medium CVSS 4 EPSS 0.20%

CVE-2026-94057·Exim vulnerability

Exim before 4.100.1 allows SMTP smuggling in which the received message does not match any sent message, and instead depends on crafted data sent after a rejection during DATA processing.

Severity
medium
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 0.20% — higher than 8% 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.

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 patch the SMTP smuggling vulnerability that allows attackers to inject crafted data during rejected DATA phase transactions. This vulnerability permits message content manipulation that bypasses normal SMTP validation checks. Administrators should prioritize this update as part of their regular patch management cycle, even though exploitation is not currently widespread.

sudo dnf update exim

Defensia detects this vulnerability

What an exploitation attempt looks like

Sample log line indicative of exploitation attempts:

SMTP_PROTOCOL_ERROR|SMTP_DATA_REJECTED|unmatched_data_injection|post_rejection_data_chunk

How to check if you are affected

  1. Step 1: Check installed Exim version with `exim -v` and compare against 4.100.1
  2. Step 2: Verify SMTP service is exposed by checking `netstat -tlnp | grep ':25'` or `ss -tlnp | grep ':25'`
  3. Step 3: Search Exim logs for suspicious DATA phase rejections followed by raw socket data: `grep -i 'data error\|smtp protocol\|reject' /var/log/exim4/mainlog | tail -100`
  4. Step 4: Confirm the patch by running `exim -v` again after upgrade and verify version is 4.100.1 or higher

FAQ

What is CVE-2026-94057?

CVE-2026-94057 is an SMTP smuggling vulnerability in Exim that allows an attacker to insert crafted message content after the server rejects a DATA phase transmission, causing a mismatch between what the server processes and what was actually sent by the client.

Is CVE-2026-94057 being actively exploited?

No, this vulnerability is not currently listed in CISA's Known Exploited Vulnerabilities catalog, and no public exploits have been disclosed as of this writing.

What versions of Exim are affected by CVE-2026-94057?

All versions of Exim prior to 4.100.1 are vulnerable; version 4.100.1 and later include the security fix.

How do I check if my server is vulnerable to CVE-2026-94057?

Run `exim -v` to display the installed version; if it shows a version number lower than 4.100.1, your system is vulnerable.

Does Defensia detect CVE-2026-94057?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If Exim is installed on a monitored server, CVE-2026-94057 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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