CVE-2021-22947Public exploitNetwork attack vector

Response injection via cached pre-TLS replies

Published Sep 29, 2021 · Updated Apr 16, 2026

Improper response validation in curl 7.20.0 through 7.78.0 allows man-in-the-middle attackers to inject fake IMAP or POP3 data via STARTTLS. The protocol state machines preserve pipelined plaintext replies in the pingpong cache across TLS negotiation and consume them as replies to encrypted commands. The attacker must control the network path; injected IMAP or POP3 content is presented as authenticated server data.

CVSS severity5.9
Medium
EPSS probability3.01%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Improper response validation in curl 7.20.0 through 7.78.0 allows man-in-the-middle attackers to inject fake IMAP or POP3 data via STARTTLS. The protocol state machines preserve pipelined plaintext replies in the pingpong cache across TLS negotiation and consume them as replies to encrypted commands. The attacker must control the network path; injected IMAP or POP3 content is presented as authenticated server data.

The record

CVE
CVE-2021-22947
Published
Sep 29, 2021
Updated
Apr 16, 2026
Vendor
curl
Product
curl
Classifications
CWE-310, CWE-349, T1557
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Sep 29, 2021CVE publishedPublication date reported by the CVE source.
  2. Apr 16, 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?

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

  1. Affected versionversion=curl 7.20.0 to and including 7.78.0

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

curl · curlversion=curl 7.20.0 to and including 7.78.0

Published CVSS scores

5.9CISACVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/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.

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 exploitPublished

Weakness, pattern, technique

CWE-310Cryptographic Issues - Generic
CWE-349Acceptance of Extraneous Untrusted Data With Trusted Data
T1557Adversary-in-the-Middle

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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