CVE-2023-25136: OpenBSD OpenSSH Double-Free RCE Vulnerability

CVE-2023-25136 Overview

CVE-2023-25136 is a double-free memory corruption vulnerability affecting OpenSSH server (sshd) version 9.1. The vulnerability was introduced in the options.kex_algorithms handling code, which improperly manages memory during key exchange algorithm processing. An unauthenticated remote attacker can exploit this vulnerability in the default configuration to potentially jump to any location in the sshd address space. While direct remote code execution remains theoretically possible according to third-party analysis, the primary confirmed impact is a denial of service condition through memory corruption.

Critical Impact

Unauthenticated remote attackers can trigger a double-free condition in the sshd process, potentially enabling arbitrary memory access and denial of service against SSH servers running OpenSSH 9.1.

Affected Products

• OpenBSD OpenSSH 9.1
• Fedora 37 and Fedora 38
• NetApp ONTAP Select Deploy Administration Utility
• NetApp A250 Firmware
• NetApp 500F Firmware
• NetApp C250 Firmware

Discovery Timeline

• 2023-02-03 - CVE-2023-25136 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2023-25136

Vulnerability Analysis

The vulnerability exists in the compat.c file of OpenSSH, specifically in the code responsible for filtering and handling key exchange algorithms. When processing the kex_algorithms option, the code fails to consistently return allocated strings from the filtering function. This inconsistency leads to a scenario where the same memory can be freed twice—a classic double-free condition.

Double-free vulnerabilities are particularly dangerous because they corrupt the memory allocator's internal data structures. When the same memory region is freed twice, it can be placed on the allocator's free list multiple times. Subsequent allocations may then return the same memory region to different parts of the program, leading to use-after-free conditions and potential arbitrary code execution.

The JFrog OpenSSH CVE-2023-25136 Analysis provides detailed technical analysis demonstrating how this vulnerability can be exploited in the pre-authentication phase, making it accessible to completely unauthenticated attackers.

Root Cause

The root cause lies in inconsistent memory management within the key exchange algorithm filtering code in compat.c. The filtering function did not always return newly allocated strings, leading to situations where the calling code would attempt to free memory that was either not dynamically allocated or had already been freed. The fix ensures that the function always returns allocated strings, providing consistent memory ownership semantics.

Attack Vector

This vulnerability is exploitable over the network without requiring authentication. An attacker can send specially crafted SSH connection requests that trigger the vulnerable code path during the key exchange negotiation phase. The attack occurs before authentication, meaning any system running an exposed OpenSSH 9.1 server is potentially vulnerable. The attacker can leverage the double-free condition to corrupt heap metadata and potentially redirect execution flow.

-/* $OpenBSD: compat.c,v 1.119 2021/09/10 05:46:09 djm Exp $ */
+/* $OpenBSD: compat.c,v 1.120 2022/07/01 03:35:45 dtucker Exp $ */
 /*
  * Copyright (c) 1999, 2000, 2001, 2002 Markus Friedl.  All rights reserved.
  *

Source: GitHub OpenSSH Commit

The patch modifies the compat.c file to ensure consistent memory allocation behavior in the kex filtering functions, eliminating the double-free condition.

Detection Methods for CVE-2023-25136

Indicators of Compromise

• Unexpected sshd process crashes or restarts in system logs
• Core dumps from sshd processes indicating heap corruption
• Anomalous SSH connection attempts with malformed key exchange parameters
• Memory corruption signatures in sshd process address space

Detection Strategies

• Monitor SSH server logs for repeated connection failures during key exchange phase
• Implement intrusion detection rules to identify unusual SSH handshake patterns
• Deploy memory corruption detection tools on systems running vulnerable OpenSSH versions
• Use version scanning to identify systems running OpenSSH 9.1

Monitoring Recommendations

• Enable verbose logging on SSH servers to capture detailed connection information
• Configure alerting for sshd service crashes or unexpected restarts
• Monitor for high volumes of failed SSH connection attempts from single sources
• Review system audit logs for signs of exploitation attempts

How to Mitigate CVE-2023-25136

Immediate Actions Required

• Upgrade OpenSSH to version 9.2 or later immediately
• Apply the official OpenBSD patch for systems running OpenBSD 7.2
• Restrict SSH access to trusted IP ranges using firewall rules
• Consider temporarily disabling SSH access on critical systems until patching is complete

Patch Information

OpenSSH 9.2 contains the fix for this vulnerability. The official patch is available from OpenBSD Patch for SSHD. The specific commit addressing this issue is available at GitHub OpenSSH Commit. Fedora users should apply updates through their package manager as documented in the Fedora Package Announcement. NetApp users should consult the NetApp Security Advisory for product-specific guidance.

Workarounds

• Implement network-level access controls to limit SSH exposure to trusted networks only
• Use a bastion host or jump server to reduce direct SSH exposure
• Deploy intrusion prevention systems (IPS) with signatures for SSH exploitation attempts
• Consider using alternative remote access methods temporarily while patching is scheduled

# Restrict SSH access to trusted networks using iptables
iptables -A INPUT -p tcp --dport 22 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p tcp --dport 22 -j DROP

# Verify OpenSSH version
ssh -V

# Check for available updates (Debian/Ubuntu)
apt-cache policy openssh-server

# Apply updates (Debian/Ubuntu)
apt update && apt upgrade openssh-server