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Overview
CVE-2023-42820 is a security vulnerability that affects JumpServer, a popular open source bastion host. This issue comes down to weak handling of random number generation—specifically, the backend carelessly exposes the random seed via its API. If an attacker grabs this seed, they can predict future "random" values, such as one-time verification codes used during password resets.
This opens up the possibility for replay attacks: attackers could use stolen or predicted codes to hijack accounts through the password reset functionality. Fortunately, users with Multi-Factor Authentication (MFA) *enabled*, or those not using local authentication, are *not* vulnerable. Unfortunately, there's no workaround—you need to upgrade.
Here’s a step-by-step breakdown of what went wrong, how the exploit works, and how to protect yourself.
What Is JumpServer?
JumpServer acts as a centralized gateway for managing and auditing access to servers, databases, and cloud systems. Organizations often use it to control who logs in where, making it a critical security tool. So, vulnerabilities here can have a big impact.
Understanding the Vulnerability: Predictable Verification Codes
When a password reset is requested in JumpServer, the user gets a verification code—usually sent via email or SMS—to prove their identity. To generate this code, JumpServer uses a pseudo-random number generator (PRNG), which should be seeded from a source the attacker can't guess.
In affected JumpServer versions, the API exposes the random seed used for these codes, either deliberately or by accident.
A Hypothetical Exploit in Python
Let’s see how an attacker might use this, in theory. Here’s what the vulnerable reset code path might look like:
(Example — simplified, insecure pattern for demonstration only!)
# Vulnerable code (pseudo-Python)
from random import Random
# Seed is exposed via some API endpoint
seed = get_seed_from_api()
rng = Random(seed)
# Server uses this to create verification code
verification_code = rng.randint(100000, 999999)
# Sends code to user (email, etc.)
send_code_email(user_email, verification_code)
Attacker's side
from random import Random
# 1. Fetch the seed from the exposed API
seed = fetch_seed()
# 2. Replay the same 'random' code
rng = Random(seed)
predicted_code = rng.randint(100000, 999999)
# 3. Use the predicted code in reset flow
submit_verification_code(predicted_code)
This is essentially a replay attack—an attacker leverages knowledge of the random seed to predict the output and bypass security.
Are You Affected?
- YES if… you use JumpServer local authentication and have *not* enabled Multi-Factor Authentication, and are running a version older than 2.28.19 or 3.6.5.
- NO if… you use MFA, or use LDAP/OAuth/other external authentication.
Download links
- JumpServer releases on GitHub
References
- Original Security Advisory
- CVE Entry at NVD
- JumpServer Project
Final Thoughts
Randomness is hard to get right in security applications. Exposing the random seed—even by mistake—breaks the entire premise of unpredictability, letting attackers easily defeat defenses like email verification codes.
If you run JumpServer, check your version right now. An easy upgrade could save your infrastructure from compromise.
Timeline
Published on: 09/27/2023 15:19:00 UTC
Last modified on: 09/29/2023 15:04:00 UTC