MEDIUM
An IMAP FETCH response line indicates the size of the returned data, in number of bytes. When that response says the data is zero bytes, libcurl would pass on that (non-existing) data with a pointer and the size (zero) to the deliver-data function. libcurl's deliver-data function treats zero as a magic number and invokes strlen() on the data to figure out the length. The strlen() is called on a heap based buffer that might not be zero terminated so libcurl might read beyond the end of it into whatever memory lies after (or just crash) and then deliver that to the application as if it was actually downloaded.
http://www.debian.org/security/2017/dsa-4007
http://www.securityfocus.com/bid/101519
http://www.securitytracker.com/id/1039644
https://access.redhat.com/errata/RHSA-2017:3263
https://access.redhat.com/errata/RHSA-2018:2486
https://access.redhat.com/errata/RHSA-2018:3558
Source: MITRE
Published: 2017-10-31
Updated: 2018-11-13
Type: CWE-119
Base Score: 6.4
Vector: AV:N/AC:L/Au:N/C:P/I:N/A:P
Impact Score: 4.9
Exploitability Score: 10
Severity: MEDIUM
Base Score: 9.1
Vector: CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:H
Impact Score: 5.2
Exploitability Score: 3.9
Severity: CRITICAL