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Exim released version 4.100.1 to remediate four security vulnerabilities in its mail transfer agent, including an SMTP smuggling flaw that can cause submitted email content to differ from the message delivered and logged by the originating system. The update also addresses a conditional GnuTLS TLS-on-connect use-after-free issue that could crash a mail reception process.
Two high-severity flaws in Exim's Proxy Protocol handling can cause heap corruption or expose server-stack data when Proxy Protocol is enabled and a configured proxy is faulty, buggy, or compromised. Some affected code dates to Exim 4.83; administrators should upgrade internet-facing servers to 4.100.1, validate signed release packages, and disable tls_early_banner_hosts as a temporary mitigation where immediate patching is not possible.
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Exim released version 4.100.1, remediating four vulnerabilities: two high-severity Proxy Protocol flaws, a GnuTLS TLS-on-connect use-after-free issue, and an SMTP smuggling flaw. The release fixes issues affecting versions as early as 4.83, including flaws reported by watchTowr researcher McCaulay Hudson.
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Exim has released **version 4.99.2** as a security update to fix four vulnerabilities in the mail transfer agent, tracked as `CVE-2026-40684`, `CVE-2026-40685`, `CVE-2026-40686`, and `CVE-2026-40687`. The flaws affect specific configurations and stem from unsafe handling of DNS PTR data, corrupt JSON in headers, malformed UTF-8 in headers, and the SPA authenticator when interacting with a hostile or compromised external SPA/NTLM service. Reported impacts include crashes, heap corruption, out-of-bounds reads and writes, and possible heap data leakage, with older Exim branches potentially affected but no longer actively maintained. One of the disclosed issues, `CVE-2026-40684`, can crash an Exim connection on systems using **musl libc** when malformed DNS PTR record data is processed; the same behavior was not reported on glibc systems. Exim said the bug is linked to an octal-printing oddity in musl libc and noted that the issue was raised with the musl maintainer. The fixes were published through Exim's security advisory process and made available in source tarballs and the project's Git repository under the `exim-4.99.2` tag.
Jun 29, 2026
Exim maintainers released **Exim 4.99.3** to fix a critical remotely reachable use-after-free flaw in the SMTP server’s **BDAT** body parsing path, tracked as **`CVE-2026-45185`** and referred to as **Dead.Letter**. The vulnerability affects **Exim 4.97 through 4.99.2/4.99.x** when built with **GnuTLS** support and configured to advertise both **`STARTTLS`** and **`CHUNKING`**, and successful exploitation can cause heap corruption with potential remote code execution. Maintainers said Linux distributions received coordinated access to patches ahead of disclosure and urged users to upgrade immediately. According to the advisory and follow-on reporting, the bug is triggered when a client sends a TLS **`close_notify`** before a BDAT transfer finishes and then sends a final cleartext byte over the same TCP connection, leaving Exim in an unsafe state. Exim said there are no effective mitigations short of upgrading, and fixed the issue in **4.99.3** by resetting the input-processing stack when TLS shutdown occurs during an active BDAT transfer. The flaw was reported by **Federico Kirschbaum of XBOW security**, with fuller technical details to be published under **`EXIM-Security-2026-05-01.1`**.
Jun 29, 2026
Exim maintainers released **Exim 4.99.5** to fix two local privilege-escalation vulnerabilities affecting multiple prior versions of the mail transfer agent. According to the security advisory, `EXIM-Security-2026-06-22.1` is a local directory traversal issue affecting Exim **4.88 through 4.99.4** that can let a local user access files outside the spool directory through queue-name transfer command-line arguments, while `EXIM-Security-2026-06-22.3` is a command-execution flaw affecting **4.82 through 4.99.4** in certain `.forward` and pipe transport configurations. The project said there is **no mitigation** for the directory traversal bug short of upgrading, and that the command-execution issue can be reduced by avoiding `force_command` on pipe transports. Release listings and distribution archives show the patched version has been published through Exim's official release channels, and administrators are being urged to update to **4.99.5** to prevent local file access and privilege-escalation risks on affected systems.
Jul 25, 2026