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CVE-2026-5773

wrong reuse of SMB connection

Project curl Security Advisory, April 29 2026 Permalink

VULNERABILITY

libcurl might in some circumstances reuse the wrong connection for SMB(S) transfers.

libcurl features a pool of recent connections so that subsequent requests can reuse an existing connection to avoid overhead.

When reusing a connection a range of criteria must be met. Due to a logical error in the code, a network transfer operation that was requested by an application could wrongfully reuse an existing SMB connection to the same server that was using a different "share" than the new subsequent transfer should.

This could in unlucky situations lead to the download of the wrong file or the upload of a file to the wrong place. When this happens, the same credentials are used and the server name is the same.

INFO

curl only supports SMB version 1 and no later version. SMB version 1 is considered insecure and deprecated and is therefore commonly disabled in servers. curl is scheduled to drop support for SMB later in 2026. SMB support is opt-in since 8.20.0.

In this flaw, the code simply erroneously did not consider the share name as a property to match for connection reuse.

The Common Vulnerabilities and Exposures (CVE) project has assigned the name CVE-2026-5773 to this issue.

CWE-488: Exposure of Data Element to Wrong Session

Severity: Low

AFFECTED VERSIONS

This flaw has existed since curl started to support SMB.

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

This bug is not considered a C mistake. It is not likely to have been avoided had we not been using C.

This flaw also affects the curl command line tool.

SOLUTION

curl 8.20.0 makes sure that connections using SMB never get reused.

RECOMMENDATIONS

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

A - Upgrade to curl and libcurl 8.20.0

B - Apply the patch and rebuild libcurl

C - Avoid using SMB

TIMELINE

It was reported to the curl project on April 5th 2026. We contacted distros@openwall on April 23.

libcurl 8.20.0 was released on April 29th 2026, coordinated with the publication of this advisory.

CREDITS

Thanks a lot!