In plain words
Imagine you have a very strong lock on your door, but someone can make a key that takes hours to try, so you cannot even open your own door because the lock is busy trying the other key. This is similar to what happens with the LDAP server when it tries to check a password that is made to be very hard to check.
What's broken
The bug is in the Apache Directory LDAP API¹, specifically in how it handles stored passwords that use the bcrypt² algorithm. The problem is that an attacker can create a password that has a very high cost factor, like 30, which means the server will spend a lot of time trying to verify it. This is known as an Asymmetric Resource Consumption vulnerability³. The CVE id for this issue is CVE-2026-103880⁴.
How it actually works
Here's how it works: when you store a password using bcrypt, you can set how much work the server has to do to check the password. Normally, this is set to a reasonable level so the server can quickly check passwords. But if an attacker sets this level very high, the server will spend a lot of time and resources checking the password, which can be used to overload the server in a denial of service attack⁵. This happens because the server is designed to try all passwords, even if they are very hard to check, which allows an attacker to cause trouble without even needing to know the password.
Proof of concept
There is a public discussion of this issue, which serves as a proof of concept⁶ that the flaw is real and can be exploited. This means that the vulnerability is not just theoretical, but can actually be used by attackers. The fact that there is a public discussion of the issue increases the urgency of fixing it, because it makes it more likely that attackers will find and exploit the vulnerability.
Is it real
It is not clear from the source if this issue is being actively exploited in the wild. However, the affected versions are Apache Directory LDAP API 2.1.0 before 2.1.9, which means that anyone using these versions should update to a fixed version to avoid the issue. The fact that the issue can be fixed by updating the software limits the impact, but it is still important to update as soon as possible.
What to do
To fix this issue, you should update your Apache Directory LDAP API to version 2.1.9 or later. This will prevent an attacker from overloading your server with passwords that have a high cost factor. Additionally, you can enforce a bounded cost for bcrypt to avoid this kind of denial of service attack.
Terms
- ¹ Apache Directory LDAP API
- , A set of libraries used to interact with LDAP directories
- ² bcrypt
- , A password hashing algorithm that is designed to be slow and computationally expensive
- ³ Asymmetric Resource Consumption vulnerability
- , A type of vulnerability where an attacker can cause a system to consume too many resources, leading to a denial of service
- ⁴ CVE-2026-103880
- , A unique identifier for a security vulnerability
- ⁵ denial of service attack
- , A type of attack where an attacker overloads a system with requests in order to make it unavailable
- ⁶ proof of concept
- , A demonstration that a security vulnerability is real and can be exploited