Vulnerability identifier: #VU56615

Vulnerability risk: Medium

CVSSv3.1: 6.2 [CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:H/A:N/E:U/RL:O/RC:C]

CVE-ID: CVE-2021-22947

CWE-ID: CWE-345

Exploitation vector: Network

Exploit availability: No

Vulnerable software:
cURL
Client/Desktop applications / Other client software

Vendor: curl.haxx.se

Description

The vulnerability allows a remote attacker to perform MitM attack.

The vulnerability exists in the way libcurl handles the STARTTLS negotiation process. 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.

Mitigation
Install updates from vendor's website.

Vulnerable software versions

cURL: 7.20.0 - 7.78.0

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External links
http://curl.haxx.se/docs/CVE-2021-22946.html

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Q & A

Can this vulnerability be exploited remotely?

Yes. This vulnerability can be exploited by a remote non-authenticated attacker via the Internet.

Is there known malware, which exploits this vulnerability?

No. We are not aware of malware exploiting this vulnerability.