CVE-2023-27533: Haxx Curl TELNET Protocol RCE Vulnerability

CVE-2023-27533 Overview

A vulnerability in input validation exists in curl versions prior to 8.0 during communication using the TELNET protocol. This flaw may allow an attacker to pass on maliciously crafted user names and "telnet options" during server negotiation. The lack of proper input scrubbing allows an attacker to send content or perform option negotiation without the application's intent. This vulnerability could be exploited if an application allows user input, thereby enabling attackers to execute arbitrary code on the system.

Critical Impact

Improper input validation in curl's TELNET protocol handling enables attackers to inject malicious content during server negotiation, potentially leading to arbitrary code execution on affected systems.

Affected Products

• Haxx curl (versions prior to 8.0)
• Fedora Project Fedora
• NetApp Active IQ Unified Manager
• NetApp Clustered Data ONTAP 9.0
• NetApp H300S, H500S, H700S, H410S (firmware and hardware)
• Splunk Universal Forwarder

Discovery Timeline

• March 30, 2023 - CVE-2023-27533 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2023-27533

Vulnerability Analysis

This vulnerability stems from insufficient input validation within curl's TELNET protocol implementation. When curl establishes a TELNET connection, it processes user-supplied data including usernames and telnet options during the negotiation phase with the remote server. Due to improper sanitization, an attacker can embed malicious content within these fields that gets passed directly to the server without adequate filtering.

The vulnerability is classified under CWE-74 (Improper Neutralization of Special Elements in Output Used by a Downstream Component) and CWE-75 (Failure to Sanitize Special Elements into a Different Plane). These classifications indicate that the core issue lies in curl's failure to properly neutralize user-controllable input before using it in protocol communications.

The attack requires user interaction, typically in scenarios where an application using libcurl accepts user-provided URLs or connection parameters and passes them to curl for processing without additional validation.

Root Cause

The root cause of CVE-2023-27533 is the absence of proper input scrubbing in curl's TELNET protocol handler. When processing user-supplied credentials and telnet options, the implementation fails to sanitize special characters and control sequences that have semantic meaning in the TELNET protocol. This allows attackers to inject arbitrary telnet commands or options that the application did not intend to send.

The vulnerable code path processes user input during the TELNET negotiation phase without verifying that the input contains only safe, expected characters. This oversight enables attackers to manipulate the protocol conversation between the client and server.

Attack Vector

The attack vector for this vulnerability is network-based and requires user interaction. An attacker can exploit this flaw through the following attack scenario:

1. The attacker crafts a malicious TELNET URL or connection parameters containing specially crafted username or telnet option values
2. A victim application or user invokes curl with these attacker-controlled parameters
3. During the TELNET negotiation with the server, the malicious payload is transmitted without sanitization
4. The injected content can alter the negotiation behavior, send unintended commands, or manipulate the session

The exploitation mechanism involves embedding control characters or protocol-specific sequences within the username or option fields. For detailed technical information about the exploitation technique, refer to the HackerOne Report #1891474 where this vulnerability was originally reported.

Detection Methods for CVE-2023-27533

Indicators of Compromise

• Unusual TELNET connection attempts originating from applications that typically don't use TELNET
• Network traffic containing malformed or suspicious TELNET option negotiation sequences
• Log entries showing curl invocations with unusually long or encoded usernames in TELNET URLs
• Unexpected telnet option negotiations containing non-standard or malicious payloads

Detection Strategies

• Monitor for curl processes initiating TELNET connections with suspicious command-line arguments
• Implement network-level inspection for anomalous TELNET protocol exchanges during connection negotiation
• Deploy endpoint detection rules to identify curl invocations targeting TELNET URLs with encoded or special characters in user fields
• Review application logs for user-supplied input being passed to curl TELNET connections

Monitoring Recommendations

• Enable verbose logging for applications utilizing libcurl to capture connection parameters
• Implement network security monitoring for outbound TELNET traffic (port 23) from unexpected sources
• Configure SIEM rules to alert on curl processes with TELNET protocol usage patterns
• Establish baseline behavior for TELNET usage in your environment to detect anomalous activity

How to Mitigate CVE-2023-27533

Immediate Actions Required

• Upgrade curl to version 8.0 or later which contains the fix for this vulnerability
• Audit applications that use libcurl to identify any that accept user input for TELNET connections
• Implement input validation at the application level before passing user data to curl
• Consider disabling TELNET protocol support in curl if not required for business operations

Patch Information

The vulnerability is resolved in curl version 8.0 and later releases. Organizations should upgrade their curl installations to the patched version as soon as possible. Multiple vendors have released updates addressing this vulnerability:

• Debian LTS Advisory April 2023
• Fedora Package Announcement
• Gentoo GLSA 202310-12
• NetApp Security Advisory NTAP-20230420-0011

Workarounds

• Disable TELNET protocol support in curl by compiling without TELNET support or using configuration options to restrict protocol usage
• Implement strict input validation for any user-supplied data before passing to curl, rejecting special characters and control sequences
• Use application-level firewalls or proxies to inspect and sanitize TELNET connection parameters
• Restrict curl's protocol options using --proto flag to allow only required protocols and explicitly exclude telnet

# Restrict curl to only use secure protocols, excluding telnet
curl --proto '=https,ftps' [URL]

# Alternatively, explicitly deny telnet protocol
curl --proto '-telnet' [URL]