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TLS session caching disaster

Project curl Security Advisory, May 26th 2021 - Permalink


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 runtime 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 accesses 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.


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

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:

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

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

  3. 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 behavior.


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


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


A - Upgrade curl to version 7.77.0

B - Apply the patch to your local version

C - Build libcurl to use another TLS backend


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

This advisory was posted on May 26, 2021.


Thanks a lot!