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CVE-2016-0777 is a prominent client-side information disclosure vulnerability affecting legacy OpenSSH versions 5.x, 6.x, and 7.x prior to version 7.1p2.
The underlying flaw originates within the experimental and poorly bounded client-side roaming feature enabled by default in many distributions.
Specifically, the vulnerability resides inside the `resend_bytes` function located in the source file roaming_common.c.
When a vulnerable OpenSSH client establishes a connection with a remote host, the roaming mechanism handles reconnection logic.
If the connection drops, the client and server attempt to resume the encrypted session state seamlessly.
However, a malicious or compromised remote server can abuse this protocol exchange by sending crafted network packets.
These crafted packets trick the client into believing a roaming reconnection is requested and negotiated.
In response, the vulnerable client executes code that commands it to resend bytes from its internal communication buffers.
Because the codebase lacks proper input validation and strict bounds checking on these memory buffers, it reads arbitrary heap data.
The client then unwittingly transmits chunks of active process memory back to the untrusted remote server over the network.
Security analysis revealed that this leaked process memory frequently contains highly sensitive cryptographic material.
Most critically, private SSH keys loaded into memory or used during authentication can be completely extracted by the rogue server.
An attacker who captures a private key can subsequently impersonate the user and access any other systems trusted by that key.
Because users inherently trust outbound administrative connections, interacting with a rogue server exposed local confidentiality entirely.
The vulnerability was discovered and disclosed by cybersecurity researchers at Qualys in January 2016.
Immediate remediation required upgrading the OpenSSH client software or explicitly disabling the vulnerable roaming configuration parameter.
DailyCVE Form:
Platform: OpenSSH client
Version: OpenSSH 5.x
Vulnerability: Information Disclosure
Severity: Medium Risk
date: January 2016
Prediction: January 2016
What Undercode Say:
To inspect your installed client version and verify exposure to legacy OpenSSH vulnerabilities, use the following bash command:
ssh -V
To permanently disable the vulnerable client roaming feature globally on your machine, append the configuration directive using this command:
echo "UseRoaming no" | sudo tee -a /etc/ssh/ssh_config
For user-specific configurations without root privileges, you can add the setting directly to your local user profile config file:
echo "UseRoaming no" >> ~/.ssh/config
Exploit: (Educational Purposes!)
An attacker sets up a rogue or modified SSH server instance to target incoming client connections.
When a vulnerable OpenSSH client (versions 5.x through 7.x < 7.1p2) connects:
1. The malicious server initiates an experimental roaming negotiation sequence during the session handshake.
2. The client executes `resend_bytes` in `roaming_common.c` without validating requested buffer lengths.
3. Raw heap memory blocks containing active session tokens and private keys are bundled into network responses.
4. The attacker captures the packet stream and parses the memory dump to extract sensitive cryptographic credentials.
Protection: from this CVE
Upgrade the OpenSSH client package to version 7.1p2 or any subsequent patched release.
Add `UseRoaming no` to your global or user-level SSH configuration files to disable roaming entirely.
Avoid initiating outbound SSH connections from administrative endpoints to untrusted or unknown external servers.
Implement network scanning tools like RedMind to continuously monitor and flag outdated legacy SSH binaries on your network.
Impact:
Direct exposure of sensitive client process heap memory to untrusted remote servers.
Unintended leakage of private cryptographic keys, passwords, and authentication tokens.
Complete compromise of secondary infrastructure trusted by the leaked private keys.
Total loss of session confidentiality for administrative workflows originating from vulnerable client machines.
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Sources:
Reported By: github.com
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