Project libssh2 Security Advisory, March 18 2019 - Permalink
A server could send a value approaching unsigned int max number of keyboard prompt requests which could result in an unchecked integer overflow. The value would then be used to allocate memory causing a possible memory write out of bounds error (CWE-130).
There are no known exploits of this flaw at this time.
The Common Vulnerabilities and Exposures (CVE) project has assigned the name CVE-2019-3856 to this issue.
libssh2 1.8.1 ensures keyboard prompt requests value is less than 100 before proceeding with the login process.
A patch for this problem is available
We suggest you take one of the following actions immediately, in order of preference:
A - Upgrade to libssh2 1.8.1 or later
B - Apply the patch and rebuild libssh2
It was first reported to the libssh2 project on Dec 3 2018 by Chris Coulson.
libssh2 1.8.1 was released on March 18 2019, coordinated with the publication of this advisory.
Reported by Chris Coulson of Canonical Ltd.