critical CVSS 10 Actively exploited EPSS 72% Public exploit available

CVE-2010-4344·Exim vulnerability

Heap-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.

Severity
critical
Software
Exim
Fixed in
4.70-1
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 72% — in the top 0.6% 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: 4.70-1

Fixed in: 4.70-1

How to fix this CVE

Upgrade Exim to version 4.70-1 or later immediately to patch a critical heap-based buffer overflow in the string formatting function. This vulnerability allows unauthenticated remote attackers to execute arbitrary code through specially crafted SMTP sessions. Deploy the patched version across all mail servers running vulnerable Exim instances.

sudo dnf update exim

Defensia detects this vulnerability

What an exploitation attempt looks like

Sample log line indicative of exploitation attempts:

SMTP session containing two sequential MAIL commands followed by DATA containing oversized or malformed header fields (e.g., Received, X-* custom headers exceeding 1024 bytes); typical log entry: "LOG: MAIN REJECT"

How to check if you are affected

  1. Run `exim -bV` to display the currently installed Exim version and compare it against version 4.70-1
  2. Check if Exim is listening on SMTP ports: `sudo netstat -tlnp | grep -E ':(25|587|465)' | grep exim`
  3. Review Exim main log for rejection entries with unusual header content: `sudo grep -i 'rejected' /var/log/exim4/main.log | tail -100`
  4. Verify the patched version is active by restarting Exim and re-running `exim -bV`: `sudo systemctl restart exim4 && exim -bV`

FAQ

What is CVE-2010-4344?

CVE-2010-4344 is a critical remote code execution vulnerability in Exim's SMTP message handling that stems from improper bounds checking in the string formatting routine, allowing attackers to overflow the heap via maliciously constructed email headers.

Is CVE-2010-4344 being actively exploited?

Yes, CVE-2010-4344 is listed in CISA's Known Exploited Vulnerabilities (KEV) catalog and public exploits are widely available, making it a high-priority target for threat actors.

What versions of Exim are affected by CVE-2010-4344?

All versions of Exim prior to and including 4.70-0 are vulnerable; patched version 4.70-1 and all subsequent releases are not affected.

How do I check if my server is vulnerable to CVE-2010-4344?

Run `exim -bV | head -1` and verify the version number is 4.70-1 or higher; any version below 4.70-1 indicates a vulnerable installation.

Does Defensia detect CVE-2010-4344?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If Exim is installed on a monitored server, CVE-2010-4344 will appear in your dashboard with remediation steps.

Related Exim CVEs

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.
CVE-2017-16944CVSS 9The receive_msg function in receive.c in the SMTP daemon in Exim 4.88 and 4.89 allows remote attackers to cause a denial of service (infinite loop and stack exhaustion) via vectors involving BDAT commands and an improper check for a '.' character signifying the end of the content, related to the bdat_getc function.

References

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