CVE-2021-34813Public exploitNetwork attack vector

Client crash via unchecked ephemeral key decoding

Published Jun 16, 2021 · Updated Aug 4, 2024

Stack-based buffer overflow in Matrix libolm before 3.2.3 allows remote attackers controlling a homeserver to crash Matrix clients. olm_pk_decrypt decodes a homeserver-supplied, oversized Base64 ephemeral key directly into a fixed-size Curve25519 public-key array on the stack without validating its length. The flaw is reached when a client restores encrypted room keys; standard native builds abort on stack-canary corruption, while code execution requires additional protection bypasses.

CVSS severity9.8
Critical
EPSS probability4.26%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Stack-based buffer overflow in Matrix libolm before 3.2.3 allows remote attackers controlling a homeserver to crash Matrix clients. olm_pk_decrypt decodes a homeserver-supplied, oversized Base64 ephemeral key directly into a fixed-size Curve25519 public-key array on the stack without validating its length. The flaw is reached when a client restores encrypted room keys; standard native builds abort on stack-canary corruption, while code execution requires additional protection bypasses.

The record

CVE
CVE-2021-34813
Published
Jun 16, 2021
Updated
Aug 4, 2024
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-787, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jun 16, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 4, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Affected buildAffected product and version details are unavailable.

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Affected products and versions are unavailable in this record.

Published CVSS scores

9.8NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
7.5NIST NVDAV:N/AC:L/Au:N/C:P/I:P/A:P

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-787Out-of-bounds Write
T1499.004Application or System Exploitation

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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