CVE-2025-14819: libcurl TLS Information Disclosure Flaw

CVE-2025-14819 Overview

CVE-2025-14819 is a Certificate Validation Bypass vulnerability in libcurl affecting TLS-related transfers when using reused easy or multi handles. The vulnerability occurs when the CURLSSLOPT_NO_PARTIALCHAIN option is altered between transfers, causing libcurl to accidentally reuse a CA store cached in memory with the partial chain option reversed. This can result in libcurl accepting trust chains that would otherwise be rejected, potentially allowing connections to untrusted or malicious servers.

Critical Impact

Applications using libcurl with handle reuse and dynamic TLS configuration changes may inadvertently accept invalid certificate chains, creating a potential pathway for man-in-the-middle attacks.

Affected Products

• libcurl (versions using TLS with handle reuse)
• Applications built with libcurl that modify CURLSSLOPT_NO_PARTIALCHAIN option
• Systems performing TLS transfers with reused curl handles

Discovery Timeline

• 2026-01-08 - CVE-2025-14819 published to NVD
• 2026-01-08 - Last updated in NVD database

Technical Details for CVE-2025-14819

Vulnerability Analysis

This vulnerability falls under CWE-295 (Improper Certificate Validation) and represents a critical flaw in how libcurl manages cached CA stores when handling TLS connections with reused handles. The issue manifests when applications dynamically alter the CURLSSLOPT_NO_PARTIALCHAIN configuration option between subsequent transfers using the same curl handle.

Under normal operation, the CURLSSLOPT_NO_PARTIALCHAIN option controls whether libcurl requires a complete certificate chain back to a trusted root CA. When this option is enabled, partial certificate chains that terminate at an intermediate CA are rejected. However, due to this vulnerability, the CA store caching mechanism does not properly invalidate or refresh when this option is toggled, resulting in the cached CA store being reused with the opposite partial chain validation setting.

The practical consequence is that trust decisions made during certificate validation may be inconsistent with the application's current security configuration, potentially accepting certificate chains that should have been rejected.

Root Cause

The root cause lies in the CA store caching mechanism within libcurl's TLS implementation. When an easy or multi handle is reused for subsequent transfers, libcurl caches the CA store in memory for performance optimization. However, the caching logic fails to account for changes to the CURLSSLOPT_NO_PARTIALCHAIN option between transfers. As a result, when this option is altered, the cached CA store reflects the previous configuration state rather than the current security requirements.

Attack Vector

This vulnerability requires a network attack vector with high complexity. An attacker would need to position themselves to intercept TLS connections from an application using the vulnerable libcurl configuration pattern. The attack scenario involves:

1. Target application uses libcurl with handle reuse
2. Application toggles the CURLSSLOPT_NO_PARTIALCHAIN option between transfers
3. Attacker presents a certificate with a partial chain that would normally be rejected
4. Due to the cached CA store misconfiguration, libcurl accepts the invalid chain

The vulnerability specifically requires user interaction in that the application must be configured in a way that modifies TLS options between handle reuses, and the user must initiate a connection that triggers the vulnerable code path.

The vulnerability can lead to confidentiality breaches as TLS connections may be established with untrusted endpoints, potentially exposing sensitive data to attackers. For detailed technical information, see the cURL CVE-2025-14819 Advisory.

Detection Methods for CVE-2025-14819

Indicators of Compromise

• Unexpected TLS certificate warnings or errors followed by successful connections
• Log entries showing connections to servers with incomplete certificate chains
• Anomalous certificate chain lengths in TLS handshake logs

Detection Strategies

• Monitor applications for patterns where CURLSSLOPT_NO_PARTIALCHAIN is modified between transfers on reused handles
• Review application code for curl handle reuse patterns combined with dynamic SSL option changes
• Implement TLS inspection to detect connections established with partial certificate chains

Monitoring Recommendations

• Enable verbose logging for libcurl TLS operations to track certificate validation decisions
• Monitor network traffic for TLS handshakes with unusual certificate chain characteristics
• Implement application-level logging around curl handle creation, reuse, and SSL option modifications

How to Mitigate CVE-2025-14819

Immediate Actions Required

• Review applications using libcurl for handle reuse patterns with dynamic CURLSSLOPT_NO_PARTIALCHAIN modifications
• Consider creating new curl handles when TLS security options need to change rather than reusing existing handles
• Update libcurl to the latest patched version when available

Patch Information

Security patches and updates are available from the curl project. Organizations should consult the cURL CVE-2025-14819 Advisory for specific version information and patch details. The Openwall OSS Security Discussion provides additional context and community discussion regarding this vulnerability.

Workarounds

• Avoid reusing curl easy or multi handles when the CURLSSLOPT_NO_PARTIALCHAIN option needs to be changed
• Create fresh curl handles with the desired SSL configuration for each distinct security context
• If handle reuse is required for performance, ensure CURLSSLOPT_NO_PARTIALCHAIN remains constant throughout the handle's lifetime

# Configuration example - ensure consistent SSL options when reusing handles
# Instead of modifying CURLSSLOPT_NO_PARTIALCHAIN on existing handles,
# create new handles with the desired configuration:

# For applications requiring partial chain rejection:
# Set CURLSSLOPT_NO_PARTIALCHAIN at handle creation and do not modify

# Example: Verify curl version includes the security fix
curl --version | grep -i "libcurl"