id: PYSEC-2026-2527
published: "2026-07-13T14:36:38.311279Z"
modified: "2026-07-13T16:04:21.211428Z"
aliases:
- CVE-2026-27469
- GHSA-9fww-8cpr-q66r
summary: Isso affected by Stored XSS via comment website field
details: "## Impact\n\nThis is a stored Cross-Site Scripting (XSS) vulnerability affecting the website and author comment fields. The website field was HTML-escaped using quote=False, which left single and double quotes unescaped. Since the frontend inserts the website value directly into a single-quoted href attribute via string concatenation, a single quote in the URL breaks out of the attribute context, allowing injection of arbitrary event handlers (e.g. onmouseover, onclick).\n\nThe same escaping was missing entirely from the user-facing comment edit endpoint (PUT /id/<id>) and the moderation edit endpoint (POST /id/<id>/edit/<key>).\n\nAny visitor to a page embedding isso comments is impacted. No authentication or interaction beyond mouse movement is required to trigger a payload — an attacker can post a comment anonymously (moderation is off by default) with a crafted website URL, and the payload persists in the database and fires on every page load. With the full-page invisible overlay technique described in the report, the victim only needs to move their mouse.\n\n## Patches\n\nThe issue is fixed in commit 3cf27c2. Users should upgrade to a version containing that commit once released. The fix applies html.escape(..., quote=True) to the website field across all three write paths (POST /new, PUT /id/<id>, POST /id/<id>/edit/<key>), and adds input validation and escaping to the moderation edit endpoint which previously had neither.\n\n## Workarounds\n\n  Enabling comment moderation (moderation = enabled = true in isso.cfg) prevents unauthenticated users from publishing comments, raising the bar for exploitation. However, it\n  does not fully mitigate the issue since a moderator activating a malicious comment would still expose visitors. There is no configuration-only workaround that fully prevents\n   the vulnerability.\n\n## Resources\n\n  - https://docs.python.org/3/library/html.html#html.escape — note the quote parameter"
affected:
- package:
    name: isso
    ecosystem: PyPI
    purl: pkg:pypi/isso
  ranges:
  - type: ECOSYSTEM
    events:
    - introduced: "0"
    - fixed: 0.13.2
  versions:
  - "0.10"
  - 0.10.1
  - 0.10.2
  - 0.10.3
  - 0.10.4
  - 0.10.5
  - 0.10.6
  - 0.11.0
  - 0.11.1
  - 0.12.0
  - 0.12.1
  - 0.12.2
  - 0.12.4
  - 0.12.5
  - 0.12.6
  - 0.12.6.1
  - 0.12.6.2
  - 0.13.0
  - 0.13.1.dev0
  - "0.8"
  - 0.8.1
  - 0.8.2
  - 0.8.3
  - "0.9"
  - 0.9.1
  - 0.9.10
  - 0.9.2
  - 0.9.3
  - 0.9.4
  - 0.9.5
  - 0.9.6
  - 0.9.7
  - 0.9.8
  - 0.9.9
references:
- type: WEB
  url: https://github.com/isso-comments/isso/security/advisories/GHSA-9fww-8cpr-q66r
- type: ADVISORY
  url: https://nvd.nist.gov/vuln/detail/CVE-2026-27469
- type: WEB
  url: https://github.com/isso-comments/isso/commit/0afbfe0691ee237963e8fb0b2ee01c9e55ca2144
- type: WEB
  url: "https://docs.python.org/3/library/html.html#html.escape"
- type: PACKAGE
  url: https://github.com/isso-comments/isso
- type: PACKAGE
  url: https://pypi.org/project/isso
- type: ADVISORY
  url: https://github.com/advisories/GHSA-9fww-8cpr-q66r
severity:
- type: CVSS_V3
  score: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N