CVE-2021-28041: OpenSSH Double Free Vulnerability

CVE-2021-28041 Overview

CVE-2021-28041 is a double free vulnerability affecting the ssh-agent component in OpenSSH versions prior to 8.5. This memory corruption flaw can be triggered in specific scenarios, including unconstrained agent-socket access on legacy operating systems or when an agent is forwarded to an attacker-controlled host. While exploitation requires specific conditions to be met, successful attacks could lead to code execution or denial of service on affected systems.

Critical Impact
Double free vulnerabilities in ssh-agent may allow attackers to corrupt memory, potentially leading to arbitrary code execution or system compromise when agent forwarding is enabled to untrusted hosts.

Affected Products

OpenBSD OpenSSH (versions before 8.5)
Fedora 33 and 34
NetApp Cloud Backup, HCI Management Node, SolidFire
NetApp HCI Compute Node and HCI Storage Node (firmware included)
Oracle Communications Offline Mediation Controller 12.0.0.3.0
Oracle ZFS Storage Appliance 8.8

Discovery Timeline

2021-03-03 - Vulnerability disclosed on Openwall OSS-Security Mailing List
2021-03-05 - CVE CVE-2021-28041 published to NVD
2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-28041

Vulnerability Analysis

This vulnerability is classified as CWE-415 (Double Free), a type of memory corruption issue that occurs when the same memory location is freed twice. In the context of ssh-agent, the double free condition can be triggered during specific agent operations, particularly when handling agent-socket access or during agent forwarding scenarios.

The vulnerability requires certain preconditions to be exploitable, making it relevant primarily in less-common deployment scenarios. These include systems with unconstrained agent-socket access, legacy operating systems with weaker memory protection, or situations where SSH agent forwarding is configured to connect to attacker-controlled hosts.

Root Cause

The root cause of CVE-2021-28041 lies in improper memory management within the ssh-agent component. Specifically, the code path fails to properly track the state of allocated memory, leading to a scenario where free() is called twice on the same memory pointer. This can corrupt the heap metadata used by the memory allocator, creating opportunities for exploitation.

Double free vulnerabilities are particularly dangerous because they can be leveraged to achieve arbitrary write primitives in memory, which attackers can then use to hijack program execution flow or modify critical data structures.

Attack Vector

Exploitation of this vulnerability requires network access with low privileges, but also depends on user interaction and specific deployment configurations. The attack vector typically involves:

Agent Forwarding to Malicious Host: An attacker controlling an SSH server could trigger the double free when a victim connects with agent forwarding enabled (-A flag or ForwardAgent yes).
Unconstrained Agent Socket Access: On systems where the agent socket (SSH_AUTH_SOCK) is accessible to other users or processes, an attacker could craft malicious requests to trigger the vulnerability.
Legacy Operating System Exploitation: Older operating systems lacking modern memory protection mechanisms (ASLR, heap hardening) are more susceptible to successful exploitation of the memory corruption.

The vulnerability mechanism involves improper handling of agent protocol messages that results in the same memory region being deallocated multiple times. Refer to the GitHub commit for the specific code changes that address this issue.

Detection Methods for CVE-2021-28041

Indicators of Compromise

Unexpected ssh-agent crashes or core dumps on systems running OpenSSH versions prior to 8.5
Anomalous memory allocation patterns in ssh-agent processes
Suspicious SSH agent forwarding activity to unknown or untrusted hosts
Evidence of heap corruption in ssh-agent memory space

Detection Strategies

Monitor for ssh-agent process crashes or abnormal terminations using system monitoring tools
Implement version scanning to identify OpenSSH installations running versions below 8.5
Review SSH configurations for agent forwarding enabled to external or untrusted hosts
Deploy endpoint detection rules to identify exploitation attempts targeting ssh-agent

Monitoring Recommendations

Enable audit logging for SSH connections, particularly those using agent forwarding
Monitor system logs for ssh-agent segmentation faults or memory-related errors
Track outbound SSH connections to identify potentially malicious agent forwarding destinations
Implement network monitoring for SSH traffic patterns indicative of agent protocol abuse

How to Mitigate CVE-2021-28041

Immediate Actions Required

Upgrade OpenSSH to version 8.5 or later on all affected systems
Disable SSH agent forwarding unless absolutely required (ForwardAgent no in SSH config)
Review and restrict access to the ssh-agent socket (SSH_AUTH_SOCK)
Audit SSH configurations to identify systems using agent forwarding to untrusted hosts

Patch Information

OpenSSH version 8.5 addresses this vulnerability with fixes to the memory management in ssh-agent. The specific fix can be reviewed in GitHub commit e04fd6dde16de1cdc5a4d9946397ff60d96568db. Organizations should prioritize upgrading to OpenSSH 8.5 or later.

For systems using distribution-packaged OpenSSH, consult vendor advisories:

Workarounds

Disable agent forwarding globally by setting ForwardAgent no in /etc/ssh/ssh_config
Use SSH's -a flag to explicitly disable agent forwarding for individual connections
Restrict agent socket permissions to prevent unauthorized access
Consider using ProxyJump (-J) instead of agent forwarding for multi-hop SSH connections

bash

# Disable agent forwarding system-wide
echo "ForwardAgent no" >> /etc/ssh/ssh_config

# Verify OpenSSH version
ssh -V

# Restrict agent socket permissions (example)
chmod 600 $SSH_AUTH_SOCK