curl / Docs / Security Problems / Inferior OCSP verification

Inferior OCSP verification

Project curl Security Advisory, December 9th 2020 - Permalink

VULNERABILITY

libcurl offers "OCSP stapling" via the CURLOPT_SSL_VERIFYSTATUS option. When set, libcurl verifies the OCSP response that a server responds with as part of the TLS handshake. It then aborts the TLS negotiation if something is wrong with the response. The same feature can be enabled with --cert-status using the curl tool.

As part of the OCSP response verification, a client should verify that the response is indeed set out for the correct certificate. This step was not performed by libcurl when built or told to use OpenSSL as TLS backend.

This flaw would allow an attacker, who perhaps could have breached a TLS server, to provide a fraudulent OCSP response that would appear fine, instead of the real one. Like if the original certificate actually has been revoked.

We are not aware of any exploit of this flaw.

INFO

This flaw has existed in curl since commit d1cf5d570663d in curl 7.41.0.

The vulnerability is present only if OpenSSL is the designated TLS backend. OCSP stapling is not enabled by default by libcurl, it needs to be explicitly enabled by the application to get used.

OCSP Stapling can be used with any of the TLS based protocols curl supports, including HTTPS, FTPS, SMTPS, POP3S, IMAPS, HTTPS-proxy and more.

The Common Vulnerabilities and Exposures (CVE) project has assigned the name CVE-2020-8286 to this issue.

CWE-299: Improper Check for Certificate Revocation

Severity: Medium

AFFECTED VERSIONS

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

THE SOLUTION

The OCSP response checker function now also verifies that the certificate id is the correct one.

A fix for CVE-2020-8286

RECOMMENDATIONS

We suggest you take one of the following actions immediately, in order of preference:

A - Upgrade libcurl to version 7.74.0

B - Don't rely on OCSP

TIMELINE

This issue was first reported to the curl project on December 2, 2020.

This advisory was posted on December 9th 2020.

CREDITS

This issue was identified and patched by an Ospoco customer who wishes to remain anonymous. Reported by Ospoco (https://ospo.co).

Thanks a lot!