CVE-2026-26028
Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.
NVD · uneditedCryptPad is an end-to-end encrypted collaborative office suite. In versions prior to 2026.2.0, the HTML sanitizer in Diffmarked.js can be bypassed due to incomplete attribute filtering on restricted tags. The sanitizer validates only the src attribute of <iframe>, <video>, and <audio> elements, leaving all other attributes unchecked. As a result, an attacker can inject arbitrary HTML through srcdoc, completely defeating CryptPad's intended bounce sandboxing and enabling link injection or other interactive content within user-controlled documents. The root cause lies in how the sanitizer classifies and enforces tag restrictions: although it defines both forbidden and restricted tag lists, <iframe> is treated as "restricted" rather than "forbidden." Enforcement then inspects only the src attribute, so pairing a benign blob: src with a malicious srcdoc results in unrestricted rendering. This issue has been fixed in version 2026.2.0.
Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.
dbcve analysis · high confidenceThe HTML sanitizer in CryptPad's Diffmarked.js only validates the src attribute on restricted elements like <iframe>, leaving other attributes unchecked. An attacker can inject malicious srcdoc attributes with a benign blob: src to bypass the sandbox and inject arbitrary HTML, defeating the intended security boundaries.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.
From the vector- Attack vector
- Network
- Complexity
- Low
- Privileges
- None
- User interaction
- Required
- Scope
- Changed
- Confidentiality
- Low
- Integrity
- Low
- Availability
- None
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.
dbcve checksWork through these to decide whether this CVE applies to you.
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Locate CryptPad installationSearch the server filesystem for the Diffmarked.js file, typically found within the cryptpad directory under /lib/ or similar paths. Also check for version.js or package.json in the root.Affected if CryptPad is installed and Diffmarked.js exists in the application path
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Check CryptPad versionOpen the version file (version.js or package.json) in the CryptPad root directory and record the version number.Affected if Version is below 2026.2.0 (e.g., 2026.1.x, 2025.x.x, etc.)
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Examine Diffmarked.js sanitizer codeOpen Diffmarked.js and locate the HTML sanitization function. Search for how restricted elements (iframe, video, audio) are handled - look for code that validates attributes on these tags.Affected if The code only checks the 'src' attribute and does not filter or reject other attributes like 'srcdoc' on restricted elements
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Confirm rich text/markdown feature is enabledVerify the CryptPad instance has rich text editor or markdown rendering enabled, which invokes Diffmarked.js to sanitize HTML content.Affected if Users can create or edit documents that render HTML through the Diffmarked.js sanitizer
If CryptPad version is below 2026.2.0 and Diffmarked.js does not validate attributes beyond 'src' on restricted elements, the environment is affected by CVE-2026-26028.
Generated from the published advisory. Verify against your own configuration.
Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.
dbcve · scopedUpgrade to CryptPad version 2026.2.0 or later, which properly filters the srcdoc attribute or reclassifies iframe as a forbidden tag.
2026.2.0
- 1. Identify the current version of CryptPad installed in your environment
- 2. Create a complete backup of the CryptPad instance including database and configuration files
- 3. Upgrade CryptPad to version 2026.2.0 or later
- 4. Verify the upgrade was successful by checking the version number in the application
- 5. Confirm that the HTML sanitizer in Diffmarked.js now properly filters all attributes including srcdoc on iframe, video, and audio elements
Generated from the published advisory — verify against the referenced sources before acting.
- Consultation2.0 h
- Implementation2.0 h
- Testing3.0 h
- Review / QA1.0 h
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Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2026-26028 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2026-26028 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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- Verified mitigations, workarounds, and config changes
- Version or environment caveats, and links to real fixes
- No weaponised exploit code, or anything meant to cause harm
- No spam, self-promotion, credentials, or personal data