CVE-2025-4947: Haxx Curl Information Disclosure Flaw

CVE-2025-4947 Overview

CVE-2025-4947 is a certificate validation bypass vulnerability in libcurl that affects QUIC protocol connections. When a user or application connects to a host specified as an IP address in the URL, libcurl accidentally skips the certificate verification process entirely. This security flaw allows potential attackers to conduct man-in-the-middle (MITM) attacks or impersonate legitimate servers without detection.

Critical Impact

Applications using libcurl with QUIC protocol support and IP address-based URLs are vulnerable to man-in-the-middle attacks, potentially exposing sensitive data transmitted over what should be secure connections.

Affected Products

• Haxx curl (libcurl with QUIC support)
• Applications and services built using affected libcurl versions
• Systems where curl is compiled with QUIC/HTTP/3 backend support

Discovery Timeline

• 2025-05-28 - CVE-2025-4947 published to NVD
• 2025-06-26 - Last updated in NVD database

Technical Details for CVE-2025-4947

Vulnerability Analysis

This vulnerability represents a critical flaw in libcurl's TLS certificate verification logic specifically affecting QUIC connections. Under normal circumstances, when establishing a secure connection, libcurl verifies the server's TLS certificate to ensure the connection is made to the intended host. However, a coding error causes this verification step to be bypassed when the target host is specified as an IP address rather than a hostname.

The vulnerability is classified under CWE-295 (Improper Certificate Validation), indicating a fundamental failure in the certificate verification process. This allows an attacker positioned between the client and server to intercept, read, and potentially modify encrypted communications without raising any security warnings.

Root Cause

The root cause lies in libcurl's handling of the certificate verification code path for QUIC connections. When a URL contains an IP address (e.g., https://192.168.1.1/) instead of a hostname, the code incorrectly skips the certificate verification step. This appears to be an oversight in the implementation where the IP address scenario was not properly handled in the QUIC-specific verification logic.

Attack Vector

The attack vector is network-based and requires no authentication or user interaction. An attacker can exploit this vulnerability by positioning themselves in the network path between a vulnerable client and its intended destination. The attack scenario proceeds as follows:

1. The victim application initiates a QUIC connection to a server using an IP address URL
2. The attacker intercepts the connection request via ARP spoofing, DNS poisoning, or network position
3. The attacker presents their own certificate to the client
4. Due to the vulnerability, libcurl does not verify the certificate and accepts the connection
5. The attacker can now intercept all traffic, potentially stealing credentials or injecting malicious content

Since no code examples are available from verified sources, organizations should refer to the official cURL security advisory for detailed technical information about the vulnerability mechanism.

Detection Methods for CVE-2025-4947

Indicators of Compromise

• Unexpected certificate warnings or errors in network monitoring tools for QUIC traffic
• Network traffic analysis showing QUIC connections to IP addresses without proper TLS handshake verification
• Anomalous QUIC/HTTP/3 connections originating from systems that typically use hostname-based URLs
• Evidence of ARP spoofing or network interception targeting systems using curl with QUIC

Detection Strategies

• Monitor network traffic for QUIC connections made directly to IP addresses rather than hostnames
• Implement certificate transparency log monitoring to detect suspicious certificate usage
• Deploy network intrusion detection systems (NIDS) configured to alert on potential MITM patterns
• Audit application configurations to identify curl usage with IP address-based URLs and QUIC enabled

Monitoring Recommendations

• Enable verbose logging in curl-based applications to track certificate verification behavior
• Implement network segmentation to limit the impact of potential MITM attacks
• Monitor for unusual patterns in QUIC traffic that may indicate certificate verification is being bypassed
• Review application logs for connections to IP addresses using QUIC protocol

How to Mitigate CVE-2025-4947

Immediate Actions Required

• Update curl and libcurl to the latest patched version as soon as available from the vendor
• Review and audit applications that use libcurl with QUIC support for IP address-based URL usage
• Consider temporarily disabling QUIC/HTTP/3 support in critical applications until patched
• Use hostname-based URLs instead of IP addresses where possible as a temporary mitigation

Patch Information

Haxx has released information about this vulnerability through their official security advisory. Organizations should consult the cURL CVE-2025-4947 Advisory for detailed patch information and apply updates according to their guidance. The vulnerability details are also available in the HackerOne Report #3150884.

Workarounds

• Avoid using IP addresses directly in URLs when making QUIC connections; use hostnames instead
• Disable QUIC/HTTP/3 support in curl by removing the --http3 flag or equivalent library option
• Implement additional certificate pinning at the application layer for critical connections
• Use network-level controls to enforce certificate validation through proxy or gateway devices

# Disable HTTP/3 (QUIC) to mitigate the vulnerability
# When using curl command line, avoid --http3 flag
curl --http2 https://example.com/

# For libcurl applications, ensure QUIC is disabled
# Set CURLOPT_HTTP_VERSION to CURL_HTTP_VERSION_2_0 or lower
# Example curl configuration to force HTTP/2
curl_easy_setopt(curl, CURLOPT_HTTP_VERSION, CURL_HTTP_VERSION_2_0);