Ubuntu Fixes Exim Regression on 22.04 LTS

Intro

Ubuntu has issued USN-6455-2 to correct an Exim regression on Ubuntu 22.04 LTS. According to the notice, the earlier advisory USN-6455-1 addressed Exim vulnerabilities, but the fix for CVE-2023-42117 introduced a regression that caused certain connections to log a Taint mismatch error.

This follow-up update is intended to resolve that issue while keeping the prior security protections in place.

Why it matters

Exim is a core mail transfer agent in many Linux environments, so even a narrow regression can create operational friction quickly. In this case, the issue was not presented as a newly disclosed vulnerability by itself, but as a stability and functionality problem introduced during a prior security fix.

That matters for two reasons:

• Security updates need validation after deployment. Even necessary fixes can create unexpected behavior in production services.
• Mail infrastructure is sensitive to regressions. Connection errors and unusual log messages can affect message handling, troubleshooting, and confidence in service health.

The original advisory context also remains important. Ubuntu notes that Exim previously had flaws involving improper validation of user-supplied data, including:

• CVE-2023-42117, which could lead to memory corruption and possible arbitrary code execution by a remote attacker
• CVE-2023-42119, which could lead to an out-of-bounds read and possible exposure of sensitive information

USN-6455-2 specifically addresses the regression introduced while fixing that earlier risk.

Who should care

This alert is most relevant to:

• Administrators running Exim on Ubuntu 22.04 LTS
• Email and messaging platform owners responsible for mail availability
• Security and patch management teams tracking post-update regressions
• SOC and operations teams reviewing unexplained Exim log anomalies

If your team applied the earlier Exim update and then noticed Taint mismatch errors or unexpected mail-related behavior, this notice deserves immediate attention.

Practical response

Defenders should take a straightforward, operations-focused approach:

1. Review affected systems and confirm whether Exim is deployed on Ubuntu 22.04 LTS.
2. Apply the updated Ubuntu package referenced in USN-6455-2 through normal patch management processes.
3. Check Exim logs for prior or recurring Taint mismatch errors tied to affected connections.
4. Validate mail flow after updating, including inbound, outbound, and relay scenarios relevant to your environment.
5. Document the regression and remediation so future patch reviews account for both security impact and service reliability.

For managed environments, this is also a good moment to verify that security updates to internet-facing services receive post-deployment health checks rather than being treated as purely routine package changes.

Bottom line

USN-6455-2 is a corrective Exim update for Ubuntu 22.04 LTS, fixing a regression introduced by a previous security patch. While the notice does not claim active exploitation, it highlights an important defensive lesson: patch quickly, but also verify service behavior after security changes—especially on critical systems like mail servers.