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Fencer Vulns
critical 9.8

CVE-2021-34813 json

Matrix libolm before 3.2.3 allows a malicious Matrix homeserver to crash a client (while it is attempting to retrieve an Olm encrypted room…

published 2021-06-16 5 y ago modified 2026-06-17 4 mo ago also EUVD-2021-21460

cvss
9.8
version 3.1, NVD
cisa kev
Not listed
not in CISA's catalog
epss
4.3%
EPSS 4.3% chance of exploitation in 30 days, percentile 91%
public exploits
0
none known

Summary

Matrix libolm before 3.2.3 allows a malicious Matrix homeserver to crash a client (while it is attempting to retrieve an Olm encrypted room key backup from the homeserver) because olm_pk_decrypt has a stack-based buffer overflow. Remote code execution might be possible for some nonstandard build configurations.

Patch / fix

  • matrix olm 3.2.3
  • olm Debian 14, Debian 13, Debian 12 3.2.3~dfsg-3

debian advisories DEBIAN-CVE-2021-34813

  • olm Ubuntu Pro 20.04 LTS 3.1.3+dfsg-2ubuntu0.1~esm1

ubuntu advisories UBUNTU-CVE-2021-34813

Ubuntu security data © Canonical Ltd., from https://github.com/canonical/ubuntu-security-notices via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

  • libolm Arch Linux 3.2.3-1

arch linux advisories AVG-2077

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2021-34813 updated cvss 7.5

libolm: allows a malicious Matrix homeserver to crash a client

  • Red Hat All currently supported Red Hat products
    not affected: vulnerable code not present

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

Exploit prediction

4.26% chance of exploitation in the next 30 days, higher than 91% of all scored CVEs (FIRST EPSS, 2026-11-08).

Exploits and checks

  • matrix.org https://matrix.org/blog/2021/06/14/adventures-in-fuzzing-libolm referenced PoC, tagged "exploit" by a source, unverified

CVSS scores

assigned by version score vector
NVD 3.1 9.8 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
NVD 2.0 7.5 AV:N/AC:L/Au:N/C:P/I:P/A:P

Classification

Weakness

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.