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CVE-2025-61600: Unbounded Memory Allocation Vulnerability in Stalwart Mail Server

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Overview

The Stalwart mail and collaboration server is facing a potential cyber threat. A critical vulnerability, CVE-2025-61600, has been identified in versions 0.13.3 and below. This vulnerability could allow a remote attacker to exhaust the server’s memory, potentially risking a system compromise or data leakage. Hence, it is vital for organizations using the Stalwart server to take immediate action to mitigate this risk.

Vulnerability Summary

CVE ID: CVE-2025-61600
Severity: High (7.5 CVSS score)
Attack Vector: Network
Privileges Required: None
User Interaction: None
Impact: Potential system compromise or data leakage due to server memory exhaustion

Affected Products

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Product | Affected Versions

Stalwart Mail Server | 0.13.3 and below

How the Exploit Works

The vulnerability resides in the IMAP protocol parser of the Stalwart server. The CommandParser implementation enforces size limits on its dynamic buffer in most parsing states, but there are several state handlers that omit these validation checks. This omission leads to an unbounded memory allocation issue, enabling a remote attacker to send specific requests that can exhaust the server’s memory. This potential exhaustion could trigger the system’s out-of-memory (OOM) killer, causing a denial of service.

Conceptual Example Code

Below is a conceptual example of how the vulnerability might be exploited. This pseudocode could be part of a script that continuously sends requests to the server without proper size validation checks, leading to memory exhaustion.

while (true) {
POST /imap/protocol HTTP/1.1
Host: target.example.com
Content-Type: application/json
{ "large_size_request": "A"*1000000000 }
}

Mitigation Guidance

The immediate solution is to update the Stalwart server to version 0.13.4, where the issue is fixed. Alternatively, you can implement rate limiting and connection monitoring at the network level as a workaround. However, this does not provide complete protection. For temporary mitigation, using a Web Application Firewall (WAF) or an Intrusion Detection System (IDS) can help until the vendor patch is applied.

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Disclaimer:

The information and code presented in this article are provided for educational and defensive cybersecurity purposes only. Any conceptual or pseudocode examples are simplified representations intended to raise awareness and promote secure development and system configuration practices.

Do not use this information to attempt unauthorized access or exploit vulnerabilities on systems that you do not own or have explicit permission to test.

Ameeba and its authors do not endorse or condone malicious behavior and are not responsible for misuse of the content. Always follow ethical hacking guidelines, responsible disclosure practices, and local laws.

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