high CVSS 7 EPSS 0.27%

CVE-2026-94054·Exim vulnerability

Exim before 4.100.1, when Proxy-Protocol is used with an attacker-controlled proxy, has an out-of-bounds write.

Severity
high
Software
Exim
Fixed in
4.100.1
Published

How likely is exploitation

FIRST's Exploit Prediction Scoring System puts the probability of this CVE being exploited in the next 30 days at 0.27% — higher than 19% of all published CVEs. EPSS is an independent forecast of exploitation, not a severity rating: it says nothing about how much damage a successful attack does, which is what a CVSS score measures.

Read the two together. CVE-2026-94054 scores 7 on CVSS, so a successful attack would be serious — but at 0.27%, almost nobody is currently attempting it. That is a different situation from a flaw under mass exploitation, and it is usually the difference between patching on your next maintenance window and patching tonight.

Affected versions

Until: 4.100.1

Fixed in: 4.100.1

How to fix this CVE

Upgrade Exim to version 4.100.1 or later to address a memory corruption vulnerability that can occur when the Proxy-Protocol extension is enabled and processing connections from an untrusted proxy source. Organizations using Exim with Proxy-Protocol should prioritize this update to prevent potential denial of service or information disclosure attacks.

sudo dnf update exim

Defensia detects this vulnerability

What an exploitation attempt looks like

Sample log line indicative of exploitation attempts:

Proxy-Protocol connection attempts with malformed or oversized headers followed by exim process crashes or segmentation faults in syslog; pattern: `exim\[\d+\]: segfault|exim\[\d+\]: out of bounds|PROXY TCP[46].*overflow`

How to check if you are affected

  1. Step 1: Check the installed Exim version by running `exim -bV | head -n 1` and verify it is 4.100.1 or later
  2. Step 2: Verify if Proxy-Protocol is enabled in your Exim configuration by checking for 'proxy_protocol_timeout' or 'proxy_protocol_networks' settings in /etc/exim4/exim4.conf.localsettings
  3. Step 3: Search Exim logs for abnormal connection patterns from proxy sources using `grep -i 'proxy' /var/log/exim4/main.log | grep -i 'error\|fail\|corruption'`
  4. Step 4: After applying the patch, re-run `exim -bV | head -n 1` to confirm the version is now 4.100.1 or higher

FAQ

What is CVE-2026-94054?

This vulnerability is an out-of-bounds write condition in Exim's Proxy-Protocol handler that can be triggered by sending specially crafted connection data through an attacker-controlled proxy, potentially leading to memory corruption and system instability.

Is CVE-2026-94054 being actively exploited?

No, this vulnerability is not currently listed on the CISA Known Exploited Vulnerabilities (KEV) catalog and no public exploits have been disclosed, though organizations should still patch promptly as the vulnerability is network-accessible.

What versions of Exim are affected by CVE-2026-94054?

All versions of Exim prior to 4.100.1 are vulnerable when Proxy-Protocol is enabled. Organizations running Exim 4.100.0 and earlier should upgrade immediately.

How do I check if my server is vulnerable to CVE-2026-94054?

Run `exim -bV` to check your version; if it shows a version before 4.100.1, your system is vulnerable. Additionally, confirm that Proxy-Protocol is configured by checking for related settings in your Exim configuration files.

Does Defensia detect CVE-2026-94054?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If Exim is installed on a monitored server, CVE-2026-94054 will appear in your dashboard with remediation steps.

Related Exim CVEs

CVE-2010-4344CVSS 10Heap-based buffer overflow in the string_vformat function in string.c in Exim before 4.70 allows remote attackers to execute arbitrary code via an SMTP session that includes two MAIL commands in conjunction with a large message containing crafted headers, leading to improper rejection logging.
CVE-2019-16928CVSS 10Exim 4.92 through 4.92.2 allows remote code execution, a different vulnerability than CVE-2019-15846. There is a heap-based buffer overflow in string_vformat in string.c involving a long EHLO command.
CVE-2019-10149CVSS 10A flaw was found in Exim versions 4.87 to 4.91 (inclusive). Improper validation of recipient address in deliver_message() function in /src/deliver.c may lead to remote command execution.
CVE-2018-6789CVSS 10An issue was discovered in the base64d function in the SMTP listener in Exim before 4.90.1. By sending a handcrafted message, a buffer overflow may happen. This can be used to execute code remotely.
CVE-2026-45185CVSS 9.8Exim before 4.99.3, in certain GnuTLS configurations, has a remotely reachable use-after-free in the BDAT body parsing path. It is triggered when a client sends a TLS close_notify mid-body during a CHUNKING transfer, followed by a final cleartext byte on the same TCP connection. This can lead to heap corruption. An unauthenticated network attacker exploiting this vulnerability could execute arbitrary code.

References

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