CVE-2020-8286: Haxx Libcurl Information Disclosure Flaw

CVE-2020-8286 Overview

CVE-2020-8286 is a certificate validation bypass vulnerability affecting curl versions 7.41.0 through 7.73.0. The vulnerability stems from insufficient verification of the Online Certificate Status Protocol (OCSP) response, which allows attackers to potentially bypass certificate revocation checks. This flaw in the widely-used libcurl library undermines the trust model of TLS/SSL connections by failing to properly validate whether a server's certificate has been revoked.

Critical Impact

Attackers can exploit this vulnerability to conduct man-in-the-middle attacks using revoked certificates, potentially intercepting sensitive data transmitted over connections that appear to be secured by valid TLS/SSL certificates.

Affected Products

• Haxx libcurl 7.41.0 through 7.73.0
• Apple macOS and Mac OS X (multiple versions including 10.14.6 and 10.15.7)
• Fedora 32 and 33
• Debian Linux 9.0 and 10.0
• NetApp Clustered Data ONTAP, HCI Management Node, SolidFire, and HCI Storage/Compute Nodes
• Siemens SIMATIC TIM 1531 IRC and SINEC Infrastructure Network Services
• Oracle Communications Billing and Revenue Management 12.0.0.3.0
• Oracle Communications Cloud Native Core Policy 1.14.0
• Oracle Essbase 21.2
• Oracle PeopleSoft Enterprise PeopleTools 8.58
• Splunk Universal Forwarder (multiple versions including 9.1.0)

Discovery Timeline

• December 14, 2020 - CVE-2020-8286 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2020-8286

Vulnerability Analysis

The vulnerability exists in curl's OCSP (Online Certificate Status Protocol) verification mechanism. When curl is configured to use OCSP stapling to verify certificate revocation status, it fails to properly validate the OCSP response received from the server. This improper check allows a malicious server to present a revoked certificate while providing a manipulated or invalid OCSP response that curl incorrectly accepts as valid.

OCSP stapling is a performance optimization where the server, rather than the client, queries the OCSP responder and "staples" the response to the TLS handshake. When this mechanism is improperly validated, the entire certificate revocation checking system becomes ineffective, allowing connections to servers with revoked certificates to succeed without warning.

The vulnerability is classified under CWE-295 (Improper Certificate Validation), which encompasses flaws where software does not adequately verify the authenticity of certificates used to establish trust.

Root Cause

The root cause of this vulnerability is insufficient verification logic in libcurl's OCSP response handling code. The library fails to perform adequate checks on the OCSP response data, including potentially accepting responses that don't properly match the certificate being verified or responses with invalid signatures. This allows an attacker controlling a server to bypass the revocation check mechanism entirely.

Attack Vector

The attack can be executed over the network without requiring authentication or user interaction. An attacker positioned as a man-in-the-middle or controlling a malicious server can serve a revoked certificate along with a crafted OCSP response. The vulnerable curl client will accept this combination, believing the connection is secured by a valid certificate.

A successful attack would proceed as follows: the attacker obtains a certificate that has been legitimately revoked (perhaps due to key compromise), then configures their server to staple a malformed or invalid OCSP response. When a vulnerable curl client connects with OCSP stapling enabled, it fails to detect that the certificate has been revoked and establishes the TLS connection, potentially exposing sensitive data to the attacker.

For detailed technical information about this vulnerability, refer to the CURL CVE-2020-8286 Documentation and the HackerOne Report #1048457.

Detection Methods for CVE-2020-8286

Indicators of Compromise

• TLS/SSL connections established with servers presenting revoked certificates when OCSP stapling is enabled
• Anomalous OCSP response handling or missing OCSP verification logs in applications using libcurl
• Network traffic patterns showing successful connections to known malicious servers using revoked certificates

Detection Strategies

• Audit installed curl and libcurl versions across all systems to identify instances running versions 7.41.0 through 7.73.0
• Monitor for unusual certificate validation patterns in application logs where curl is used for HTTPS connections
• Implement network-level certificate transparency monitoring to detect connections using revoked certificates
• Use vulnerability scanning tools to identify systems with vulnerable curl/libcurl versions

Monitoring Recommendations

• Enable detailed TLS/SSL logging in applications using libcurl to capture certificate validation events
• Monitor certificate revocation list (CRL) and OCSP traffic for anomalies indicating potential bypass attempts
• Implement alerting for connections to endpoints with certificates approaching or past expiration that may also be revoked
• Deploy endpoint detection solutions capable of identifying vulnerable library versions in running processes

How to Mitigate CVE-2020-8286

Immediate Actions Required

• Upgrade curl to version 7.74.0 or later, which contains the fix for this vulnerability
• Update all applications and products that bundle libcurl to patched versions
• Review and apply vendor-specific patches for affected products including Apple macOS, Debian, Fedora, NetApp, Siemens, Oracle, and Splunk products
• As a temporary measure, consider disabling OCSP stapling verification if immediate patching is not possible (note: this reduces security and should only be temporary)

Patch Information

The curl project released version 7.74.0 to address this vulnerability. Multiple vendors have released patches for their affected products:

• Apple: Security updates available via HT212325, HT212326, and HT212327
• Debian: DSA-4881 and LTS announcement available
• Fedora: Package updates available via Fedora package repositories
• NetApp: Security Advisory NTAP-20210122-0007
• Siemens: SSA-389290 and SSA-200951
• Oracle: Patches included in CPU April 2021, CPU July 2021, and CPU April 2022
• Gentoo: GLSA 202012-14

Workarounds

• Disable OCSP stapling in curl by not using the --cert-status option or CURLOPT_SSL_VERIFYSTATUS (note: this removes revocation checking entirely)
• Implement alternative certificate revocation checking mechanisms at the application or network level
• Use network-level TLS inspection to validate certificates independently of client-side checking
• Consider implementing certificate pinning for critical connections to known endpoints as an additional layer of protection

# Configuration example
# Check curl version to verify if affected
curl --version

# Upgrade curl on Debian/Ubuntu systems
sudo apt update && sudo apt upgrade curl libcurl4

# Upgrade curl on RHEL/CentOS/Fedora systems
sudo dnf update curl libcurl

# Verify the updated version is 7.74.0 or later
curl --version | grep -i "libcurl"