Exim has released security updates to address a severe security issue affecting certain configurations that could enable memory corruption and potential code execution.
Exim is an open-source Mail Transfer Agent (MTA) designed for Unix-like systems to receive, route, and deliver email.
The vulnerability, tracked as CVE-2026-45185 (CVSS score: 9.8), aka Dead.Letter, has been described as a use-after-free vulnerability in Exim's binary data transmission (BDAT) message body parsing when a TLS connection is handled by GnuTLS.
"The vulnerability is triggered during BDAT message body handling when a client sends a TLS close_notify alert before the body transfer is complete, and then follows up with a final byte in cleartext on the same TCP connection," Exim said in an advisory released today.
"This sequence of events can cause Exim to write into a memory buffer that has already been freed during the TLS session teardown, leading to heap corruption. An attacker only needs to be able to establish a TLS connection and use the CHUNKING (BDAT) SMTP extension."
The issue impacts all Exim versions from 4.97 up to and including 4.99.2. That said, it only affects builds that use USE_GNUTLS=yes, meaning builds that rely on other TLS libraries like OpenSSL are not impacted.
Federico Kirschbaum, head of Security Lab at XBOW, an autonomous cybersecurity testing platform, has been credited with discovering and reporting the flaw on May 1, 2026.
"During TLS shutdown, Exim frees its TLS transfer buffer – but a nested BDAT receive wrapper can still process incoming bytes and end up calling ungetc(), which writes a single character (\n) into the freed region," Kirschbaum said. "That one-byte write lands on Exim's allocator metadata, corrupting the allocator's internal shape; the exploit then leverages that corruption to gain further primitives."
XBOW described the vulnerability as "one of the highest-caliber bugs" discovered in Exim to date, adding that triggering it requires almost no special configuration on the server.
The shortcoming has been addressed in version 4.99.3. All users are advised to upgrade as soon as possible. There are no mitigations that resolve the vulnerability.
"The fix ensures that the input processing stack is cleanly reset when a TLS close notification is received during an active BDAT transfer, preventing the stale pointers from being used," Exim noted.
This is not the first time critical use-after-free bugs in Exim have been disclosed. In late 2017, Exim patched a use-after-free vulnerability in the SMTP daemon (CVE-2017-16943, CVSS score: 9.8) that unauthenticated attackers could have exploited to achieve remote code execution via specially crafted BDAT commands and seize control of the email server.
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