CVE-2021-26911: core/imap/MCIMAPSession.cpp in Canary Mail before 3.22 has Missing SSL Certificate Validation for IMAP in STAR

This week

HIGH 7.4REMOTEDOWNGRADEDOWNGRADE

CVE-2021-26911 is a high-severity flaw (CVSS 7.4) affecting canary mail, mailcore2. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

core/imap/MCIMAPSession.cpp in Canary Mail before 3.22 has Missing SSL Certificate Validation for IMAP in STARTTLS mode.

Why this priority

High severity
Public exploit reference
Remotely exploitable, no login, no user action

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

Risk facts

CVSS score	7.4 (High)
Attack vector	Network (remote)
Attack complexity	High
Privileges needed	None
User interaction	None
EPSS exploit chance	1.1% (top 35%)
CISA exploited list	No
Weakness	CWE-295
Published	2021-02-17
Last modified	2026-06-17

Publisher (CNA): MITRE · Vendor: canarymail · libmailcore

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

Am I affected?

Timeline

2021-02-17 21:15 UTCPublished CVE record published
2026-06-17 03:43 UTCUpdated Record last modified

canarymail canary mail DOWNGRADE

Affected: 3.20 · 3.21

Fixed in: no fixed release known

Safe to downgrade to: a release before 3.20

1. Check installed version

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

2. No fixed release: DOWNGRADE to a release before 3.20 (Debian / Ubuntu)

apt-cache madison canary-mail sudo apt install canary-mail=<VERSION_BEFORE_3.20> sudo apt-mark hold canary-mail

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

dnf --showduplicates list canary-mail sudo dnf downgrade canary-mail-<VERSION_BEFORE_3.20> sudo dnf versionlock add canary-mail

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.

libmailcore mailcore2 DOWNGRADE

Affected: 0.6.4

Fixed in: no fixed release known

Safe to downgrade to: a release before 0.6.4

1. Check installed version

apt-cache policy mailcore2 || rpm -q mailcore2 || mailcore2 --version

2. No fixed release: DOWNGRADE to a release before 0.6.4 (Debian / Ubuntu)

apt-cache madison mailcore2 sudo apt install mailcore2=<VERSION_BEFORE_0.6.4> sudo apt-mark hold mailcore2

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

dnf --showduplicates list mailcore2 sudo dnf downgrade mailcore2-<VERSION_BEFORE_0.6.4> sudo dnf versionlock add mailcore2

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-2021-26911?

canary mail: downgrade or apply the listed mitigation; mailcore2: downgrade or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2021-26911 being exploited?

CVE-2021-26911 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-2021-26911 affect?

CVE-2021-26911 affects canary mail, mailcore2. Severity is high (CVSS 7.4). core/imap/MCIMAPSession.cpp in Canary Mail before 3.22 has Missing SSL Certificate Validation for IMAP in STARTTLS mode.

How urgent is CVE-2021-26911?

Priority 52/100. High severity; Public exploit reference; Remotely exploitable, no login, no user action

Related CVEs

CVE-2025-65318 CRITICAL 9.1 UPGRADE When using the attachment interaction functionality, Canary Mail 5.1.40 and below saves documents to a file sy

References

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