In early 2023, a serious authentication bypass vulnerability was discovered in libcurl, a widely used library for transferring data with URLs. Identified as CVE-2023-27538, this bug could let attackers reuse SSH connections even when the SSH configuration had changed—potentially opening the door to unauthorized data access or manipulation.

In this article, we’ll break down what happened, how the bug works, and what you can do to keep your systems safe. No fancy jargon—just plain talk and clear code.

What Is CVE-2023-27538?

CVE-2023-27538 is an authentication bypass vulnerability that affects libcurl versions before v8... It has to do with how libcurl manages SSH connections. When you use libcurl to talk to servers over SSH (for example, using HTTPS, SFTP, or SCP URLs), the library usually tries to reuse old SSH connections to make things faster.

The critical part? libcurl checks whether the SSH configuration for a new transfer matches the configuration of a previously used connection. If it thinks they're the same, it skips opening a new connection—making the reuse.

The issue: libcurl accidentally left out checks for two SSH options in this process. That means a connection could be reused, even if some important settings had changed. This opens the door to potential authentication bypass where a session could be hijacked across different credentials or connection parameters.

If no, it makes a new one.

libcurl only compares certain fields in the connection config. Before v8.., it did not check two SSH-related options:

The "knownhosts" file setting

Missing these meant two transfers could be treated as "identical" when they actually *shouldn't* be.

Example: Reproducing The Vulnerability

Let’s see this in action. Suppose we connect to two different servers with two different authentication settings, but with the same host and port.

CURL *curl = curl_easy_init();

curl_easy_setopt(curl, CURLOPT_URL, "sftp://yourhost/file1.txt");
curl_easy_setopt(curl, CURLOPT_SSH_AUTH_TYPES, CURLSSH_AUTH_PASSWORD);
curl_easy_setopt(curl, CURLOPT_PASSWORD, "password123");
curl_easy_perform(curl);

curl_easy_setopt(curl, CURLOPT_URL, "sftp://yourhost/file2.txt");
// Switch to public key authentication, but libcurl might not notice!
curl_easy_setopt(curl, CURLOPT_SSH_AUTH_TYPES, CURLSSH_AUTH_PUBLICKEY);
curl_easy_perform(curl);

curl_easy_cleanup(curl);

Before libcurl v8.., the second request could reuse the first SSH connection even though the auth type has changed! This breaks the expectation that each authentication setting gets its own secure channel.

Attack Scenario

Imagine a shared libcurl process serving multiple users, maybe as part of a backend file transfer service. If user A connects with one set of SSH credentials, then user B connects with a different set, libcurl may re-use the first connection for both—even though their credentials and security policies differ.

Real risk: This can let one user access or control files as another—an authentication bypass.

How Was It Fixed?

libcurl v8.. release notes detail the fix. The maintainers added the missing SSH fields to the connection-matching checks, preventing accidental connection reuse in such cases.

Here’s the specific commit that fixed the bug.

Affected versions: All libcurl versions prior to 8..

- How it’s exploited: By establishing an SSH connection with certain settings, then changing those settings and reusing the same CURL handle or multi interface. If the host/port is the same, libcurl may not distinguish between the different security requirements.
- What an attacker needs: Access to trigger multiple transfers (such as via a multi-user application, or code that dynamically changes SSH parameters).

Update: Upgrade to libcurl v8.. (or later).

2. Audit: If you have apps that use SSH features in libcurl and allow changing of settings, check they *don’t reuse* the same CURL handle across different credentials.
3. Isolation: Consider isolating user contexts to prevent unwanted sharing at the process or handle level.

References

- CVE Details: CVE-2023-27538
- libcurl security advisory
- GitHub commit fixing the bug
- libcurl changelog

Final Thoughts

CVE-2023-27538 is a reminder that even small omissions in security checks can have a big impact—especially in widely used libraries like curl. Always keep your dependencies up to date, and dig deep into how they manage connections and credentials for you.

Timeline

Published on: 03/30/2023 20:15:00 UTC
Last modified on: 04/21/2023 23:15:00 UTC