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Multiple flaws in the Exim mail transfer agent include CVE-2023-42115, a critical unauthenticated remote-code-execution vulnerability rated CVSS 9.8. The out-of-bounds write affects Exim's SMTP service when the EXTERNAL authentication scheme is configured and available, potentially enabling an unauthenticated remote attacker to execute code as the Exim service account.
Exim versions 4.96.1 and 4.97 address CVE-2023-42115 and the related CVE-2023-42114 and CVE-2023-42116 vulnerabilities. Organizations should upgrade promptly; where immediate patching is not possible, they should disable exposed authentication features and limit remote SMTP access. Other disclosed issues depend on configurations such as SPA/NTLM authentication, an untrusted Proxy Protocol proxy, SPF ACL use, or reliance on an untrusted DNS resolver.
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2 events from the most recent confirmed update back to the earliest known activity.
Exim versions 4.96.1 and 4.97 fixed CVE-2023-42114, CVE-2023-42115, and CVE-2023-42116. At the time described, CVE-2023-42117 and CVE-2023-42118 had no available fixes and were assumed to affect all Exim versions.
The Zero Day Initiative publicly disclosed multiple Exim MTA vulnerabilities, including critical unauthenticated RCE flaw CVE-2023-42115. The issue is an out-of-bounds write reachable when EXTERNAL authentication is configured and can allow code execution as the Exim service account.
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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out-of-bounds reads and writes, and possible heap data leakage, with older Exim branches potentially affected but no longer actively maintained.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 disclosed and fixed **CVE-2026-48840**, a pre-authentication information disclosure flaw in Exim’s PROXY protocol handling that affects versions **4.88 through 4.99.3** and is fixed in **4.99.4**. The bug is triggered during SMTP `HELO`/greeting processing when Exim parses a specially crafted **PROXYv2** header with insufficient length, causing uninitialized stack data to be copied into an IP address string and exposed in the SMTP banner. Advisories say the leak can disclose **16 bytes** on `TCPv6` or **4 bytes** on `TCPv4`, including userspace pointers that could help attackers bypass **ASLR** and support further exploitation. The issue affects deployments built with **`SUPPORT_PROXY`** and configured with a non-empty **`hosts_proxy`** value, a combination reported as enabled by default in some downstream builds including Debian, Ubuntu, RHEL EPEL, and Fedora. Exim said exposure is highest where attackers can reach the service from trusted proxy IPs or through a trusted proxy path, and recommended upgrading to **4.99.4**, restricting **`hosts_proxy`** to exact load balancer addresses, or disabling PROXY protocol parsing entirely by unsetting that option. Related downstream advisories were also issued by **cPanel** and **Debian** for packaged Exim deployments.
Jun 29, 2026