CVE-2026-26028

Description

CryptPad 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.

CVSS Details

CVSS Score

6.1

Severity

MEDIUM

CVSS Vector

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

Configurations (Affected Products)

No configuration data available.

CryptPad < 2026.2.0

PoC / Exploit Code

⚠ For Security Research Only

The following code is for security research and authorized testing only.

python

<!-- PoC: Bypass CryptPad sanitizer using srcdoc -->
<!-- The sanitizer checks 'src' but ignores 'srcdoc' -->
<iframe src="blob:https://example.com/valid-blob" srcdoc="<img src=x onerror=alert(1)>"></iframe>

References

[1] CVE.org https://www.cve.org/CVERecord?id=CVE-2026-26028
[2] NVD NIST https://nvd.nist.gov/vuln/detail/CVE-2026-26028
[3] CVE Details https://www.cvedetails.com/cve/CVE-2026-26028/
[4] VulDB https://vuldb.com/cve/CVE-2026-26028
[5] https://github.com/cryptpad/cryptpad/releases/tag/2026.2.0
[6] https://github.com/cryptpad/cryptpad/security/advisories/GHSA-g2g4-47gv-p72v

Raw JSON Data

JSON

{"cve": {"id": "CVE-2026-26028", "sourceIdentifier": "[email protected]", "published": "2026-05-20T20:16:36.760", "lastModified": "2026-05-21T16:04:53.813", "vulnStatus": "Deferred", "cveTags": [], "descriptions": [{"lang": "en", "value": "CryptPad 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."}], "metrics": {"cvssMetricV31": [{"source": "[email protected]", "type": "Secondary", "cvssData": {"version": "3.1", "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N", "baseScore": 6.1, "baseSeverity": "MEDIUM", "attackVector": "NETWORK", "attackComplexity": "LOW", "privilegesRequired": "NONE", "userInteraction": "REQUIRED", "scope": "CHANGED", "confidentialityImpact": "LOW", "integrityImpact": "LOW", "availabilityImpact": "NONE"}, "exploitabilityScore": 2.8, "impactScore": 2.7}]}, "weaknesses": [{"source": "[email protected]", "type": "Primary", "description": [{"lang": "en", "value": "CWE-79"}, {"lang": "en", "value": "CWE-116"}]}], "references": [{"url": "https://github.com/cryptpad/cryptpad/releases/tag/2026.2.0", "source": "[email protected]"}, {"url": "https://github.com/cryptpad/cryptpad/security/advisories/GHSA-g2g4-47gv-p72v", "source": "[email protected]"}]}}