CVE-2021-3449Public exploitNetwork attack vector

TLS server crash via stale signature-algorithm length

Published Mar 25, 2021 · Updated Sep 17, 2024

Denial of service in OpenSSL 1.1.1 before 1.1.1k allows remote attackers to crash TLS servers via a crafted renegotiation ClientHello. During rehandshake, init_sig_algs frees peer_sigalgs but leaves peer_sigalgslen stale, so tls12_shared_sigalgs walks a NULL buffer. Exposure requires a TLS 1.2 server with renegotiation enabled, the default OpenSSL configuration; TLS clients are unaffected.

CVSS severity5.9
Medium
EPSS probability63.54%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Denial of service in OpenSSL 1.1.1 before 1.1.1k allows remote attackers to crash TLS servers via a crafted renegotiation ClientHello. During rehandshake, init_sig_algs frees peer_sigalgs but leaves peer_sigalgslen stale, so tls12_shared_sigalgs walks a NULL buffer. Exposure requires a TLS 1.2 server with renegotiation enabled, the default OpenSSL configuration; TLS clients are unaffected.

The record

CVE
CVE-2021-3449
Published
Mar 25, 2021
Updated
Sep 17, 2024
Vendor
OpenSSL Project
Product
OpenSSL
Classifications
CWE-476, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Mar 25, 2021CVE publishedPublication date reported by the CVE source.
  2. Sep 17, 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?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=Fixed in OpenSSL 1.1.1k (Affected 1.1.1-1.1.1j)

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

OpenSSL Project · OpenSSLversion=Fixed in OpenSSL 1.1.1k (Affected 1.1.1-1.1.1j)

Published CVSS scores

5.9NIST NVDCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
4.3NIST NVDAV:N/AC:M/Au:N/C:N/I:N/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.

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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 exploitPublished

Weakness, pattern, technique

CWE-476NULL Pointer Dereference
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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