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CVE-2026-45185 – Critical Unauthenticated RCE in Exim Mail Server

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Overview

A critical use-after-free vulnerability has been discovered in Exim, one of the most widely deployed mail transfer agents on the internet.

Technical Details

The flaw is triggered when a client sends a TLS close_notify mid-body during a CHUNKING (BDAT) transfer, followed by a final cleartext byte on the same TCP connection. This causes Exim to free its TLS transfer buffer while a nested BDAT receive wrapper continues processing incoming bytes — writing a single newline character into the freed region and corrupting the allocator’s internal metadata.

An unauthenticated remote attacker can exploit this heap corruption to achieve remote code execution on the mail server.

Impact

  • No authentication required
  • No user interaction required
  • Full system compromise (confidentiality, integrity, availability)
  • CVSS v3.1: AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Affected Versions

Exim < 4.99.3 when compiled with GnuTLS support (affects Exim 4.97+).

Remediation

Upgrade Exim to version 4.99.3 or later. If an immediate upgrade is not possible, consider restricting inbound SMTP access at the network level as a temporary mitigation.

References

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1

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