NSA warns about Sandworm APT exploiting Exim flaw
Vulnerabilities mentionedAll →
| CVE | Vulnerability | CVSS | EPSS | Flags | Affected | Exposure | Published |
|---|---|---|---|---|---|---|---|
| CVE-2019-10149 | Unvalidated Email Recipient Enables Remote Command Execution in Exim MTA CVE-2019-10149 is an improper input validation flaw (CWE-78, OS command injection) in the deliver_message() function in /src/deliver.c of the Exim mail transfer agent, where email recipient addresses are not safely validated before use. A remote attacker triggers it by sending mail whose recipient address contains crafted Exim string-expansion syntax (such as ${run{...}}), causing Exim to expand and execute attacker-supplied operating-system commands during recipient verification or delivery. An attacker gains arbitrary remote command execution on the mail server, commonly with elevated privileges, enabling full system compromise, data theft, ransomware deployment (e.g., the Trinity ransomware campaign), or installation of botnet implants. Any organization running an internet-exposed Exim MTA in the affected version range is at risk, and Exim is widely deployed as the default mail server on many Linux distributions. Exploitation is confirmed in the wild: mass scanning and compromise campaigns began shortly after the June 2019 disclosure, CISA added the flaw to its Known Exploited Vulnerabilities catalog on 2022-01-10, and its EPSS score of 100% (100th percentile) reflects near-certain near-term exploitation risk. Do: Upgrade Exim to 4.92 or later, or install your distribution's patched Exim package, as required by CISA's KEV action ('apply updates per vendor instructions'); if patching must be delayed, restrict internet-facing SMTP access and tighten recipient-verification configuration. Check mail logs for suspicious MAIL FROM/RCPT TO addresses containing expansion syntax such as ${run{...}} or shell-like commands, and audit patched servers for post-compromise artifacts such as unusual cron jobs, downloaded binaries, or new authorized SSH keys. | 9.8 | 100% | KEV PoC ×5 |
| mass≈500,000+ internet-exposed Exim SMTP servers, with hundreds of thousands likely running vulnerable versions |
Full article564 words · extracted from helpnetsecurity.com · click to collapse
The Russian APT group Sandworm has been exploiting a critical Exim flaw (CVE-2019-10149) to compromise mail servers since August 2019, the NSA has warned in a security advisory published on Thursday.
“When CVE-2019-10149 is successfully exploited, an actor is able to execute code of their choosing. When Sandworm exploited CVE-2019-10149, the victim machine would subsequently download and execute a shell script from a Sandworm-controlled domain,” they said.
The script would then attempt to add privileged users, disable network security settings, update SSH configurations to enable additional remote access, and execute an additional script to enable follow-on exploitation.
About Exim and the flaw
Exim is a mail transfer agent (MTA) that is commonly used for Unix-based systems and comes pre-installed on some Linux distributions.
It is the most widely used MTA and is deployed on over half of all Internet-facing mail servers.
While it’s efficient and highly configurable, its widespread use makes it a common target for attackers, who are always on the lookout for vulnerabilities that can be exploited. And, in Q2 2019, there were a few, including the most critical one: CVE-2019-10149.
Its existence was disclosed in June 2019, after a patch was provided for the supported versions and for a few that are now supported anymore.
Soon after, attackers started exploiting it to compromise Linux servers and instal cryptocoin miners on them, and Microsoft warned about a Linux worm leveraging the flaw to target Azure virtual machines (VMs) running affected versions of Exim.
Mitigation and detection
The NSA has provided mitigation advice as well as indicators of compromise so that organizations can protect themselves and check whether they’ve been targeted by the Sandworm attackers (aka the BlackEnergy APT, aka Telebots), which have in the past been linked to cyber-espionage campaigns targeting NATO, the EU, the White House, a variety of US ICS operators, and Ukranian energy companies, organizations in the financial sector and news media companies.
First and foremost, admins are advised to update their Exim installations to the latest stable release (v4.93) to mitigate this and other vulnerabilities.
“Other vulnerabilities exist and are likely to be exploited, so the latest fully patched version should be used,” the NSA said, and advised system administrators to continually check software versions and update as new versions become available.
“The actors exploited victims using Exim software on their public facing MTAs by sending a command in the “MAIL FROM” field of an SMTP (Simple Mail Transfer Protocol) message,” they explained.
Admins and IT security employees can detect and/or block exploitation of the flaw by using specific Snort rules, check traffic logs for emails with a recipient containing “${run”, and routinely check for unauthorized system modifications.
The NSA provided IP addresses and domains that were associated with the Sandworm attacks and offered additional advice on how to apply multiple defensive layers to protect public facing software such as MTAs.
UPDATE (June 2, 2020, 2:30 a.m. PT):
Though the number of vulnerable Exim instances has been steadily falling due to organizations upgrading their Exim mail servers, it was still around 900,000 in May 2020.
“While the NSA Advisory only calls out CVE-2019-10149 by name, there were three critical vulnerabilities announced in 2019 involving remote code/command execution (RCE). All three were leveraged in this broader attack campaign,” RiskIQ noted.
They are all fixed in the latest stable latest Exim release (v4.93).
Text extracted automatically; images, tables and formatting may be missing. Original: https://www.helpnetsecurity.com/2020/05/29/sandworm-cve-2019-10149/