CVE-2026-116738 in VMime
Summary
by MITRE • 10/27/2026
VMime through 0.9.2 contains an integer underflow in the uuEncoder::decode function that allows remote attackers to trigger a stack-based buffer overflow via crafted uuencoded MIME parts. Attackers can send an email with a uuencoded line whose length character is 'b' but is not a 'begin' line, corrupting stack memory to crash the application or potentially execute code.
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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 security flaw rooted in improper input validation and arithmetic handling within the uuEncoder::decode function. This component is responsible for decoding UU-encoded data, a legacy encoding scheme often found in email attachments to ensure binary files can be transmitted over text-based protocols like SMTP. The core technical issue stems from an integer underflow condition that occurs when processing specific malformed inputs. Specifically, the vulnerability is triggered by crafted uuencoded MIME parts where the length character of a line is set to 'b' without being preceded by a proper begin header. In standard UU encoding, the first byte of each data block indicates the number of bytes in that block, with 'b' representing eleven decimal units. However, when this value appears outside the context of a valid begin command or within an improperly structured stream, the decoding logic fails to correctly calculate buffer sizes or offsets. This miscalculation leads to negative values being used as array indices or memory allocation parameters, resulting in an integer underflow that compromises the integrity of the stack-based buffer management.
From a technical perspective, this flaw allows for a stack-based buffer overflow, which is one of the most dangerous classes of software vulnerabilities due to its potential for arbitrary code execution. When the application processes the maliciously crafted email content, the incorrect length calculation causes the decoder to write data beyond the boundaries of the allocated memory space on the call stack. This overwriting can corrupt critical return addresses, saved frame pointers, and other control flow data structures stored in that region of memory. An attacker leveraging this vulnerability does not need authentication or prior access; they simply need to send a specially constructed email containing the malformed uuencoded part. The remote nature of the attack vector significantly increases the risk profile, as it can be exploited over network boundaries without any user interaction beyond receiving and processing the message, depending on how the mail client handles incoming data automatically.
The operational impact of this vulnerability is severe, encompassing both availability and confidentiality concerns. In its most immediate manifestation, the buffer overflow causes a segmentation fault or similar memory access violation, leading to the crash of the VMime application or any software utilizing it for email processing. This denial-of-service condition disrupts mail services and can cause instability in systems relying on continuous uptime. More critically, if an attacker successfully manipulates the overwritten stack data with precise shellcode or return-oriented programming gadgets, they may achieve remote code execution. This would allow full compromise of the underlying system, granting the attacker elevated privileges to install malware, exfiltrate sensitive information, or use the compromised host as a pivot point for further network intrusion. The severity is compounded by the fact that uu encoding remains in common use despite its age, meaning many legacy systems and applications are still susceptible to this type of attack.
To mitigate these risks, organizations must prioritize immediate patching to upgrade VMime to version 0.9.3 or later, where the integer underflow has been addressed through rigorous input validation and bounds checking. In environments where upgrading is not immediately feasible, defensive measures should include deploying intrusion detection systems configured to detect anomalous patterns in email headers and body content indicative of malformed uu encoding attempts. Additionally, implementing strict MIME parsing policies that reject any encoded data lacking proper begin-end delimiters can prevent the exploitation vector from being triggered. It is also advisable to disable automatic processing of uu-encoded attachments if such functionality is not strictly required by business operations. From a broader security architecture standpoint, this vulnerability aligns with CWE-190 Integer Overflow or Wrap Around and CWE-121 Stack-based Buffer Overflow, while its exploitation method corresponds to MITRE ATT&CK technique T1568.002 Dynamic Resolution, where attackers use encoded payloads to evade signature-based detection mechanisms. Regular security audits of email processing libraries and adherence to secure coding standards that mandate explicit validation of all input lengths are essential preventive measures against similar vulnerabilities in the future.