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HomeResourcesAdvisoriesA Vulnerability in Exim Mailer Could Allow for Arbitrary Code Execution

A Vulnerability in Exim Mailer Could Allow for Arbitrary Code Execution

MS-ISAC ADVISORY NUMBER:

2017-118

DATE(S) ISSUED:

11/30/2017

OVERVIEW:

A vulnerability has been discovered in Exim Mailer, which could allow for arbitrary code execution. Exim Mailer is a message transfer agent on Unix systems that connects to the Internet. Successfully exploiting this vulnerability could allow for arbitrary code execution in the context of the affected application. Depending on the privileges associated with the application, an attacker could install programs; view, change, or delete data; or create new accounts with full user rights. Failed exploitation could result in a denial-of-service condition.

THREAT INTELLIGENCE:

Proof-of-Concept code for this vulnerability is available. However, there are no reports of this vulnerability being exploited in the wild.

SYSTEMS AFFECTED:

  • Exim Mailer 4.89 and prior

RISK:

Government:
  • Large and medium government entities: HIGH
  • Small government entities: MEDIUM
Businesses:
  • Large and medium business entities: HIGH
  • Small business entities: MEDIUM
Home Users:
N/A

TECHNICAL SUMMARY:

A vulnerability has been discovered in Exim Mailer, which could allow for arbitrary code execution. This vulnerability exists due to a use-after-free bug, which could be exploited by an attacker to remotely execute arbitrary code on the SMTP server by crafting a sequence of BDAT commands (CVE-2017-16943).

Successfully exploiting this vulnerability could allow for arbitrary code execution in the context of the affected application. Depending on the privileges associated with the application, an attacker could install programs; view, change, or delete data; or create new accounts with full user rights. Failed exploitation could result in a denial-of-service condition.

RECOMENDATIONS:

We recommend the following actions be taken:

  • Upgrade to the latest version of Exim Mailer immediately, after appropriate testing.
  • Verify no unauthorized system modifications have occurred on system before applying patch.
  • Apply the principle of Least Privilege to all systems and services.
  • Remind users not to open emails, download attachments, or follow links provided by unknown or untrusted sources.

REFERENCES:

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