How secure is your Proxy? Step-by-step guide to exploit Tinyproxy zero-day vulnerability

2024-05-06 20:46:06

Cisco Talos has reported a major security flaw, identified as CVE-2023-49606, in the widely used Tinyproxy software. This vulnerability, resulting from improper handling of HTTP connection headers, exposes systems to potential remote code execution, which could allow attackers to take control of affected servers.

OVERVIEW OF VULNERABILITY

A minimalist, open source HTTP proxy, Tinyproxy is designed for simplicity and efficiency, making it popular in environments where lightweight proxy servers are required. The vulnerability in question arises in the parsing of HTTP connection headers, a critical component intended to specify which headers are persistent during an HTTP session.

According to Cisco Talos Vulnerability Report (TALOS-2023-1889), specific versions of Tinyproxy, namely 1.11.1 and 1.10.0, are affected. The flaw exists in Tinyproxy’s remove_connection_headers function, where a use-after-free error may occur. This type of software error is particularly dangerous as it can cause memory corruption, providing the attacker with an avenue to execute arbitrary code on the host machine.

TECHNICAL BREAKDOWN

The problem arises when Tinyproxy handles HTTP headers that are supposed to be removed according to the Connection header directives. If an attacker manipulates the Connectiono headers, it can cause reuse of previously freed memory. Proxy-ConnectionThis after-free usage scenario is triggered as follows:

• Tinyproxy iterates over the headers specified in the Connectiono Proxy-Connection header.
• Each specified header is removed from a key-value store (hashofheaders) and its memory is subsequently freed.
• However, due to an error in the handling of these headers, the freed memory can be reused before the function completes, causing memory corruption.

This vulnerability is particularly concerning because it can be exploited with a simple malformed HTTP request, making it easy for an attacker to exploit and potentially take over the system running Tinyproxy.

EXPLOTABILITY AND IMPACT

The CVSSv3 score for this vulnerability is 9.8 (critical), underscoring its severity. An attacker does not need authenticated access to the target system, making it accessible to any malicious entity that can send HTTP requests to the affected proxy. Potential impacts include unauthorized access, data breaches, service interruption, and in severe cases, complete system compromise.

A use-after-free problem occurs when an application continues to use a memory reference (pointer) after it has freed the memory referenced by the pointer. This can lead to unpredictable behavior, including application crashes, data corruption, and arbitrary code execution.

EXPLOITATION EXAMPLE FOR CVE-2023-49606

Stage setup: Imagine a scenario where a server is running Tinyproxy version 1.11.1. This version is known to have a use-after-free vulnerability in the way it processes HTTP connection headers.

Step-by-step exploitation:

Initial HTTP request by attacker:

• The attacker creates an HTTP request that includes specially crafted Connection headers. The goal is to manipulate these headers to trigger the use condition after release.
• HTTP request example:
  • GET / HTTP/1.1
  • Host: vulnerable-proxy.com
  • Connection: keep-alive, maliciousHeader
  • maliciousHeader: freeThis

Processing the Request:

• Tinyproxy receives the request and parses the headers. According to the specifications, you must remove the headers listed in the Connection header after processing the request.
• The remove_connection_headers function is called to handle this task.

Activating use after release:

• During processing, Tinyproxy removes maliciousHeader and frees the memory associated with it. However, due to a failure in proxy memory management, this freed memory could be reallocated for new data while still being referenced by the current process.
• Suppose the freed memory is quickly reallocated and filled with data controlled by the attacker via another part of the application (or another concurrently processed request).

Exploiting freed memory:

• The attacker anticipates that Tinyproxy will access this freed (and now reallocated) memory again before the function completes. If this memory now contains executable code or data created by the attacker, it may be executed or misused by the application.
• For example, if the memory location is used to store a function pointer, the attacker could overwrite this pointer with the address of their malicious code.

Running arbitrary code:

• When Tinyproxy tries to access the “freed” memory as part of its normal operations (for example, to record details or continue processing headers), it inadvertently executes the code pointed to by the overwritten pointer.
• This could allow the attacker to execute arbitrary code on the server running Tinyproxy, potentially giving them full control over the server.

Example of malicious impact:

• The attacker could install backdoors, leak sensitive data, launch further attacks from the compromised server, or disrupt server operations.

MITIGATION AND RESPONSE

As of the latest Cisco Talos update, there has been no official patch or maintenance release from the Tinyproxy maintainers to address this vulnerability. Organizations using Tinyproxy are advised to closely monitor their systems for unusual activity and implement additional security measures until a patch is available. This may include:

• Implement web application firewalls (WAF) to filter malicious HTTP headers.
• Isolate the proxy server from critical internal networks.
• Periodically update all software to the latest versions whenever possible.

For Tinyproxy users and administrators, it is critical to understand the technical mechanics of this vulnerability and remain vigilant in monitoring your network traffic. It is recommended that immediate action be taken to mitigate the risks associated with this critical vulnerability until a permanent fix is ​​issued.

He is a well-known expert in mobile security and malware analysis. He studied Computer Science at NYU and started working as a cybersecurity analyst in 2003. He actively works as an antimalware expert. He also worked for security companies such as Kaspersky Lab. His daily work includes investigating new malware and cybersecurity incidents. He also has a deep level of knowledge in mobile security and mobile vulnerabilities.

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