelling that worked (#2284) `linebreaks` emits `<p>`/`<br>` but neither escaped its content nor reported its output safe. The plain spelling therefore escaped the filter's **own** tags and printed a literal `<p>` on the page, so `|safe` was the only form that rendered at all — and that form emitted the content live: ``` {{ bio|linebreaks }} renders literal '<p>' text (visibly broken) {{ bio|linebreaks|safe }} '<img src=x onerror=alert(1)>' <- executes ``` Because the broken spelling is the one a developer discards, the vulnerable spelling is the one that ships. Any application rendering user-entered text with paragraph breaks is written that way. ## Impact Stored or reflected XSS in any djust application that renders untrusted input through the affected filters, or that reuses a context variable which was previously marked safe. Exploitation requires no special configuration. Defects 3, 4 and 5 require no unusual template construct at all — defect 5 needs only that the application use component slots, and defect 6's vulnerable spelling is the only one that renders correctly. ## Patches Fixed in **1.1.1**, and in **1.2.0** (`main`). 1.1.1 re-implements each fix against 1.1.0's own code rather than back-porting `main`'s, which depends on a value-level safety model 1.1.0 does not have. Three consequences are documented in the 1.1.1 CHANGELOG and are all in the **over-escaping** direction: `{{ p|escape|F }}` double-escapes for a plain following filter `F`; `{% render_slot slot.content %}` over-escapes; and a `mark_safe`d value passed through `|escape` is escaped rather than passed through. **Not fixed in 1.1.1**, and tracked separately: application-written tag handlers that return attacker data as a plain `str` (only djust's own `render_slot` is covered by defect 5), and a `{% with %}`/`{% for %}` bind inheriting a safety grant it never earned. Both are fixed in 1.2.0. ## Workarounds None complete. Before upgrading, avoid `|safe` after any filter in a chain, avoid `safeseq`/`unordered_list` on values that may be strings, and avoid reusing a context variable for both `mark_safe` content and untrusted input. ## Credit Found during an internal Django-parity audit by a registry-wide differential that compares djust's escaping capabilities against Django's across every filter chain, rather than by inspection.