CVE-2021-34558Network attack vector

Client crash via unchecked certificate-key type cast

Published Jul 15, 2021 · Updated Aug 4, 2024

Improper certificate validation in Go crypto/tls before 1.15.14 and 1.16.0 through 1.16.5 allows attackers to crash TLS clients. rsaKeyAgreement.generateClientKeyExchange assumes the certificate public key is RSA and performs an unchecked type assertion when TLS_RSA is negotiated, so an ECDSA or Ed25519 key triggers a panic. Exploitation requires a privileged network position plus a trusted non-RSA server certificate, or a client using Config.InsecureSkipVerify; TLS 1.3-only clients are unaffected.

CVSS severity6.5
Medium
EPSS probability6.98%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Improper certificate validation in Go crypto/tls before 1.15.14 and 1.16.0 through 1.16.5 allows attackers to crash TLS clients. rsaKeyAgreement.generateClientKeyExchange assumes the certificate public key is RSA and performs an unchecked type assertion when TLS_RSA is negotiated, so an ECDSA or Ed25519 key triggers a panic. Exploitation requires a privileged network position plus a trusted non-RSA server certificate, or a client using Config.InsecureSkipVerify; TLS 1.3-only clients are unaffected.

The record

CVE
CVE-2021-34558
Published
Jul 15, 2021
Updated
Aug 4, 2024
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-295, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jul 15, 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

2.6NIST NVDAV:N/AC:H/Au:N/C:N/I:N/A:P
6.5NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H

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 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-295Improper Certificate Validation
T1499.004Endpoint Denial of Service: Application or System Exploitation

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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