GHSA-9FWW-8CPR-Q66R
Vulnerability from github – Published: 2026-02-24 16:03 – Updated: 2026-02-24 16:03Impact
This 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).
The same escaping was missing entirely from the user-facing comment edit endpoint (PUT /id/) and the moderation edit endpoint (POST /id//edit/).
Any 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.
Patches
The 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/, POST /id//edit/), and adds input validation and escaping to the moderation edit endpoint which previously had neither.
Workarounds
Enabling comment moderation (moderation = enabled = true in isso.cfg) prevents unauthenticated users from publishing comments, raising the bar for exploitation. However, it 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 the vulnerability.
Resources
- https://docs.python.org/3/library/html.html#html.escape — note the quote parameter
{
"affected": [
{
"package": {
"ecosystem": "PyPI",
"name": "isso"
},
"ranges": [
{
"events": [
{
"introduced": "0"
},
{
"fixed": "0.13.2"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"aliases": [
"CVE-2026-27469"
],
"database_specific": {
"cwe_ids": [
"CWE-116",
"CWE-79"
],
"github_reviewed": true,
"github_reviewed_at": "2026-02-24T16:03:04Z",
"nvd_published_at": "2026-02-21T08:16:11Z",
"severity": "MODERATE"
},
"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/\u003cid\u003e) and the moderation edit endpoint (POST /id/\u003cid\u003e/edit/\u003ckey\u003e).\n\nAny visitor to a page embedding isso comments is impacted. No authentication or interaction beyond mouse movement is required to trigger a payload \u2014 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/\u003cid\u003e, POST /id/\u003cid\u003e/edit/\u003ckey\u003e), 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 \u2014 note the quote parameter",
"id": "GHSA-9fww-8cpr-q66r",
"modified": "2026-02-24T16:03:04Z",
"published": "2026-02-24T16:03:04Z",
"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"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N",
"type": "CVSS_V3"
}
],
"summary": "Isso affected by Stored XSS via comment website field"
}
Sightings
| Author | Source | Type | Date |
|---|
Nomenclature
- Seen: The vulnerability was mentioned, discussed, or observed by the user.
- Confirmed: The vulnerability has been validated from an analyst's perspective.
- Published Proof of Concept: A public proof of concept is available for this vulnerability.
- Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
- Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
- Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
- Not confirmed: The user expressed doubt about the validity of the vulnerability.
- Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.