CVE-2023-27538: Haxx Libcurl Auth Bypass Vulnerability

CVE-2023-27538 Overview

An authentication bypass vulnerability exists in libcurl prior to version 8.0.0 where it improperly reuses a previously established SSH connection despite the fact that an SSH option was modified, which should have prevented reuse. libcurl maintains a pool of previously used connections to reuse them for subsequent transfers if the configurations match. However, two SSH settings were omitted from the configuration check, allowing them to match easily, potentially leading to the reuse of an inappropriate connection with different authentication credentials than intended.

Critical Impact

This vulnerability could allow local attackers to bypass authentication mechanisms by exploiting improper SSH connection reuse, potentially gaining unauthorized access to sensitive information through reused connections that should have required re-authentication.

Affected Products

• Haxx libcurl (versions prior to 8.0.0)
• Fedora 36
• Debian Linux 10.0
• NetApp Active IQ Unified Manager (VMware vSphere)
• NetApp Clustered Data ONTAP 9.0
• Broadcom Brocade Fabric Operating System Firmware
• NetApp H300S/H500S/H700S/H410S Firmware
• Splunk Universal Forwarder (multiple versions including 9.1.0)

Discovery Timeline

• 2023-03-30 - CVE-2023-27538 published to NVD
• 2025-06-09 - Last updated in NVD database

Technical Details for CVE-2023-27538

Vulnerability Analysis

This authentication bypass vulnerability (CWE-305, CWE-287) stems from a flaw in libcurl's connection pooling mechanism specifically related to SSH connections. When libcurl establishes an SSH connection, it maintains this connection in a pool for potential reuse by subsequent transfer requests. The intent is to improve performance by avoiding the overhead of establishing new connections when the same server and configuration are used.

The vulnerability arises because two critical SSH configuration options were inadvertently excluded from the checks that determine whether a pooled connection can be safely reused. This oversight means that when an application modifies certain SSH authentication settings between transfers, libcurl may incorrectly determine that an existing connection is suitable for reuse when it should instead establish a new connection with the updated authentication parameters.

The impact of this vulnerability is primarily information disclosure, as an attacker with local access could potentially access data intended for a different authentication context. The local attack vector requires the attacker to have some level of access to the system running the vulnerable libcurl version.

Root Cause

The root cause of CVE-2023-27538 lies in incomplete validation logic within libcurl's connection reuse mechanism. When comparing the configuration of a new transfer request against pooled connections, the code failed to include all relevant SSH authentication parameters in the comparison. Specifically, two SSH settings were omitted from the configuration matching algorithm.

This incomplete comparison allowed connections to appear compatible when they were actually configured with different SSH authentication options, leading to inappropriate connection reuse that bypasses the intended authentication isolation between transfers.

Attack Vector

The attack requires local access to the system running the vulnerable libcurl library. An attacker could exploit this vulnerability by manipulating the sequence of SSH transfers in an application that uses libcurl's connection pooling feature. By carefully timing requests and modifying SSH authentication parameters, an attacker could cause the library to reuse a connection authenticated with different credentials than intended.

The exploitation scenario involves:

1. An initial SSH connection being established with specific authentication credentials
2. The connection being returned to the pool for potential reuse
3. A subsequent request with modified SSH settings being incorrectly matched to the pooled connection
4. The new request inheriting the authentication context of the original connection

Technical details are available in the HackerOne Report #1898475.

Detection Methods for CVE-2023-27538

Indicators of Compromise

• Monitor for applications using libcurl versions prior to 8.0.0 with SSH transfer capabilities enabled
• Review audit logs for unexpected SSH connection patterns or authentication anomalies
• Check for processes making multiple SSH transfers with varying authentication configurations that may indicate exploitation attempts
• Look for applications that utilize libcurl's connection pooling feature with SSH protocols

Detection Strategies

• Implement software composition analysis (SCA) to identify systems running vulnerable libcurl versions
• Deploy endpoint detection solutions capable of monitoring libcurl-based SSH connection behaviors
• Use vulnerability scanners to identify affected systems across your infrastructure
• Review application logs for signs of improper connection reuse or authentication context violations

Monitoring Recommendations

• Configure logging to capture SSH connection establishment and reuse events in applications using libcurl
• Monitor for unusual patterns in SSH authentication attempts that may indicate exploitation
• Establish baseline metrics for SSH connection behavior to detect anomalies
• Implement network monitoring to track SSH session durations and reuse patterns

How to Mitigate CVE-2023-27538

Immediate Actions Required

• Upgrade libcurl to version 8.0.0 or later immediately on all affected systems
• Identify all applications and systems using vulnerable libcurl versions through software inventory
• Prioritize patching systems that perform SSH transfers using libcurl
• Review and update any embedded systems or firmware that include vulnerable libcurl versions

Patch Information

The vulnerability has been addressed in libcurl version 8.0.0 and later. Organizations should apply vendor-specific patches according to their infrastructure:

• Debian Linux: Security updates available via Debian LTS Announcement
• Gentoo Linux: Patch available via Gentoo GLSA 202310-12
• NetApp Products: Security advisory and patches available at NetApp Security Advisory NTAP-20230420-0010
• Fedora: Update through standard package management channels

Workarounds

• Disable SSH connection pooling in libcurl by setting CURLOPT_FRESH_CONNECT option to force new connections for each transfer
• Implement application-level controls to manually manage SSH connection lifecycle
• Consider using alternative SSH client libraries for critical authentication-sensitive operations until patching is complete
• Restrict local access to systems running vulnerable libcurl versions to minimize attack surface

# Configuration example for disabling connection reuse in curl CLI
curl --no-keepalive sftp://example.com/path/to/resource

# For applications using libcurl API, set CURLOPT_FRESH_CONNECT
# to force new connections and prevent connection pool reuse