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CVE-2021-44538: The olm_session_describe function in Matrix libolm before 3.2.7 is vulnerable to a buffer overflow

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This week
CRITICAL 9.8REMOTEWATCHLISTUPGRADEUPGRADEUPGRADE

CVE-2021-44538 is a critical-severity flaw (CVSS 9.8) affecting element, javascript sdk, olm. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

The olm_session_describe function in Matrix libolm before 3.2.7 is vulnerable to a buffer overflow. The Olm session object represents a cryptographic channel between two parties. Therefore, its state is partially controllable by the remote party of the channel. Attackers can construct a crafted sequence of messages to manipulate the state of the receiver's session in such a way that, for some buffer sizes, a buffer overflow happens on a call to o

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 score9.8 (Critical)
Attack vectorNetwork (remote)
Attack complexityLow
Privileges neededNone
User interactionNone
EPSS exploit chance2.0% (top 19%)
CISA exploited listNo
WeaknessCWE-119
Published2021-12-14
Last modified2026-06-17
Publisher (CNA): MITRE · Vendor: cinny project · debian · matrix · schildi

Share: LinkedIn · X · Reddit

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

Am I affected?

Timeline

matrix element UPGRADE
Affected: < 1.9.7
Fixed in: 1.9.7
Safe to downgrade to: no unaffected older release
1. Check installed version
apt-cache policy element || rpm -q element || element --version
2. UPGRADE to 1.9.7 (Debian / Ubuntu)
sudo apt update && sudo apt install --only-upgrade element
2. UPGRADE (RHEL / Amazon Linux / Fedora)
sudo dnf upgrade element
Note
Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.
matrix javascript sdk UPGRADE
Affected: >= 2.4.2 and < 15.2.1
Fixed in: 15.2.1
Safe to downgrade to: a release before 2.4.2
1. Check installed version
apt-cache policy javascript-sdk || rpm -q javascript-sdk || javascript-sdk --version
2. UPGRADE to 15.2.1 (Debian / Ubuntu)
sudo apt update && sudo apt install --only-upgrade javascript-sdk
2. UPGRADE (RHEL / Amazon Linux / Fedora)
sudo dnf upgrade javascript-sdk
Note
Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.
3. If no update is available yet, DOWNGRADE to a release before 2.4.2 (Debian / Ubuntu)
apt-cache madison javascript-sdk sudo apt install javascript-sdk=<VERSION_BEFORE_2.4.2> sudo apt-mark hold javascript-sdk
3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)
dnf --showduplicates list javascript-sdk sudo dnf downgrade javascript-sdk-<VERSION_BEFORE_2.4.2> sudo dnf versionlock add javascript-sdk
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.
matrix olm UPGRADE
Affected: >= 3.1.4 and < 3.2.8
Fixed in: 3.2.8
Safe to downgrade to: a release before 3.1.4
1. Check installed version
apt-cache policy olm || rpm -q olm || olm --version
2. UPGRADE to 3.2.8 (Debian / Ubuntu)
sudo apt update && sudo apt install --only-upgrade olm
2. UPGRADE (RHEL / Amazon Linux / Fedora)
sudo dnf upgrade olm
Note
Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.
3. If no update is available yet, DOWNGRADE to a release before 3.1.4 (Debian / Ubuntu)
apt-cache madison olm sudo apt install olm=<VERSION_BEFORE_3.1.4> sudo apt-mark hold olm
3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)
dnf --showduplicates list olm sudo dnf downgrade olm-<VERSION_BEFORE_3.1.4> sudo dnf versionlock add olm
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.
schildi schildichat UPGRADE
Affected: < 1.9.7-sc1
Fixed in: 1.9.7-sc1
Safe to downgrade to: no unaffected older release
1. Check installed version
apt-cache policy schildichat || rpm -q schildichat || schildichat --version
2. UPGRADE to 1.9.7-sc1 (Debian / Ubuntu)
sudo apt update && sudo apt install --only-upgrade schildichat
2. UPGRADE (RHEL / Amazon Linux / Fedora)
sudo dnf upgrade schildichat
Note
Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.
cinny project cinny UPGRADE
Affected: < 1.6.0
Fixed in: 1.6.0
Safe to downgrade to: no unaffected older release
1. Check installed version
apt-cache policy cinny || rpm -q cinny || cinny --version
2. UPGRADE to 1.6.0 (Debian / Ubuntu)
sudo apt update && sudo apt install --only-upgrade cinny
2. UPGRADE (RHEL / Amazon Linux / Fedora)
sudo dnf upgrade cinny
Note
Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.
debian debian linux MITIGATE
Affected: 9.0 · 10.0 · 11.0
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-2021-44538?

element: upgrade to 1.9.7; javascript sdk: upgrade to 15.2.1; olm: upgrade to 3.2.8. Exact commands for each OS are on this page.

Is CVE-2021-44538 being exploited?

CVE-2021-44538 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-2021-44538 affect?

CVE-2021-44538 affects element, javascript sdk, olm. Severity is critical (CVSS 9.8). The olm_session_describe function in Matrix libolm before 3.2.7 is vulnerable to a buffer overflow. The Olm session object represents a cryptographic channel between two parties. Therefore, its state is partially controllable by the remote

How urgent is CVE-2021-44538?

Priority 64/100. Critical severity; Remotely exploitable, no login, no user action; On your watchlist

Software pages

Debian vulnerabilities

Related CVEs

References

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