Missing Security Headers Enable Stored XSS in SiYuan

SiYuan before v3.7.4 fails to set Content-Disposition and X-Content-Type-Options headers when serving arbitrary file assets, allowing stored cross-site scripting attacks. Authenticated attackers can upload HTML files as assets and execute scripts with full kernel API access when the workspace owner opens the asset link.

Casky was already ahead

This CVE exploits attack patterns that Casky's 322matched skills already investigate — long before this vulnerability was disclosed. Claude's reasoning model maps these techniques to MITRE ATT&CK, so practitioners who ran these skills have already seen the threat behaviour in their findings.

Analysis

SiYuan before v3.7.4 contains a critical stored cross-site scripting vulnerability stemming from improper handling of HTTP security headers when serving file assets. The application fails to set Content-Disposition and X-Content-Type-Options headers, which normally instruct browsers to download files rather than render them inline. This oversight allows authenticated attackers to upload malicious HTML files as workspace assets. When a workspace owner accesses the asset link, the browser executes embedded scripts with full access to SiYuan's kernel API, potentially compromising the entire workspace. The vulnerability affects all SiYuan users and is particularly dangerous because it requires no victim interaction beyond opening a link—a social engineering vector that threatens data integrity, confidentiality, and availability across collaborative note-taking environments.

Casky's 322 mapped skills leverage Claude's extended reasoning to detect the attack patterns underlying this vulnerability by analyzing MITRE ATT&CK techniques TA0001 (Initial Access—specifically T1566 Phishing for credential harvesting or T1195 Supply Chain Compromise of workspace assets) and TA0043 (Reconnaissance—T1592 Gather Victim Host Information through kernel API reconnaissance). Practitioners using Casky would identify findings showing unsafe asset delivery mechanisms, missing HTTP security headers in response patterns, and HTML/script file uploads by authenticated users to shared repositories. The platform's skill-driven analysis would flag the absence of Content-Disposition: attachment directives and X-Content-Type-Options: nosniff headers in asset serving responses, correlating these gaps with DOM-based XSS patterns and privilege escalation to API access—enabling teams to prioritize patching and implement compensating controls like strict Content-Security-Policy headers before exploitation occurs.

Live Threat IntelligenceComing soon

Casky Risk Score

—

EPSS Probability

—

Shodan Exposure

—

GreyNoise Signal

—

Composite risk scoring from EPSS, CISA KEV, Shodan, and GreyNoise — 21 security APIs correlated into a single Casky Risk Score. Coming in Casky Pro. Join early access →

322 Casky skills cover this attack pattern

These skills use Claude AI's reasoning model to surface findings in the same attack categories as CVE-2026-74800.

Abuse of Public-Facing API: Mobile API Abuse

mobile security · low

Run

abusing-dpapi-for-credential-access

red teaming · high

Run

abusing-shadow-credentials-for-privesc

red teaming · high

Run

MITRE ATT&CK Techniques

TA0001 TA0043

Investigate the attack patterns behind CVE-2026-74800

Casky has 322 skills that investigate the attack patterns behind CVE-2026-74800. Run one and get CVSS-scored findings in 3 minutes.

Run the skill that detects this →

X Instagram LinkedIn