CVE-2017-3737

Published: 7 December 2017

OpenSSL 1.0.2 (starting from version 1.0.2b) introduced an "error state" mechanism. The intent was that if a fatal error occurred during a handshake then OpenSSL would move into the error state and would immediately fail if you attempted to continue the handshake. This works as designed for the explicit handshake functions (SSL_do_handshake(), SSL_accept() and SSL_connect()), however due to a bug it does not work correctly if SSL_read() or SSL_write() is called directly. In that scenario, if the handshake fails then a fatal error will be returned in the initial function call. If SSL_read()/SSL_write() is subsequently called by the application for the same SSL object then it will succeed and the data is passed without being decrypted/encrypted directly from the SSL/TLS record layer. In order to exploit this issue an application bug would have to be present that resulted in a call to SSL_read()/SSL_write() being issued after having already received a fatal error. OpenSSL version 1.0.2b-1.0.2m are affected. Fixed in OpenSSL 1.0.2n. OpenSSL 1.1.0 is not affected.

Notes

this issue does not affect OpenSSL 1.1.0

1.0.2b introduced a security hardening mechanism designed to
protect against bugs in application code.
https://git.openssl.org/?p=openssl.git;a=commit;h=e4f77bf1833245d2b6aa4ce6a16c85e1cdf78589
This CVE applies to the hardening mechanism being incomplete.
openssl versions older than 1.0.2b don't have the hardening
mechanism at all.

Status

Severity score breakdown

References

• https://www.openssl.org/news/secadv/20171207.txt
• https://ubuntu.com/security/notices/USN-3512-1
• https://www.cve.org/CVERecord?id=CVE-2017-3737
• NVD
• Launchpad
• Debian