{"id":"CVE-2026-54011","title":"Open WebUI: Stored XSS in Mermaid Markdown Preview","summary":"Open WebUI: Stored XSS in Mermaid Markdown Preview","severity":"high","cvss":8.7,"cwe":["CWE-79"],"vendor":"open-webui","product":"open-webui","ecosystem":"pip","affected":["open-webui <= 0.9.5"],"patched":["open-webui 0.9.6"],"published":"2026-06-17","updated":"2026-06-17","source":"GHSA","sourceUrl":"https://github.com/advisories/GHSA-v8qj-hxv7-mgvv","references":[{"url":"https://github.com/open-webui/open-webui/security/advisories/GHSA-v8qj-hxv7-mgvv"},{"url":"https://github.com/advisories/GHSA-v8qj-hxv7-mgvv"}],"tags":["ghsa","pip"],"epss":0.00336,"epssPercentile":0.24202,"ingestedAt":"2026-06-29T14:31:47.243Z","slug":"CVE-2026-54011","body":"## Overview\n\n## Summary\n\nOpen WebUI renders Mermaid blocks from Markdown files in the file preview panel and inserts the generated SVG into the DOM using `innerHTML`.\n\nBecause Mermaid is configured with `securityLevel: 'loose'`, attacker-controlled Mermaid content can be rendered unsafely in this flow. A working payload was validated through the Markdown preview path, resulting in JavaScript execution in the victim’s browser under the application origin.\n\nThis is a confirmed stored XSS vulnerability reachable through normal product functionality.\n\n## Affected Version\n\n- `main`\n- Reproduced on `v0.8.12`\n\n## Affected Code\n\nMermaid is initialized in permissive mode:\n\nhttps://github.com/open-webui/open-webui/blob/9bd84258d09eefe7bf975878fb0e31a5dadfe0f8/src/lib/utils/index.ts#L1698\nThe file preview path renders Mermaid output and injects the returned SVG into the DOM:\n\nhttps://github.com/open-webui/open-webui/blob/9bd84258d09eefe7bf975878fb0e31a5dadfe0f8/src/lib/components/chat/FileNav/FilePreview.svelte#L133\n\n## Impact\n\nA successful exploit allows JavaScript execution in the victim’s browser under the Open WebUI origin when a malicious Markdown file is opened in the preview panel.\n\n## PoC\n\nA malicious `.md` file containing the follwowing contents can be used to trigger the bug:\n````\n```mermaid\nflowchart LR\n  A[click me]\n  click A href \"javascript:alert(document.domain)\" \"x\"\n```\n````\nSteps to reproduce: \n1- Create a new chat \n2- Enable Code Interpreter and browse and upload the file with `.md` extension. \n<img width=\"331\" height=\"258\" alt=\"image\" src=\"https://github.com/user-attachments/assets/bce2b754-56d1-4da1-90a9-22bcb93269f2\" />\n3- Clicking on the file, and clicking `click me` should pop an alert\n<img width=\"1103\" height=\"485\" alt=\"image\" src=\"https://github.com/user-attachments/assets/18754486-799b-434e-a2fc-dd7c09956a29\" />\n \n\n## Remediation\n\nSince `mermaid` has `DOMPurify` as a built-in, it is recommended to use the `strict` mode instead of `loose`.\n\n## Affected packages\n\n- `open-webui <= 0.9.5`\n\n## Remediation\n\nUpgrade to a patched release:\n\n- `open-webui 0.9.6`","depth":"twilight","depthScore":48,"depthScoreParts":{"impact":47.8,"likelihood":0.1,"exploitation":0,"ransomware":0},"changes":[]}