CVE-2026-27469 Isso: Stored XSS via comment website field
Attackers can inject malicious scripts via comment fields in Isso, potentially stealing user sessions or executing code in victim browsers.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- A user has to click or open something
- Fix
- Available
What is CVE-2026-27469?
CVE-2026-27469 is a medium-severity cross-site scripting vulnerability in isso-comments isso, published on 21 Feb 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Official description
Isso is a lightweight commenting server written in Python and JavaScript. In commits before 0afbfe0691ee237963e8fb0b2ee01c9e55ca2144, there 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 is missing entirely from the user-facing comment edit endpoint (PUT /id/) and the moderation edit endpoint (POST /id//edit/). This issue has been patched in commit 0afbfe0691ee237963e8fb0b2ee01c9e55ca2144. To workaround, nabling comment moderation (moderation = enabled = true in isso.cfg) prevents unauthenticated users from publishing comments, raising the bar for exploitation, but it does not fully mitigate the issue since a moderator activating a malicious comment would still expose visitors.
Which versions are affected by CVE-2026-27469?
3 open source packages ship an affected version, including isso.
| Package | Affected | Fixed in |
|---|---|---|
isso PyPI | All versions | 0.13.2 |
isso PyPI | All versions | 0.13.2 |
isso PyPI | All versions | 0.13.2 |
How do I fix CVE-2026-27469?
Upgrade isso to 0.13.2 or later.
Escape output for the context it lands in and set a strict Content Security Policy. Upgrading the affected component is the real fix. More about Cross-site scripting.
Is CVE-2026-27469 being exploited?
There is no known exploitation of CVE-2026-27469 in the wild and no public exploit code.
Sources
Also known as GHSA-9fww-8cpr-q66r, PYSEC-2026-2527.