SiYuan has a SVG Sanitizer Bypass via Whitespace in `javascript:` URI — Unauthenticated XSS
Date: 2026-03
Affected: go/github.com/siyuan-note/siyuan/kernel
Attack vector: xss
Category: vulnerability-disclosure
Severity: Medium
CVE: CVE-2026-31807
CVE-2026-31809
CWE: CWE-79

# SVG Sanitizer Bypass via Whitespace in `javascript:` URI — Unauthenticated XSS ## Summary SiYuan's SVG sanitizer (`SanitizeSVG`) checks `href` attributes for the `javascript:` prefix using `strings.HasPrefix()`. However, inserting ASCII tab (`&#9;`), newline (`&#10;`), or carriage return (`&#13;`) characters inside the `javascript:` string bypasses this prefix check. Browsers strip these characters per the WHATWG URL specification before parsing the URL scheme, so the JavaScript still executes. This allows an attacker to inject executable JavaScript into the unauthenticated `/api/icon/getDynamicIcon` endpoint, creating a reflected XSS. This is a second bypass of the fix for CVE-2026-29183 (fixed in v3.5.9), [distinct from the `<animate>` element bypass](https://github.com/siyuan-note/siyuan/security/advisories/GHSA-5hc8-qmg8-pw27). ## Affected Component - **File:** `kernel/util/misc.go` - **Function:** `SanitizeSVG()` (lines 234-319) - **Specific check:** Line 271 — `strings.HasPrefix(val, "javascript:")` - **Endpoint:** `GET /api/icon/getDynamicIcon?type=8&content=...` (unauthenticated) - **Version:** SiYuan <= 3.5.9 ## Root Cause The sanitizer uses Go's `html.Parse` which decodes HTML entities in attribute values. When the input contains `java&#9;script:alert(1)`, the parser decodes `&#9;` to a literal tab character (U+0009). The sanitizer then checks: ```go val := strings.TrimSpace(strings.ToLower(a.Val)) // val is now "java\tscript:alert(1)" if strings.HasPrefi


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Architectural layers (MAESTRO):