CVE-2024-6197: Haxx Libcurl Buffer Overflow Vulnerability

CVE-2024-6197 Overview

CVE-2024-6197 is a memory corruption vulnerability in libcurl's ASN.1 parser that affects the utf8asn1str() function. When this function detects an invalid field during the parsing of an ASN.1 UTF-8 string, it incorrectly invokes free() on a 4-byte local stack buffer. This erroneous memory operation can lead to stack memory corruption when certain malloc implementations accept the invalid pointer and add the memory to their available chunks list.

Critical Impact

This vulnerability can cause application crashes and potential stack memory overwrites. While denial of service through crashes is the most likely outcome, more serious exploitation cannot be ruled out in specific circumstances.

Affected Products

• haxx libcurl (all versions prior to patch)

Discovery Timeline

• 2024-07-24 - CVE-2024-6197 published to NVD
• 2024-11-29 - Last updated in NVD database

Technical Details for CVE-2024-6197

Vulnerability Analysis

The vulnerability resides in libcurl's ASN.1 parsing functionality, specifically within the utf8asn1str() function. This function is responsible for parsing ASN.1 UTF-8 strings during TLS certificate processing. When the parser encounters an invalid field, the error handling path contains a critical flaw: it calls free() on a 4-byte buffer that was allocated on the stack rather than the heap.

Stack buffers should never be passed to free() as the memory allocator expects heap-allocated memory with specific metadata structures. The behavior when free() receives a stack pointer varies by implementation:

• Most modern malloc implementations detect this invalid pointer and immediately abort the process
• Some implementations may accept the pointer and add the memory to their free list, leading to heap metadata corruption
• In the latter case, subsequent memory operations may overwrite nearby stack memory with heap management data

The content of any overwrite is determined by the malloc implementation's internal structures, typically consisting of memory pointers and control flags.

Root Cause

The root cause is improper memory management in the error handling path of the utf8asn1str() function. A local stack buffer is incorrectly passed to free(), violating the fundamental rule that only heap-allocated memory should be freed. This represents a classic programming error where the code path that handles errors does not properly distinguish between stack-allocated and heap-allocated buffers.

Attack Vector

An attacker can exploit this vulnerability by providing a malicious TLS certificate with a specially crafted ASN.1 UTF-8 string that triggers the vulnerable error path. The attack is network-based and can be initiated by:

1. Setting up a malicious server that presents a crafted certificate during TLS handshake
2. Luring a victim application using libcurl to connect to the malicious server
3. The malformed ASN.1 data triggers the parsing error in utf8asn1str()
4. The vulnerable error handler calls free() on the stack buffer

The attack requires no authentication or user interaction beyond initiating a connection to the malicious server. Technical details regarding the specific ASN.1 malformation required can be found in the cURL security advisory and the associated HackerOne report.

Detection Methods for CVE-2024-6197

Indicators of Compromise

• Unexpected application crashes in processes using libcurl during TLS connections
• Memory corruption errors or segmentation faults related to curl or libcurl libraries
• Crash dumps showing stack corruption in ASN.1 parsing functions
• AddressSanitizer (ASan) reports indicating invalid free operations on stack addresses

Detection Strategies

• Deploy AddressSanitizer builds during testing to detect invalid free() calls on stack memory
• Monitor application crash logs for signatures related to libcurl ASN.1 parsing
• Implement network traffic analysis to detect malformed TLS certificates
• Use SentinelOne Singularity platform to detect memory corruption attempts and anomalous process behavior

Monitoring Recommendations

• Configure crash reporting systems to alert on libcurl-related memory errors
• Monitor network connections for TLS handshake failures that may indicate exploitation attempts
• Implement logging for certificate validation errors in applications using libcurl
• Deploy endpoint detection and response (EDR) solutions to identify post-exploitation behavior

How to Mitigate CVE-2024-6197

Immediate Actions Required

• Update libcurl to the latest patched version immediately
• Audit all applications and systems using libcurl to identify vulnerable deployments
• Implement network segmentation to limit exposure of vulnerable systems to untrusted networks
• Consider temporarily restricting connections to untrusted TLS endpoints if patching is delayed

Patch Information

The cURL project has released patches to address this vulnerability. Detailed information about the fix is available in the official cURL security advisory. Organizations using libcurl should update to the latest version as soon as possible. Additional vendor-specific advisories include the NetApp Security Advisory NTAP-20241129-0008 for affected NetApp products.

Workarounds

• Restrict application connections to trusted TLS endpoints only where possible
• Deploy network-level controls to block connections to known malicious servers
• Implement certificate pinning to reduce exposure to rogue certificates
• Monitor for vendor patches if using bundled versions of libcurl in third-party software

# Check current libcurl version
curl --version | head -1

# On Debian/Ubuntu systems, update libcurl
sudo apt update && sudo apt upgrade libcurl4

# On RHEL/CentOS systems
sudo yum update libcurl

# Verify the update
curl --version | head -1