[SECURITY ADVISORY] curl: TLS session caching disaster

[Daniel Stenberg via curl-library]

TLS session caching disaster
============================

Project curl Security Advisory, May 26th 2021 -
[Permalink](https://curl.se/docs/CVE-2021-22901.html)

VULNERABILITY
-------------

libcurl can be tricked into using already freed memory when a new TLS session
is negotiated or a client certificate is requested on an existing connection.
For example, this can happen when a TLS server requests a client certificate
on a connection that was established without one. A malicious server can use
this in rare unfortunate circumstances to potentially reach remote code
execution in the client.

OpenSSL can declare a "new session" for different reasons, including the
initial TLS handshake completion, TLS 1.2 (or earlier) renegotiation, or TLS
1.3 client certificate requests. When libcurl at run-time sets up support for
session ID caching on a connection using OpenSSL, it stores pointers to the
transfer in-memory object for later retrieval when OpenSSL considers a new
session to be established.

However, if the connection is used by multiple transfers (like with a reused
HTTP/1.1 connection or multiplexed HTTP/2 connection) that first transfer
object might be freed before the new session is established on that connection
and then the function will access a memory buffer that might be freed. When
using that memory, libcurl might even call a function pointer in the object,
making it possible for a remote code execution if the server could somehow
manage to get crafted memory content into the correct place in memory.

We are not aware of any exploit of this flaw.

INFO
----

The flaw can only happen in libcurl built to use OpenSSL (or one of its forks).

This flaw has existed in curl since commit
[a304051620b92](https://github.com/curl/curl/commit/a304051620b92) in libcurl
7.75.0, released on February 3, 2021.

The Common Vulnerabilities and Exposures (CVE) project has assigned the name
CVE-2021-22901 to this issue.

CWE-416: Use After Free

Severity: High

## Steps to remote code execution

1. libcurl built to use OpenSSL (BoringSSL and libressl work the same)

2. A multi interface using application

3. One of the following:

- create and use a first easy handle to do HTTP/1.1 over TLS to a malicious
server

- free that easy handle with `curl_easy_cleanup()`

- create and use a second easy handle to do HTTP/1.1 over TLS with to the
same server such that the TLS connection is reused

or

- more than one concurrent easy handle created that do HTTP/2 over a TLS
connection to a malicious server,

- the *first* easy handle to use the connection must be freed with
`curl_easy_cleanup()`

- at least one easy handle remaining in use of the same connection

4. The attacking server needs to figure out heap address details in order to
know what payload contents to provide

5. The necessary exact memory address in the heap gets populated by memory
contents controlled by the server

6. The attacker starts a new handshake (on TLS 1.2 or earlier), or sends a TLS
1.3 client certificate request, or otherwise triggers OpenSSL to consider a
new session to be established

For a remote code execution, the client needs to perform (potentially many)
more transfers (and thus have more easy handles) to allow the server to place
crafted contents into heap memory. Instead of remote code execution, the
client could instead crash or otherwise experience undefined behaviour.

AFFECTED VERSIONS
-----------------

- Affected versions: curl 7.75.0 to and including 7.76.1
- Not affected versions: curl < 7.75.0 and curl >= 7.77.0

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

THE SOLUTION
------------

When the transfer is detached from the connection, it clears the association
to it from the session ID cache logic.

A [fix for CVE-2021-22901](https://github.com/curl/curl/commit/7f4a9a9b2a49547eae24d2e19bc5c346e9026479)

RECOMMENDATIONS
--------------

A - Upgrade curl to version 7.77.0

B - Apply the patch to your local version

C - Build libcurl to use another TLS backend

TIMELINE
--------

This issue was reported to the curl project on April 29, 2021.

This advisory was posted on May 26, 2021.

CREDITS
-------

This issue was reported by Harry Sintonen. Patched by Harry Sintonen and
Daniel Stenberg. Help and research by Brad Spencer.

Thanks a lot!