One of the vulnerabilities that Microsoft addressed on June 2020 Patch Tuesday is a Server Message Block (SMB) protocol bug that could allow an attacker to leak kernel memory remotely, without authentication.
Called SMBleed and tracked as CVE-2020-1206, the vulnerability could be chained with SMBGhost (CVE-2020-0796), a flaw addressed in March 2020, to achieve pre-authentication remote code execution, security researchers with ZecOps reveal.
The same as SMBGhost, SMBleed is an issue that resides in the compression mechanism of SMBv3.1.1 and affects the manner in which the protocol handles certain requests. Microsoft has addressed the bug by correcting the way in which those requests are handled.
“To exploit the vulnerability against a server, an unauthenticated attacker could send a specially crafted packet to a targeted SMBv3 server. To exploit the vulnerability against a client, an unauthenticated attacker would need to configure a malicious SMBv3 server and convince a user to connect to it,” Microsoft notes in an advisory.
SMBleed impacts Windows 10 and Windows Server, versions 1903, 1909 and 2004 (but not previous versions).
While no mitigating factors have been identified, Microsoft has detailed workarounds that may help defend against exploitation, such as disabling SMBv3 compression. However, the tech giant recommends installing the available patches.
ZecOps’ researchers, who discovered both SMBGhost and SMBleed, already published proof-of-concept (POC) code for exploiting the newly disclosed bug, but explain that credentials and a writable share are required for the POC to work.
However, they also note that it is possible to exploit the vulnerability without authentication, and that they used it with the SMBGhost bug to achieve RCE (Remote Code Execution). They released a POC for this scenario as well, and plan on publishing technical details soon.
The researchers note that users can remediate both SMBleed and SMBGhost by blocking TCP port 445 (which is used to initiate a connection with the affected component), by enforcing host isolation, and by disabling SMB 3.1.1 compression (although they do not recommend this solution).
Installing the available patches remains the recommended solution for the issue, as it would completely remove the vulnerability, ZecOps notes.