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UAF in SSH sha256 fingerprint check

Project curl Security Advisory, May 17th 2023 - Permalink


libcurl offers a feature to verify an SSH server's public key using a SHA 256 hash. When this check fails, libcurl would free the memory for the fingerprint before it returns an error message containing the (now freed) hash.

This flaw risks inserting sensitive heap-based data into the error message that might be shown to users or otherwise get leaked and revealed.


This only applies to users of the CURLOPT_SSH_HOST_PUBLIC_KEY_SHA256 option, which is only supported for libcurl built with libssh2 (curl optionally supports other SSH backends). Either of the options CURLOPT_VERBOSE or CURLOPT_ERRORBUFFER also need to be set to trigger the problem.

The damage is somewhat limited by the extremely short time window between the free and the use of the freed memory.

The largest possible info leak that can happen due to this flaw per trigger occasion, is limited to CURL_ERROR_SIZE - the error message prefix length (69) = 186 bytes. It will also stop at the first null byte within those 186 bytes.

The Common Vulnerabilities and Exposures (CVE) project has assigned the name CVE-2023-28319 to this issue.

CWE-416: Use After Free

Severity: Medium


libcurl is used by many applications, but not always advertised as such!



A - Upgrade curl to version 8.1.0

B - Apply the patch to your local version



This issue was reported to the curl project on March 21 2023. We contacted distros@openwall on May 9, 2023.

curl 8.1.0 was released on May 17 2023, coordinated with the publication of this advisory.


Thanks a lot!