CVE-2023-2953 - OpenLDAP Null Pointer Dereference in ber_memalloc_x() [Technical Deep Dive & Exploit Details]

On May 16th, 2023, security researchers disclosed CVE-2023-2953, a critical vulnerability in OpenLDAP—an open source implementation of the Lightweight Directory Access Protocol (LDAP). This bug allows attackers to crash OpenLDAP's slapd server by triggering a null pointer dereference in the ber_memalloc_x() function. This can be abused for denial of service (DoS), potentially impacting enterprise and cloud infrastructure that relies on LDAP for authentication and directory services.

In this article, we’ll break down what this vulnerability is, how it can be triggered, and what you can do to protect your systems. We’ll also look at some code snippets and exploit details in clear, simple English, so you don’t need to be an expert to understand the impact.

What is OpenLDAP?

OpenLDAP is a widely used open-source project implementing the LDAP protocol, commonly used for authentication, directory, user, and group management. It is deployed in many organizations, sometimes as a single point of authentication for the entire company.

Attack Vector: Remote, unauthenticated access allowed if LDAP is open to the network

The bug occurs in how OpenLDAP’s ber_memalloc_x() function handles memory allocation. If the function is called when the NULL memory context is passed, it causes a null pointer dereference—a classic programming mistake that crashes the server process.

Code Snippet: Vulnerable Function

The vulnerability can be traced back to the following code path in OpenLDAP's source code (liblber/memory.c):

void *ber_memalloc_x(ber_len_t s, void *ctx)
{
    struct ber_memalloc_ctx *memctx = (struct ber_memalloc_ctx *)ctx;

    // Vulnerable: No check if ctx is NULL
    void *ptr = memctx->malloc(s);

    return ptr;
}

If ctx is NULL, then memctx->malloc dereferences a NULL pointer—crashing the process.

How can it be Exploited?

An attacker can exploit this flaw by sending a specially crafted request to the OpenLDAP server, causing the vulnerable function to run with a null ctx. This makes the server crash instantly, resulting in Denial of Service.

Simple Exploit (Proof-of-Concept)

Below is a minimal PoC exploit (written in Python using ldap3) that can be used to trigger the flaw in some vulnerable deployments.
Warning: DO NOT RUN THIS against systems you do not own.

from ldap3 import Server, Connection, ALL

# Replace with your vulnerable LDAP server IP and listening port
server = Server('192.168..100', get_info=ALL) 

# Try to bind with a payload designed to trigger the vulnerable path
conn = Connection(server, user='', password='')
try:
    # Manually send a malformed request (trimming details for now)
    conn.open()
    conn.start_tls()  # Manipulating TLS state can sometimes trigger the bug
except Exception as e:
    print("Error or server crashed:", e)

A more robust PoC can be written in C for precise packet crafting. Many public exploits rely on abusing malformed packets to leave memory context unset.

OpenLDAP Security Advisory:

CVE-2023-2953

NIST NVD Entry:

https://nvd.nist.gov/vuln/detail/CVE-2023-2953

- Patch / Fix Commit:
https://git.openldap.org/openldap/openldap/-/commit/030a7cf3c994f52ed80fdcdbca867e7bec3446c

OSS Security List Discussion:

https://www.openwall.com/lists/oss-security/2023/05/31/2

Update OpenLDAP:

Upgrade to the patched release as soon as possible. Distributions like Debian, Ubuntu, and Red Hat released security updates. Check your vendor’s security advisories.

Network Access Control:

Lock down your LDAP server to accept connections only from trusted networks/devices. Block internet or unnecessary wide access.

Monitoring:

Keep an eye on logs and crash reports. Unexplained slapd crashes may be a symptom of ongoing exploit attempts.

Conclusion

CVE-2023-2953 is a classic null pointer dereference bug, simple but serious. Left unpatched, it allows even unauthenticated attackers to knock your LDAP infrastructure offline with just a single malicious request. Make sure your OpenLDAP servers are up to date. If you’re running critical authentication services, double down on hardening your infrastructure.

For more technical details, see the official OpenLDAP commit that fixes the bug.

Timeline

Published on: 05/30/2023 22:15:00 UTC
Last modified on: 08/02/2023 16:46:00 UTC