CVE-2020-15953: LibEtPan through 1.9.4, as used in MailCore 2 through 0.6.3 and other products, has a STARTTLS buffering issue

Q: How do I fix CVE-2020-15953?

libetpan: upgrade to newer than 1.9.4; mailcore2: upgrade to newer than 0.6.3; fedora: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Q: Is CVE-2020-15953 being exploited?

CVE-2020-15953 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2020-15953 affect?

CVE-2020-15953 affects libetpan, mailcore2, fedora. Severity is high (CVSS 7.4). LibEtPan through 1.9.4, as used in MailCore 2 through 0.6.3 and other products, has a STARTTLS buffering issue that affects IMAP, SMTP, and POP3. When a server sends a "begin TLS" response, the client reads additional data (e.g., from a med

Q: How urgent is CVE-2020-15953?

Priority 62/100. High severity; Public exploit reference; Remotely exploitable, no login, no user action; On your watchlist

This week

HIGH 7.4REMOTEWATCHLISTUPGRADEUPGRADEMITIGATE

CVE-2020-15953 is a high-severity flaw (CVSS 7.4) affecting libetpan, mailcore2, fedora. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

LibEtPan through 1.9.4, as used in MailCore 2 through 0.6.3 and other products, has a STARTTLS buffering issue that affects IMAP, SMTP, and POP3. When a server sends a "begin TLS" response, the client reads additional data (e.g., from a meddler-in-the-middle attacker) and evaluates it in a TLS context, aka "response injection."

Why this priority

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

A fixed release or distro patch exists.

Risk facts

CVSS score	7.4 (High)
Attack vector	Network (remote)
Attack complexity	High
Privileges needed	None
User interaction	None
EPSS exploit chance	2.4% (top 16%)
CISA exploited list	No
Weakness	CWE-74
Published	2020-07-27
Last modified	2026-06-17

Publisher (CNA): MITRE · Vendor: debian · fedoraproject · libetpan project · libmailcore

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

Am I affected?

Timeline

2020-07-27 07:15 UTCPublished CVE record published
2026-06-17 02:57 UTCUpdated Record last modified

libetpan project libetpan UPGRADE

Affected: <= 1.9.4

Fixed in: newer than 1.9.4

Safe to downgrade to: no unaffected older release

1. Check installed version

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

2. UPGRADE to newer than 1.9.4 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade libetpan

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade libetpan

Note

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

libmailcore mailcore2 UPGRADE

Affected: <= 0.6.3

Fixed in: newer than 0.6.3

Safe to downgrade to: no unaffected older release

1. Check installed version

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

2. UPGRADE to newer than 0.6.3 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade mailcore2

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade mailcore2

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.

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.

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-2020-15953?

libetpan: upgrade to newer than 1.9.4; mailcore2: upgrade to newer than 0.6.3; fedora: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2020-15953 being exploited?

CVE-2020-15953 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-2020-15953 affect?

CVE-2020-15953 affects libetpan, mailcore2, fedora. Severity is high (CVSS 7.4). LibEtPan through 1.9.4, as used in MailCore 2 through 0.6.3 and other products, has a STARTTLS buffering issue that affects IMAP, SMTP, and POP3. When a server sends a "begin TLS" response, the client reads additional data (e.g., from a med

How urgent is CVE-2020-15953?

Priority 62/100. High severity; Public exploit reference; Remotely exploitable, no login, no user action; On your watchlist

Software pages

Debian vulnerabilities

Related CVEs

CVE-2022-4121 MEDIUM 5.5 MITIGATE In libetpan a null pointer dereference in mailimap_mailbox_data_status_free in low-level/imap/mailimap_types.c
CVE-2017-8825 HIGH 7.5 UPGRADE A null dereference vulnerability has been found in the MIME handling component of LibEtPan before 1.8, as used

References

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