CVE-2011-1506: The STARTTLS implementation in Kerio Connect 7.1.4 build 2985 and MailServer 6.x does not properly restrict I/

This month

MEDIUM 6.8DOWNGRADEDOWNGRADE

CVE-2011-1506 is a medium-severity flaw (CVSS 6.8) affecting connect, kerio mailserver. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

The STARTTLS implementation in Kerio Connect 7.1.4 build 2985 and MailServer 6.x does not properly restrict I/O buffering, which allows man-in-the-middle attackers to insert commands into encrypted SMTP sessions by sending a cleartext command that is processed after TLS is in place, related to a "plaintext command injection" attack, a similar issue to CVE-2011-0411. NOTE: some of these details are obtained from third party information.

Why this priority

No elevated risk signals yet

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

Risk facts

CVSS score	6.8 (Medium)
EPSS exploit chance	2.5% (top 15%)
CISA exploited list	No
Weakness	CWE-20
Published	2011-03-22
Last modified	2026-06-16

Publisher (CNA): MITRE · Vendor: kerio

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

Am I affected?

Timeline

2011-03-22 17:55 UTCPublished CVE record published
2026-06-16 23:29 UTCUpdated Record last modified

kerio connect DOWNGRADE

Affected: 7.1.4

Fixed in: no fixed release known

Safe to downgrade to: a release before 7.1.4

1. Check installed version

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

2. No fixed release: DOWNGRADE to a release before 7.1.4 (Debian / Ubuntu)

apt-cache madison connect sudo apt install connect=<VERSION_BEFORE_7.1.4> sudo apt-mark hold connect

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

dnf --showduplicates list connect sudo dnf downgrade connect-<VERSION_BEFORE_7.1.4> sudo dnf versionlock add connect

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.

kerio kerio mailserver DOWNGRADE

Affected: 5.0 · 5.1 · 5.1.1 · 5.6.3 · 5.6.4 · 5.6.5 · 5.7.0 · 5.7.1

Fixed in: no fixed release known

Safe to downgrade to: a release before 5.0

1. Check installed version

apt-cache policy kerio-mailserver || rpm -q kerio-mailserver || kerio-mailserver --version

2. No fixed release: DOWNGRADE to a release before 5.0 (Debian / Ubuntu)

apt-cache madison kerio-mailserver sudo apt install kerio-mailserver=<VERSION_BEFORE_5.0> sudo apt-mark hold kerio-mailserver

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

dnf --showduplicates list kerio-mailserver sudo dnf downgrade kerio-mailserver-<VERSION_BEFORE_5.0> sudo dnf versionlock add kerio-mailserver

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-2011-1506?

connect: downgrade or apply the listed mitigation; kerio mailserver: downgrade or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2011-1506 being exploited?

CVE-2011-1506 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-2011-1506 affect?

CVE-2011-1506 affects connect, kerio mailserver. Severity is medium (CVSS 6.8). The STARTTLS implementation in Kerio Connect 7.1.4 build 2985 and MailServer 6.x does not properly restrict I/O buffering, which allows man-in-the-middle attackers to insert commands into encrypted SMTP sessions by sending a cleartext comma

How urgent is CVE-2011-1506?

Priority 45/100. No elevated risk signals yet.

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References

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