CVE-2017-16943: The receive_msg function in receive.c in the SMTP daemon in Exim 4.88 and 4.89 allows remote attackers to exec

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CRITICAL 9.8REMOTEWATCHLISTDOWNGRADEMITIGATE

CVE-2017-16943 is a critical-severity flaw (CVSS 9.8) affecting exim, debian linux. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

The receive_msg function in receive.c in the SMTP daemon in Exim 4.88 and 4.89 allows remote attackers to execute arbitrary code or cause a denial of service (use-after-free) via vectors involving BDAT commands.

Why this priority

Critical severity
EPSS top 1%: very likely to be exploited
Public exploit reference
Remotely exploitable, no login, no user action
On your watchlist

A fixed release or distro patch exists.

Risk facts

CVSS score	9.8 (Critical)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	47.1% (top 1%)
CISA exploited list	No
Weakness	CWE-416
Published	2017-11-25
Last modified	2026-06-17

Publisher (CNA): MITRE · Vendor: debian · exim

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2017-11-25 17:29 UTCPublished CVE record published
2026-06-17 01:10 UTCUpdated Record last modified

exim exim DOWNGRADE

Affected: 4.88 · 4.89

Fixed in: no fixed release known

Safe to downgrade to: a release before 4.88

1. Check installed version

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

2. No fixed release: DOWNGRADE to a release before 4.88 (Debian / Ubuntu)

apt-cache madison exim sudo apt install exim=<VERSION_BEFORE_4.88> sudo apt-mark hold exim

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

dnf --showduplicates list exim sudo dnf downgrade exim-<VERSION_BEFORE_4.88> 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.

debian debian linux MITIGATE

Affected: 9.0

Fixed in: no fixed release known

Safe to downgrade to: a release before 9.0

Check your OS release

cat /etc/os-release; uname -r

Debian / Ubuntu

sudo apt update && sudo apt full-upgrade

RHEL / Amazon Linux / Fedora

sudo dnf upgrade --refresh

Windows

Install the latest cumulative update from Windows Update

If no patch exists yet

OS downgrades are not safe. Apply the vendor mitigation from the advisory and restrict exposure.

Exact fixes by distribution

A distro patch exists: prefer upgrading over downgrading.

Fedora 46 · shotcut · FIXED · 26.10.30-1.fc46 · FEDORA-2026-7787c7fc2d · source

sudo dnf upgrade --advisory=FEDORA-2026-7787c7fc2d

Debian 12 · exim4 · FIXED · 4.89-12

sudo apt update && sudo apt install --only-upgrade exim4 dpkg --compare-versions "$(dpkg-query -W -f='${Version}' exim4)" ge "4.89-12" && echo PATCHED || echo STILL-VULNERABLE

Debian 13 · exim4 · FIXED · 4.89-12

sudo apt update && sudo apt install --only-upgrade exim4 dpkg --compare-versions "$(dpkg-query -W -f='${Version}' exim4)" ge "4.89-12" && echo PATCHED || echo STILL-VULNERABLE

RHEL 5 · exim · NOT AFFECTED

Alpine 3.20 · exim · FIXED · exim-4.89-r7

apk add --upgrade exim apk info -v exim

Alpine 3.21 · exim · FIXED · exim-4.89-r7

apk add --upgrade exim apk info -v exim

Alpine 3.22 · exim · FIXED · exim-4.89-r7

apk add --upgrade exim apk info -v exim

Alpine 3.23 · exim · FIXED · exim-4.89-r7

apk add --upgrade exim apk info -v exim

Amazon Linux 1 · see advisory · FIXED · ALAS-2017-932 · source

sudo yum update --advisory=ALAS-2017-932

Public exploits and detection

Public code exists for this CVE. Treat unpatched systems as at risk, and only run PoCs in a lab.

GitHub PoC: beraphin/CVE-2017-16943 (0 stars)
Write-up: bugs.exim.org/show_bug.cgi?id=2199
Write-up: github.com/LetUsFsck/PoC-Exploit-Mirror/tree/master/CVE-2017-16944
Write-up: hackerone.com/reports/296991

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-2017-16943?

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

Is CVE-2017-16943 being exploited?

CVE-2017-16943 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-2017-16943 affect?

CVE-2017-16943 affects exim, debian linux. Severity is critical (CVSS 9.8). The receive_msg function in receive.c in the SMTP daemon in Exim 4.88 and 4.89 allows remote attackers to execute arbitrary code or cause a denial of service (use-after-free) via vectors involving BDAT commands.

How urgent is CVE-2017-16943?

Priority 74/100. Critical severity; EPSS top 1%: very likely to be exploited; Public exploit reference; Remotely exploitable, no login, no user action; On your watchlist

Software pages

Debian vulnerabilities · Exim vulnerabilities

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References

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