CVE-2021-44538

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 olm_session_describe. Furthermore, safe buffer sizes were undocumented. The overflow content is partially controllable by the attacker and limited to ASCII spaces and digits. The known affected products are Element Web And SchildiChat Web.

Published: 2021-12-14 Last update: 2024-11-21 Assigner: [email protected] Source: [email protected]
NVD Status：Modified，CVE State: published
View at NVD, CVE.org, EUVD

CVE-2021-44538 is rated High Risk (70/100): CVSS Critical severity, with medium exploitation likelihood (EPSS 2.38%). High exploitation likelihood—assess exposure and prioritize remediation.

Risk is dynamic; we continuously reassess and refresh what is shown on this page as upstream context changes.

Exploit prediction scoring system (EPSS) score for CVE-2021-44538

EPSS lead: Daily EPSS estimates relative likelihood of exploitation; percentile ranks this CVE among scored vulnerabilities (higher = more severe relative rank).

# Date Old EPSS score New EPSS score Delta (New - Old)
1 2026-05-26 1.42% 2.38% +0.96%
2 2025-11-21 2.38% 1.42% -0.96%
3 2025-11-18 — 2.38% —

Full EPSS history (18 records total)

Common vulnerability scoring system (CVSS) metrics for CVE-2021-44538

CVSS metrics for this CVE.

Base score Version Severity Vector Exploitability Impact Score source
9.8 3.1 CRITICAL
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:N)
No account or special rights needed—anonymous or random user is enough.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:U)
Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:H)
Serious risk that confidential data gets exposed in a big way.
Integrity (I:H)
They could widely tamper with or forge data—trust in the data is badly hurt.
Availability (A:H)
Could take the service down hard or make it unusable for people who depend on it.
 3.9 5.9 [email protected]
7.5 2.0 HIGH
AV:N/AC:L/Au:N/C:P/I:P/A:P Click to expand
Access vector (AV:N)
Can be exploited remotely over network reachability.
Access complexity (AC:L)
Exploitation conditions are straightforward and predictable.
Authentication (AU:N)
No authentication is required.
Confidentiality impact (C:P)
Partial confidentiality impact.
Integrity impact (I:P)
Partial integrity impact.
Availability impact (A:P)
Partial availability impact.
 10.0 6.4 [email protected]

Weakness enumeration for CVE-2021-44538

OS Trackers for CVE-2021-44538

vendor priority summary link
alpine critical CVE-2021-44538: 4 source package rows (element-web, olm, riot-web, thunderbird); 16 state rows across 7 repos (3.17-community, 3.18-community, 3.19-community, 3.20-community, 3.21-community, 3.22-community, edge-community); fixed 16, open 0. https://security.alpinelinux.org/vuln/CVE-2021-44538
debian not yet assigned CVE-2021-44538 not yet assigned priority: Debian including 2 source packages (olm, thunderbird), 10 status rows across 5 suites (bookworm, bullseye, forky, sid, trixie): resolved 9, open 1. https://security-tracker.debian.org/tracker/CVE-2021-44538
suse high — https://www.suse.com/security/cve/CVE-2021-44538/
ubuntu medium CVE-2021-44538 medium priority: Ubuntu including 3 source packages (node-matrix-js-sdk, olm, thunderbird), 44 status rows across 15 suites (bionic, focal, hirsute, impish, jammy, kinetic, lunar, mantic, noble, oracular, plucky, questing, trusty, upstream, xenial): ignored 20, released 12, needs-triage 10, DNE 2. https://ubuntu.com/security/CVE-2021-44538

Affected software / configurations for CVE-2021-44538

Vendor Product Version Raw CPE
matrix element < 1.9.7 cpe:2.3:a:matrix:element:*:*:*:*:desktop:*:*:*
matrix element < 1.9.7 cpe:2.3:a:matrix:element:*:*:*:*:web:*:*:*
matrix javascript_sdk >= 2.4.2, < 15.2.1 cpe:2.3:a:matrix:javascript_sdk:*:*:*:*:*:*:*:*
matrix olm >= 3.1.4, < 3.2.8 cpe:2.3:a:matrix:olm:*:*:*:*:*:*:*:*
schildi schildichat < 1.9.7-sc1 cpe:2.3:a:schildi:schildichat:*:*:*:*:desktop:*:*:*
schildi schildichat < 1.9.7-sc1 cpe:2.3:a:schildi:schildichat:*:*:*:*:web:*:*:*
cinny_project cinny < 1.6.0 cpe:2.3:a:cinny_project:cinny:*:*:*:*:*:*:*:*
debian debian_linux 9.0 cpe:2.3:o:debian:debian_linux:9.0:*:*:*:*:*:*:*
debian debian_linux 10.0 cpe:2.3:o:debian:debian_linux:10.0:*:*:*:*:*:*:*
debian debian_linux 11.0 cpe:2.3:o:debian:debian_linux:11.0:*:*:*:*:*:*:*

References for CVE-2021-44538

cvelogic Threat Intelligence