CVE-2026-116739 in VMime
Summary
by MITRE • 10/27/2026
VMime through 0.9.2 contains an information disclosure vulnerability in the uuEncoder::decode function that writes uninitialized stack memory into decoded output for over-long lines. Attackers can send a crafted uuencode or x-uuencode MIME part with a large line length byte to leak up to 15 bytes of stack contents per line in decoded attachments.
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Analysis
by VulDB Data Team • 10/27/2026
The vulnerability identified in VMime versions through 0.9.2 represents a critical information disclosure flaw rooted in improper memory management within the uuEncoder::decode function. This component is responsible for processing MIME parts encoded using the UUencode or x-uuencode formats, which are legacy encoding schemes often used to transmit binary data over text-based protocols like email. The core technical failure occurs when the decoder processes lines that exceed a specific length threshold defined by the line length byte in the encoded stream. Instead of properly initializing the buffer space allocated for decoding these oversized segments, the function writes directly into uninitialized stack memory. This lack of initialization means that whatever data previously resided on the call stack remains present and accessible to be written out as part of the decoded output.
From a technical perspective, this is a classic instance of an uncontrolled resource consumption leading to sensitive information exposure via side channels or direct leakage. When an attacker crafts a malicious MIME part with an artificially large line length byte, they trigger the decoder's path that handles over-long lines. Because the memory buffer used for decoding has not been zeroed out or otherwise initialized before use, any bytes in the decoded output beyond what is actually provided by the encoded data will contain residual values from previous stack operations. These residual values can include sensitive context such as return addresses, local variable contents, pointers to heap allocations, or other application-specific secrets that were recently placed on the stack during normal execution flow.
The operational impact of this vulnerability allows remote attackers to exfiltrate up to 15 bytes of stack memory per line in decoded attachments. While fifteen bytes may seem limited, it is sufficient for significant security breaches depending on the context. In many applications, even small amounts of leaked stack data can reveal pointers that facilitate further exploitation through buffer overflow attacks or use-after-free scenarios by allowing an attacker to map out the memory layout of the process. Additionally, if sensitive configuration details, authentication tokens, or internal state information happen to reside in those specific stack locations at the time of decoding, they are directly exposed to the recipient of the malicious email attachment. This transforms a simple parsing error into a potent vector for reconnaissance and potential remote code execution preparation.
This vulnerability aligns with CWE-200: Information Exposure, specifically falling under categories related to exposure of sensitive information through error messages or improper output handling. It also relates closely to CWE-134: Use of Externally-Controlled Format String if the uninitialized memory contents are interpreted in unexpected ways by subsequent processing steps. In terms of attack vectors and tactics, this behavior is consistent with ATT&CK technique T1082: System Information Discovery, where an adversary gathers information about the target system to aid in further exploitation. The specific mechanism of leaking stack data via malformed input also shares characteristics with CWE-94: Improper Control of Generation of Code (Code Injection) if the leaked pointers are used for subsequent injection attacks, though primarily it serves as a reconnaissance step under T1059: Command and Scripting Interpreter or general information gathering.
Mitigation strategies must focus on both immediate patching and long-term defensive coding practices. The primary remediation is to upgrade VMime to version 0.9.3 or later, where the uuEncoder::decode function has been corrected to properly initialize stack buffers before use. For systems that cannot immediately update due to dependency constraints, input validation at the mail server level can provide a layer of defense by rejecting MIME parts with line lengths exceeding standard limits defined in RFC standards for UUencoding. Developers should also implement strict bounds checking when processing encoded data and ensure that all memory buffers are explicitly initialized before being populated with decoded content. Adopting static analysis tools configured to detect uninitialized variable usage during the development lifecycle can help prevent similar vulnerabilities from being introduced into other parts of the codebase, ensuring a more robust security posture against information disclosure attacks.