libssh2 SSH_MSG_CHANNEL_REQUEST Packet Handling Integer Overflow Vulnerability [CVE-2019-3857]

CVE Number – CVE-2019-3857

A vulnerability in libssh2 could allow an unauthenticated, remote attacker to execute arbitrary code or cause a denial of service (DoS) condition on a targeted system.

The vulnerability is due to improper parsing of SSH_MSG_CHANNEL_REQUEST packets with an exit signal message with a length of max unsigned integer value by the affected software, which could trigger an integer overflow condition. An attacker could exploit this vulnerability by persuading a user to connect to an attacker-controlled SSH server, which would allow the attacker to send SSH_MSG_CHANNEL_REQUEST packets that submit malicious input to the targeted system. A successful exploit could trigger an integer overflow condition, which could lead to an out-of-bounds write condition and zero-byte allocation that the attacker could use to execute arbitrary code or cause a DoS condition. 

libssh2 has confirmed the vulnerability and released software updates.


  • To exploit this vulnerability, an attacker may use misleading language or instructions to persuade a user to connect to a malicious SSH server in addition to sending packets that submits malicious input to the system. These requirements may reduce the likelihood of a successful exploit.


  • Administrators are advised to apply the appropriate updates.

    Administrators are advised to allow only trusted users to have network access.

    Administrators are advised to use an unprivileged account when browsing the Internet.

    Administrators are advised to monitor critical systems.

    Users are advised not to open email messages from suspicious or unrecognized sources. If users cannot verify that links or attachments included in email messages are safe, they are advised not to open them.

    Users are advised not to visit websites or follow links that have suspicious characteristics or cannot be verified as safe.

Vendor Announcements

  • libssh2 has released a security advisory at the following link: CVE-2019-3857

Fixed Software

  • libssh2 has released software updates at the following link: libssh2 1.8.1

Duncan Newell

Duncan is a technology professional with over 20 years experience of working in various IT roles. He has a interest in cyber security, and has a wide range of other skills in radio, electronics and telecommunications.

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