CVE-2023-28319: Haxx Curl Use After Free Vulnerability

CVE-2023-28319 Overview

A use after free vulnerability exists in curl versions prior to v8.1.0 within libcurl's SSH server public key verification feature. When verifying an SSH server's public key using a SHA-256 hash, if the verification check fails, libcurl incorrectly frees the memory containing the fingerprint before constructing an error message that references this now-freed hash. This flaw creates a risk of inserting sensitive heap-based data into the error message, which may be displayed to users or otherwise leaked and revealed.

Critical Impact

This use after free vulnerability can expose sensitive heap memory contents through error messages, potentially leaking credentials, cryptographic keys, or other sensitive data stored in memory.

Affected Products

• Haxx curl (versions prior to 8.1.0)
• Apple macOS (multiple versions)
• NetApp Clustered Data ONTAP
• NetApp ONTAP Antivirus Connector
• NetApp H300S/H500S/H700S/H410S Firmware

Discovery Timeline

• 2023-05-26 - CVE-2023-28319 published to NVD
• 2025-01-15 - Last updated in NVD database

Technical Details for CVE-2023-28319

Vulnerability Analysis

This vulnerability is classified as CWE-416 (Use After Free), a memory corruption issue that occurs when a program continues to use a pointer after the memory it references has been freed. In the context of curl, the vulnerable code path is triggered during SSH server public key verification using SHA-256 hash comparison.

When a user configures curl to verify an SSH server's public key fingerprint and the verification fails, libcurl enters an error handling routine. The implementation flaw causes the fingerprint memory to be freed prematurely—before the error message string is constructed. The error message then attempts to include the fingerprint value by dereferencing the already-freed memory pointer, resulting in the inclusion of arbitrary heap data in the output.

The network-accessible nature of this vulnerability means that an attacker who can influence SSH connection failures or intercept network traffic could potentially trigger this condition to extract sensitive information from the curl process's heap memory.

Root Cause

The root cause lies in improper memory management within libcurl's SSH key verification error handling path. The fingerprint buffer is deallocated before the error message referencing it is fully constructed and returned. This ordering violation means the pointer becomes dangling, and subsequent use results in reading freed heap memory. Proper implementation would require retaining the fingerprint memory until after the error message has been fully constructed and copied.

Attack Vector

The attack vector for CVE-2023-28319 requires an attacker to trigger SSH server public key verification failures in a curl session. This can be achieved through:

1. Man-in-the-Middle Position: An attacker positioned between the curl client and the legitimate SSH server can present a different host key, causing the verification to fail
2. DNS Hijacking: Redirecting the target hostname to an attacker-controlled server with a different SSH key
3. Server Compromise: If an attacker controls or compromises the target SSH server, they can deliberately present incorrect keys

Once the verification fails, the error message containing heap data may be logged, displayed to the user, or transmitted to monitoring systems, providing the attacker with potential access to sensitive memory contents.

The vulnerability operates without requiring authentication and can be exploited remotely over the network, making it particularly concerning for automated systems that log curl error output.

Detection Methods for CVE-2023-28319

Indicators of Compromise

• Unusual or malformed error messages from curl operations containing unexpected binary or ASCII data
• SSH connection failures with abnormally long or garbled fingerprint values in error output
• Log entries showing curl SSH verification errors with non-hexadecimal characters in fingerprint fields
• Memory access violations or crashes in applications using vulnerable libcurl versions

Detection Strategies

• Monitor application logs for curl SSH verification failures containing suspicious or malformed data patterns
• Implement version scanning across infrastructure to identify systems running curl versions prior to 8.1.0
• Deploy network monitoring to detect SSH connection anomalies that may indicate exploitation attempts
• Use memory analysis tools to detect use-after-free conditions in applications linked against vulnerable libcurl

Monitoring Recommendations

• Enable verbose logging for curl operations in production environments to capture potential exploitation attempts
• Set up alerting for SSH key verification failures, particularly those with unusual error message content
• Conduct regular software inventory audits to ensure all curl installations are updated to patched versions
• Monitor for network traffic patterns indicative of man-in-the-middle attacks targeting SSH connections

How to Mitigate CVE-2023-28319

Immediate Actions Required

• Upgrade curl to version 8.1.0 or later immediately on all affected systems
• Review application logs for evidence of prior exploitation or unusual error messages from SSH operations
• Assess all systems using libcurl for SSH operations and prioritize patching based on exposure
• Consider temporarily disabling SSH public key verification features if patching cannot be immediately applied

Patch Information

The vulnerability has been addressed in curl version 8.1.0 and later. Multiple vendors have released security updates:

• Haxx (curl maintainers): Fixed in curl 8.1.0 - reported via HackerOne Report #1913733
• Apple: Security updates HT213843, HT213844, and HT213845
• NetApp: Security Advisory NTAP-20230609-0009
• Gentoo Linux: GLSA 202310-12

Workarounds

• Avoid using the SSH public key verification feature (CURLOPT_SSH_KNOWNHOSTS or --hostpubsha256) until systems can be patched
• Implement network segmentation to limit exposure of systems running vulnerable curl versions
• Ensure error messages from curl operations are not logged or displayed in user-accessible locations
• Use alternative SSH client implementations for critical operations until curl can be updated

# Configuration example
# Verify installed curl version
curl --version

# If version is below 8.1.0, upgrade using package manager
# For Debian/Ubuntu:
sudo apt update && sudo apt install curl

# For RHEL/CentOS:
sudo yum update curl

# For macOS (using Homebrew):
brew upgrade curl

# Verify the upgrade was successful
curl --version | grep -E "^curl [0-9]"