CVE-2026-109438 in 3proxy
Summary
by MITRE • 10/15/2026
3proxy from 0.9.7 through 1.0.1, fixed in commit 206c923, contains an uninitialized stack memory disclosure vulnerability that allows unauthenticated attackers to leak stack bytes via the SOCKS4 reply path when the external-NAT option (-Ne) is configured. Attackers can send a client probe that fails the SOCKS version check to trigger a reply containing uninitialized bytes from the sin structure, exposing high bytes of a userspace pointer that weakens ASLR protections.
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Analysis
by VulDB Data Team • 10/15/2026
The vulnerability identified in 3proxy versions ranging from 0.9.7 through 1.0.1 represents a critical information disclosure flaw rooted in improper memory initialization within the application's network handling logic. Specifically, this issue resides in the SOCKS4 reply path when the external-NAT option is enabled via the -Ne configuration flag. The core technical deficiency involves the failure to properly initialize stack-allocated structures before they are transmitted back to a client. In C-based applications like 3proxy, local variables on the stack often retain residual data from previous function calls or system operations if not explicitly zeroed out prior to use. When an attacker interacts with the vulnerable component, this uninitialized memory state is inadvertently exposed rather than being sanitized, leading directly to sensitive information leakage.
The operational mechanism of exploitation relies on a specific interaction sequence involving protocol validation and response generation. An unauthenticated remote attacker can initiate a connection by sending a client probe that intentionally fails the SOCKS version check required for valid communication. Instead of rejecting the request cleanly or returning an error code with properly initialized fields, the application proceeds to construct a reply packet using a socket address structure, typically sin, which contains uninitialized bytes. Because these memory locations were not cleared before being copied into the outgoing network buffer, they contain whatever data previously existed at that stack location. This results in the transmission of arbitrary stack contents back to the attacker over the network connection.
The security implications of this vulnerability are significant due to its potential impact on system-level protections such as Address Space Layout Randomization (ASLR). ASLR is a memory protection process designed to prevent exploitation of memory corruption vulnerabilities by randomizing the locations where program code and data are loaded in memory, thereby making it difficult for attackers to predict addresses. By leaking high bytes of user-space pointers from the stack, an attacker can gather partial information about the current memory layout of the 3proxy process. This leaked pointer information reduces the entropy required to bypass ASLR, effectively weakening this critical defense mechanism and paving the way for more sophisticated attacks such as return-oriented programming (ROP) or direct code execution exploits that rely on precise address knowledge.
From a classification perspective, this vulnerability aligns with CWE-200: Exposure of Sensitive Information to an Unauthorized Actor, specifically falling under the sub-category of information leakage through uninitialized memory variables. In terms of adversarial tactics, it maps to ATT&CK technique T1597.002: Probe Remote Services, as the initial step involves probing a service for vulnerabilities or misconfigurations that lead to data exposure. The vulnerability also touches upon CWE-457: Use of Uninitialized Variable, highlighting the fundamental coding error where variables are used before being assigned a value by the program logic.
Mitigation strategies must prioritize immediate patching and configuration hardening. Organizations running affected versions should upgrade 3proxy to version 1.0.2 or later, which includes commit 206c923 that addresses this memory initialization flaw. For environments where upgrading is not immediately feasible, disabling the external-NAT option (-Ne) if it is not strictly required can mitigate the risk by removing the vulnerable code path from execution. Additionally, deploying network-level controls such as intrusion detection systems to monitor for anomalous traffic patterns associated with SOCKS protocol anomalies may provide an additional layer of defense against exploitation attempts while patching efforts are underway.