Security readout for executives and security teams
Plain-English summary
SiYuan versions before 3.7.3 can let a signed-in attacker place malicious SVG content that runs script in another user’s browser. Successful exploitation could expose sensitive application data or allow unauthorized actions with the victim’s privileges. A victim must interact with the crafted content.
Executive priority
Treat this as an urgent remediation item for affected SiYuan deployments. Prioritize externally reachable and multi-user systems because compromise could affect application confidentiality and integrity. Balance the critical technical severity with the absence of reported active exploitation in the supplied sources.
Technical view
The HTML parser-based sanitizer treats script-like content inside SVG desc, style, or noscript elements as raw text. When the resulting file is served as image/svg+xml, browsers can interpret that content as executable SVG. The issue supports stored and reflected XSS in the SiYuan application origin.
Likely exposure
SiYuan deployments earlier than 3.7.3 are potentially exposed, particularly network-accessible, multi-user instances where authenticated users can submit content later rendered as SVG. The supplied evidence does not identify specific vulnerable routes, configurations, or indicators of compromise.
Exploitation context
Exploitation is network-accessible and low complexity but requires an authenticated attacker and victim interaction. The CVSS 4.0 score is 9.3. The source bundle does not report CISA KEV listing or confirmed active exploitation, so exploitation in the wild should not be claimed.
Researcher notes
The parser-context mismatch is central: content considered inert during HTML sanitization becomes active under SVG MIME interpretation. Assessment should cover both persisted and reflected rendering flows. The bundle does not provide affected endpoint names, detection signatures, exploit telemetry, or configuration-specific exceptions.
Mitigation direction
- Inventory SiYuan instances and prioritize every deployment running a version earlier than 3.7.3.
- Upgrade to SiYuan 3.7.3 or later, following the vendor advisory.
- Until upgraded, restrict untrusted authenticated users and SVG content handling where operationally feasible.
- Review application logs and user reports for suspicious SVG submissions or unexpected browser script activity.
Validation and detection
- Confirm every SiYuan instance reports version 3.7.3 or later.
- Verify stored and reflected SVG rendering paths no longer expose attacker-supplied active content.
- Use approved defensive testing to confirm sanitized SVG content cannot execute scripts in the application origin.
- Review content permissions to determine which authenticated users could submit affected SVG content.
Public sources used
Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.
Conservative CVE-to-ATT&CK context
These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.
ATT&CK lookup starting points
Use these exact CWE pages and searches to review the Glexia ATT&CK library from this CVE's weakness and description context.
CWE-79: User-session and phishing behavior lookup
Client-side and session-facing weaknesses should be reviewed alongside initial-access and user-execution behaviors. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.
Open ATT&CK lookupCVE-2026-66394 mapping review
Open the CVE-to-ATT&CK bridge for reviewed, inferred, or future official mappings tied to this CVE.
Open ATT&CK lookup- Severity
- Critical
- CVSS
- 9.3 (4.0)
- Known Exploited
- No
- Published
Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:P/VC:H/VI:H/VA:N/SC:H/SI:H/SA:N
CNA and ADP enrichment extracted from CVE v5
These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.
SSVC decision data
CVSS vector scores
2 official scoresWe collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.
CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:P/VC:H/VI:H/VA:N/SC:H/SI:H/SA:N——VulnCheckCVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:H/I:H/A:N2.35.8VulnCheckVulnerability scoring details
Base CVSS 4.0 score
9.3CriticalVector: CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:P/VC:H/VI:H/VA:N/SC:H/SI:H/SA:N
Base CVSS 3.1 score
8.7HighVector: CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:H/I:H/A:N
Vulnerability timeline
Timeline events are normalized from CVE metadata, CNA source timelines, ADP timelines, and KEV metadata when present.
- CVE reservedCVE Program
The CVE ID was reserved by the assigning CNA.
- CVE publishedCVE Program
The CVE record was published.
- CVE updatedCVE Program
The CVE record metadata indicates this as the latest update time.
ADP provider summaries
Source materials
- CVE List V5 sourceCVE List V5
- GitHub Security Advisory (GHSA-99rq-75j6-5j9f)CVE reference, VulnCheck · vendor-advisory
- VulnCheck Advisory: SiYuan before v3.7.3 Stored and Reflected XSS via SVG Sanitizer BypassCVE reference, VulnCheck · third-party-advisory
Products and packages named in the record
CWE details
CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.