CVE-2020-8177: Haxx Curl Path Traversal Vulnerability

CVE-2020-8177 Overview

CVE-2020-8177 is a local file overwrite vulnerability affecting curl versions 7.20.0 through 7.70.0. The vulnerability stems from improper restriction of names for files and other resources, allowing an attacker to overwrite local files when the -J (Content-Disposition header) flag is used in conjunction with -O (remote output).

Critical Impact

Attackers can leverage this vulnerability to overwrite arbitrary local files on the victim's system, potentially leading to code execution, privilege escalation, or denial of service through corruption of critical system files.

Affected Products

• Haxx curl versions 7.20.0 through 7.70.0
• Debian Linux 10.0
• Fujitsu M10/M12 series firmware (M10-1, M10-4, M10-4s, M12-1, M12-2, M12-2s)
• Siemens SINEC Infrastructure Network Services
• Splunk Universal Forwarder (including version 9.1.0)

Discovery Timeline

• 2020-12-14 - CVE-2020-8177 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-8177

Vulnerability Analysis

This vulnerability is classified under CWE-99 (Improper Control of Resource Identifiers) and CWE-74 (Improper Neutralization of Special Elements in Output Used by a Downstream Component). The flaw exists in curl's handling of the -J flag, which instructs curl to use the filename specified in the server's Content-Disposition header for saving downloaded content.

When combined with the -O flag for remote output, curl fails to properly validate and sanitize the filename provided by the server. This allows a malicious server to craft a Content-Disposition header containing path traversal sequences or special characters that can cause curl to overwrite files outside the intended download directory.

The attack requires local access, meaning an attacker must either control a malicious server that a local user connects to, or be able to inject malicious content into a trusted server's responses.

Root Cause

The root cause lies in insufficient input validation of server-supplied filenames in the Content-Disposition header. When processing this header with the -J flag enabled, curl versions 7.20.0 through 7.70.0 do not adequately sanitize filenames to prevent directory traversal or overwriting of existing files. The filename parsing logic fails to reject dangerous path components such as .. sequences or absolute paths, allowing attackers to escape the current working directory.

Attack Vector

The attack requires a local user to initiate a curl request to a malicious or compromised server while using the -J (or --remote-header-name) flag combined with -O (or --remote-name). The malicious server responds with a crafted Content-Disposition header containing a specially constructed filename. When curl processes this header, it writes the downloaded content to the attacker-specified location rather than the intended directory.

For exploitation to succeed, the victim must:

1. Use a vulnerable version of curl (7.20.0 - 7.70.0)
2. Execute curl with both -J and -O flags
3. Connect to an attacker-controlled or compromised server
4. Have write permissions to the target file location

The attacker can leverage this to overwrite configuration files, scripts, or other sensitive files that may lead to further compromise when executed or read by privileged processes.

Detection Methods for CVE-2020-8177

Indicators of Compromise

• Unexpected modifications to configuration files, scripts, or binaries with timestamps correlating to curl download activity
• Curl processes executing with -J and -O flags downloading from untrusted or suspicious servers
• Log entries showing curl connections to unusual domains followed by file system changes outside expected download directories
• Presence of files with unusual names or paths that match patterns typically seen in Content-Disposition exploitation attempts

Detection Strategies

• Monitor process execution for curl invocations using the -J or --remote-header-name flag in combination with -O or --remote-name
• Implement file integrity monitoring (FIM) on critical system files and configuration directories to detect unauthorized modifications
• Deploy network monitoring to identify curl connections to untrusted or newly registered domains
• Create SIEM correlation rules linking curl process execution with subsequent file modification events outside expected paths

Monitoring Recommendations

• Enable comprehensive logging of curl command executions including full command-line arguments
• Configure endpoint detection solutions to alert on file writes following curl processes, particularly those targeting sensitive directories
• Monitor for curl binary usage patterns that deviate from baseline behavior in your environment
• Implement network segmentation and egress filtering to limit curl connections to approved external resources

How to Mitigate CVE-2020-8177

Immediate Actions Required

• Upgrade curl to version 7.71.0 or later immediately on all affected systems
• Audit systems for curl versions in the vulnerable range (7.20.0 - 7.70.0) using package management tools or direct version queries
• Review scripts and automation that use curl with -J flag and assess the trust level of destination servers
• Implement application whitelisting or restrictions on curl flag usage where possible

Patch Information

The vulnerability was addressed in curl version 7.71.0, released by the curl project. Organizations should update to version 7.71.0 or later. For detailed information on the fix, consult the official curl security advisory. Additional vendor advisories are available from Siemens, Debian, and Oracle.

Firmware updates for affected Fujitsu M10/M12 series hardware should be obtained through Fujitsu support channels. Splunk Universal Forwarder users should upgrade to patched versions as specified in Splunk security advisories.

Workarounds

• Avoid using the -J flag with curl when downloading from untrusted or unverified servers
• Use explicit local filenames with the -o flag instead of relying on server-provided names via -J
• Implement network-level controls to restrict curl access to trusted, verified destinations only
• Run curl operations in sandboxed environments or containers with limited filesystem write access

# Recommended: Use explicit filename instead of -J flag
curl -o known_safe_filename.tar.gz https://trusted-server.com/download

# Avoid this pattern with untrusted servers:
# curl -J -O https://untrusted-server.com/file

# Verify curl version to ensure patched release
curl --version
# Ensure version is 7.71.0 or later