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Date: Wed, 15 Sep 2021 08:20:53 +0200 (CEST)
From: Daniel Stenberg <>
To: curl security announcements -- curl users <>,, libcurl hacking <>,
Subject: [SECURITY ADVISORY] curl: STARTTLS protocol injection via MITM

STARTTLS protocol injection via MITM

Project curl Security Advisory, September 15th 2021 -


When curl connects to an IMAP, POP3, SMTP or FTP server to exchange data
securely using STARTTLS to upgrade the connection to TLS level, the server can
still respond and send back multiple responses before the TLS upgrade. Such
multiple "pipelined" responses are cached by curl. curl would then upgrade to
TLS but not flush the in-queue of cached responses and instead use and trust
the responses it got *before* the TLS handshake as if they were authenticated.

Using this flaw, it allows a Man-In-The-Middle attacker to first inject the
fake responses, then pass-through the TLS traffic from the legitimate server
and trick curl into sending data back to the user thinking the attacker's
injected data comes from the TLS-protected server.

Over POP3 and IMAP an attacker can inject fake response data.

We are not aware of any case of this flaw having been exploited in the wild.


This flaw was first introduced in commit

The Common Vulnerabilities and Exposures (CVE) project has assigned the name
CVE-2021-22947 to this issue.

CWE-349: Acceptance of Extraneous Untrusted Data With Trusted Data

Severity: Medium


- Affected versions: curl 7.20.0 to and including 7.78.0
- Not affected versions: curl < 7.20.0 and curl >= 7.79.0

Also note that libcurl is used by many applications, and not always advertised
as such.


A [fix for CVE-2021-22947](


  A - Upgrade curl to version 7.79.0

  B - Apply the patch to your local version

  C - Do not use IMAP, POP3, SMTP or FTP with explicit TLS


This issue was reported to the curl project on September 7, 2021.

This advisory was posted on September 15, 2021.


This issue was reported and patched by Patrick Monnerat.

Thanks a lot!


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