CVE-2026-34873Network attack vector

Client impersonation via zero-secret session resumption

Published Apr 1, 2026 · Updated Apr 1, 2026

Improper authentication in Mbed TLS 3.5.0 through 3.6.5 and 4.0.0 allows man-in-the-middle attackers to impersonate TLS clients. During TLS 1.3 ticket resumption after HelloRetryRequest, the server accepts a TLS 1.2 ClientHello and resumes with an all-zero master secret. Exploitation requires an intercepted handshake, dual TLS-version support, client authentication, session tickets, and a HelloRetryRequest; successful attackers inherit the ticket holder's identity and, with custom ticket data, its encoded privileges.

CVSS severity9.1
Critical
EPSS probability0.24%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Improper authentication in Mbed TLS 3.5.0 through 3.6.5 and 4.0.0 allows man-in-the-middle attackers to impersonate TLS clients. During TLS 1.3 ticket resumption after HelloRetryRequest, the server accepts a TLS 1.2 ClientHello and resumes with an all-zero master secret. Exploitation requires an intercepted handshake, dual TLS-version support, client authentication, session tickets, and a HelloRetryRequest; successful attackers inherit the ticket holder's identity and, with custom ticket data, its encoded privileges.

The record

CVE
CVE-2026-34873
Published
Apr 1, 2026
Updated
Apr 1, 2026
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-287, T1557
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Apr 1, 2026CVE publishedPublication date reported by the CVE source.
  2. Apr 1, 2026Record 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

9.1CISA-ADPCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

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.

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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 exploitUnconfirmed

Weakness, pattern, technique

CWE-287Improper Authentication
T1557Adversary-in-the-Middle

Public exploit references

No public exploit references are available in this record.

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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