CVE-2026-70368 in Red Hat
Summary
by MITRE • 08/04/2026
A stack-based out-of-bounds read vulnerability exists in the "s_vlog" function of stunnel, when handling oversized log messages via "vsnprintf". A remote attacker with network access to a stunnel service can send protocol inputs that trigger a log message longer than 1024 bytes, leading to an out-of-bounds stack read and a potential crash. In certain corner cases, the same vulnerability could be used to replace a series of trailing "\n" characters with "\0".
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Analysis
by VulDB Data Team • 08/04/2026
The vulnerability under discussion represents a critical stack-based out-of-bounds read condition within the stunnel secure socket layer implementation specifically affecting the s_vlog function. This flaw manifests when the application processes log messages that exceed the configured buffer size limit of 1024 bytes through the vsnprintf function call, creating a scenario where memory beyond the allocated stack space becomes accessible. The vulnerability operates at the intersection of improper input validation and inadequate buffer management practices, creating a pathway for malicious actors to exploit the application's logging mechanism.
The technical exploitation of this vulnerability occurs when network-based attackers send protocol inputs designed to generate log messages exceeding the predetermined 1024-byte threshold. When stunnel processes these oversized inputs through its logging subsystem, the vsnprintf function fails to properly bounds-check the incoming data before writing to the stack-based buffer. This results in a read operation that accesses memory locations beyond the intended buffer boundaries, potentially exposing sensitive stack content or causing application instability. The out-of-bounds read vulnerability maps directly to CWE-125, which specifically addresses out-of-bounds read conditions in software implementations.
From an operational perspective, this vulnerability presents significant risk to stunnel deployments as it can be triggered remotely without requiring authentication or specialized privileges beyond network access to the target service. The potential for application crash represents a denial of service threat that could disrupt secure communication channels managed by stunnel. Additionally, the secondary impact involving null byte replacement within trailing newline characters introduces memory corruption possibilities that could theoretically be leveraged for more sophisticated exploitation techniques. The vulnerability's remote attack surface and lack of authentication requirements align with ATT&CK technique T1190 for exploitation through network services.
The implementation flaw stems from inadequate input sanitization within the logging subsystem, where stunnel fails to enforce proper bounds checking when processing variable-length formatted strings. This represents a classic buffer overflow precursor that manifests as an out-of-bounds read rather than a write operation, though both scenarios present similar security implications. The vulnerability's manifestation through the vsnprintf function indicates that the application relies on standard C library functions without implementing additional safety mechanisms to prevent buffer overflows during logging operations.
Mitigation strategies should prioritize immediate patching of affected stunnel versions to address the root cause through proper bounds checking implementation. Network segmentation and access controls can help limit exposure by restricting direct network access to stunnel services where possible. Additionally, implementing log message length limits within the application configuration can provide an additional defensive layer against oversized inputs reaching the vulnerable code path. Monitoring for unusual log message patterns or excessive logging activity may help detect exploitation attempts, while regular security assessments of logging subsystems can identify similar vulnerabilities in other applications.
The vulnerability highlights fundamental security weaknesses in how legacy secure communication tools handle input validation and memory management, particularly within critical subsystems like logging that are often overlooked during security reviews. Proper implementation of input sanitization, bounds checking, and defensive programming practices would prevent this class of vulnerability from manifesting in production environments. Organizations should conduct comprehensive audits of their secure communication infrastructure to identify similar issues that could enable remote code execution or information disclosure attacks through analogous pathways in other network services.
The relationship between this vulnerability and established security frameworks demonstrates how seemingly minor implementation flaws can create significant risk profiles when combined with network accessibility and application functionality. The vulnerability's characteristics align with both CWE categorization standards for memory safety issues and ATT&CK framework concepts regarding service exploitation, emphasizing the need for comprehensive security approaches that address multiple attack vectors simultaneously. Proper input validation and bounds checking remain fundamental defensive measures that should be integrated throughout all application development processes to prevent such vulnerabilities from reaching production environments.