CVE-2003-0743: Heap-based buffer overflow in smtp_in.c for Exim 3 (exim3) before 3.36 and Exim 4 (exim4) before 4.21 may allo

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HIGH 7.5DOWNGRADE

CVE-2003-0743 is a high-severity flaw (CVSS 7.5) affecting exim. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Heap-based buffer overflow in smtp_in.c for Exim 3 (exim3) before 3.36 and Exim 4 (exim4) before 4.21 may allow remote attackers to execute arbitrary code via an invalid (1) HELO or (2) EHLO argument with a large number of spaces followed by a NULL character and a newline, which is not properly trimmed before the "(no argument given)" string is appended to the buffer.

Why this priority

High severity
EPSS top 7%: very likely to be exploited

No fix is published yet: follow the downgrade or mitigation steps.

Risk facts

CVSS score	7.5 (High)
EPSS exploit chance	5.7% (top 7%)
CISA exploited list	No
Published	2003-10-20
Last modified	2026-06-16

Publisher (CNA): MITRE · Vendor: university of cambridge

Last updated 2026-06-16 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2003-10-20 04:00 UTCPublished CVE record published
2026-06-16 22:02 UTCUpdated Record last modified

university of cambridge exim DOWNGRADE

Affected: 3.0 · 3.3 · 3.3.1 · 3.3.2 · 3.11 · 3.12 · 3.13 · 3.14

Fixed in: no fixed release known

Safe to downgrade to: a release before 3.0

1. Check installed version

apt-cache policy exim || rpm -q exim || exim --version

2. No fixed release: DOWNGRADE to a release before 3.0 (Debian / Ubuntu)

apt-cache madison exim sudo apt install exim=<VERSION_BEFORE_3.0> sudo apt-mark hold exim

2. No fixed release: DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list exim sudo dnf downgrade exim-<VERSION_BEFORE_3.0> sudo dnf versionlock add exim

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2003-0743?

exim: downgrade or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2003-0743 being exploited?

CVE-2003-0743 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2003-0743 affect?

CVE-2003-0743 affects exim. Severity is high (CVSS 7.5). Heap-based buffer overflow in smtp_in.c for Exim 3 (exim3) before 3.36 and Exim 4 (exim4) before 4.21 may allow remote attackers to execute arbitrary code via an invalid (1) HELO or (2) EHLO argument with a large number of spaces followed b

How urgent is CVE-2003-0743?

Priority 49/100. High severity; EPSS top 7%: very likely to be exploited

Software pages

Exim vulnerabilities

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References

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