CVE-2024-6874: Haxx Libcurl Buffer Overflow Vulnerability

CVE-2024-6874 Overview

CVE-2024-6874 is an out-of-bounds read vulnerability affecting libcurl's URL API function curl_url_get(). The vulnerability occurs during punycode conversions when handling Internationalized Domain Names (IDN). When converting a name that is exactly 256 bytes in length, libcurl reads outside of a stack-based buffer when built to use the macidn IDN backend. The conversion function fills the provided buffer exactly but fails to null-terminate the string, potentially causing stack contents to be accidentally returned as part of the converted string.

Critical Impact

This out-of-bounds read vulnerability can lead to unintended information disclosure, where sensitive stack contents may be leaked through the converted string output.

Affected Products

• haxx libcurl version 8.8.0
• Applications built using libcurl with the macidn IDN backend
• Systems performing punycode/IDN conversions with specific 256-byte input lengths

Discovery Timeline

• 2024-07-24 - CVE-2024-6874 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2024-6874

Vulnerability Analysis

This vulnerability represents a classic out-of-bounds read condition (CWE-125) triggered by a boundary edge case in libcurl's punycode conversion functionality. The curl_url_get() function provides punycode conversions for handling Internationalized Domain Names (IDN), converting between human-readable domain names and their ASCII-compatible encoding.

The flaw manifests specifically when the input name is exactly 256 bytes in length. Under these precise conditions, when libcurl is compiled with the macidn IDN backend, the conversion routine reads beyond the allocated stack buffer boundaries. Additionally, the function fails to properly null-terminate the output string after filling the buffer to capacity.

This combination of boundary condition errors creates a scenario where data residing on the stack—potentially including sensitive information such as return addresses, function parameters, or other local variables—can be inadvertently appended to the converted string and returned to the caller. This constitutes an information disclosure vulnerability that could reveal internal memory contents to an attacker.

Root Cause

The root cause of CVE-2024-6874 stems from improper boundary checking and missing null termination in the punycode conversion logic within libcurl's macidn IDN backend. Specifically:

1. Boundary Condition Error: The code fails to properly handle the edge case where the input is exactly 256 bytes, causing it to read one byte beyond the allocated stack buffer.

2. Missing Null Termination: After the conversion function fills the buffer to its maximum capacity, it does not append a null terminator character, leaving the string improperly terminated.

3. Backend-Specific Issue: This vulnerability only affects builds using the macidn IDN backend, indicating the flaw is localized to this specific implementation rather than the core libcurl URL handling code.

Attack Vector

The attack vector for CVE-2024-6874 requires network access and involves crafting a malicious URL or domain name that, when processed through libcurl's punycode conversion, triggers the out-of-bounds read condition.

An attacker could exploit this vulnerability by:

1. Crafting a specially designed domain name that is exactly 256 bytes when processed
2. Inducing an application using the vulnerable libcurl version (with macidn backend) to convert this domain name
3. Receiving the converted string which may contain leaked stack memory contents

While exploitation requires specific conditions (exact input length, specific IDN backend configuration), the network-accessible attack surface in applications performing URL/domain processing increases the exposure risk. The vulnerability does not allow code execution but could leak sensitive memory information that might aid further attacks.

Detection Methods for CVE-2024-6874

Indicators of Compromise

• Unusual or malformed domain names exactly 256 bytes in length being processed by applications
• Unexpected data appearing in converted URL/domain name outputs
• Applications performing high volumes of punycode/IDN conversions with boundary-length inputs
• Memory access anomalies detected in libcurl processes

Detection Strategies

• Monitor for applications using libcurl version 8.8.0 compiled with the macidn IDN backend
• Implement runtime memory access monitoring for libcurl processes handling URL conversions
• Deploy application-level logging for curl_url_get() calls with IDN conversion flags enabled
• Utilize Software Composition Analysis (SCA) tools to identify vulnerable libcurl versions in your environment

Monitoring Recommendations

• Enable verbose logging for applications performing URL parsing and domain name conversions
• Monitor network traffic for unusual domain name patterns with specific length characteristics
• Implement memory sanitizer tools (ASan, MSan) in development and testing environments to detect out-of-bounds access
• Review application logs for unexpected string length mismatches in URL conversion operations

How to Mitigate CVE-2024-6874

Immediate Actions Required

• Identify all systems and applications using libcurl version 8.8.0 with the macidn IDN backend
• Upgrade libcurl to a patched version as soon as available from the vendor
• Review the official curl security advisory for patch information
• Assess applications for exposure to untrusted URL/domain name inputs

Patch Information

The curl project has released information regarding this vulnerability. Organizations should consult the CURL CVE-2024-6874 Documentation for official patch details and updated versions.

Additional vendor advisories:

• NetApp Security Advisory NTAP-20240822-0004 for NetApp product-specific guidance
• HackerOne Report #2604391 contains the original vulnerability report

Workarounds

• Consider using an alternative IDN backend (not macidn) if available and compatible with your environment
• Implement input validation to reject domain names exactly 256 bytes in length before passing to libcurl
• Deploy web application firewalls or input filters to screen unusual domain name inputs
• Isolate applications performing untrusted URL processing in sandboxed environments to limit information disclosure impact

For environments where immediate patching is not feasible, rebuilding libcurl with a different IDN backend may provide temporary mitigation while preserving IDN functionality.