CVE-2020-15917: common/session.c in Claws Mail before 3.17.6 has a protocol violation because suffix data after STARTTLS is mi

This week

CRITICAL 9.8REMOTEWATCHLISTUPGRADEMITIGATEDOWNGRADE

CVE-2020-15917 is a critical-severity flaw (CVSS 9.8) affecting claws-mail, fedora, backports sle. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

common/session.c in Claws Mail before 3.17.6 has a protocol violation because suffix data after STARTTLS is mishandled.

Why this priority

Critical severity
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	2.6% (top 15%)
CISA exploited list	No
Published	2020-07-23
Last modified	2026-06-17

Publisher (CNA): MITRE · Vendor: claws-mail · fedoraproject · opensuse

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

Am I affected?

Timeline

2020-07-23 19:15 UTCPublished CVE record published
2026-06-17 02:57 UTCUpdated Record last modified

claws-mail claws-mail UPGRADE

Affected: < 3.17.6

Fixed in: 3.17.6

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy claws-mail || rpm -q claws-mail || claws-mail --version

2. UPGRADE to 3.17.6 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade claws-mail

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade claws-mail

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

fedoraproject fedora MITIGATE

Affected: 31 · 32

Fixed in: no fixed release known

Safe to downgrade to: a release before 31

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.

opensuse backports sle DOWNGRADE

Affected: 15.0 sp1 · 15.0 sp2

Fixed in: no fixed release known

Safe to downgrade to: a release before 15.0

1. Check installed version

apt-cache policy backports-sle || rpm -q backports-sle || backports-sle --version

2. No fixed release: DOWNGRADE to a release before 15.0 (Debian / Ubuntu)

apt-cache madison backports-sle sudo apt install backports-sle=<VERSION_BEFORE_15.0> sudo apt-mark hold backports-sle

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

dnf --showduplicates list backports-sle sudo dnf downgrade backports-sle-<VERSION_BEFORE_15.0> sudo dnf versionlock add backports-sle

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.

opensuse leap MITIGATE

Affected: 15.1 · 15.2

Fixed in: no fixed release known

Safe to downgrade to: a release before 15.1

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.

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-2020-15917?

claws-mail: upgrade to 3.17.6; fedora: mitigate or apply the listed mitigation; backports sle: downgrade or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2020-15917 being exploited?

CVE-2020-15917 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-2020-15917 affect?

CVE-2020-15917 affects claws-mail, fedora, backports sle. Severity is critical (CVSS 9.8). common/session.c in Claws Mail before 3.17.6 has a protocol violation because suffix data after STARTTLS is mishandled.

How urgent is CVE-2020-15917?

Priority 66/100. Critical severity; Remotely exploitable, no login, no user action; On your watchlist

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References

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