CVE-2026-70368: Stunnel Buffer Overflow Vulnerability

CVE-2026-70368 Overview

CVE-2026-70368 is a stack-based out-of-bounds read vulnerability in the s_vlog function of stunnel. The flaw occurs when the function handles oversized log messages through vsnprintf. A remote attacker with network access to a stunnel service can send protocol inputs that produce log messages exceeding 1024 bytes. The result is an out-of-bounds stack read that can crash the service. In certain corner cases, the same defect can replace a trailing sequence of \n characters with \0, corrupting logged data. The weakness is classified under [CWE-125] (Out-of-bounds Read).

Critical Impact
Remote, unauthenticated attackers can trigger stunnel service crashes and induce integrity issues in log output by sending crafted protocol traffic that produces log lines longer than 1024 bytes.

Affected Products

stunnel (versions containing the vulnerable s_vlog implementation)
Distributions packaging vulnerable stunnel builds, tracked in the Red Hat CVE-2026-70368 Advisory
Any service fronted by an affected stunnel instance exposed to untrusted networks

Discovery Timeline

2026-08-04 - CVE-2026-70368 published to NVD
2026-08-06 - Last updated in NVD database

Technical Details for CVE-2026-70368

Vulnerability Analysis

The defect resides in s_vlog, the logging routine that formats messages before dispatching them to configured log sinks. s_vlog uses vsnprintf to render variadic input into a fixed 1024-byte stack buffer. When the formatted output would exceed that buffer, subsequent handling reads past the intended boundary on the stack. The out-of-bounds read produces process termination in the common case. Denial of service against a TLS termination or tunneling proxy directly affects the availability of the services that depend on it.

Root Cause

The root cause is unsafe post-processing of the vsnprintf return value. vsnprintf returns the number of bytes that would have been written if the buffer were large enough, not the number actually written. When s_vlog uses that return value to index into the fixed-size stack buffer, for example when trimming trailing newline characters, it reads bytes beyond the buffer. The same indexing error is what allows a trailing \n run to be overwritten with \0 outside the intended region in edge cases.

Attack Vector

Exploitation is remote and requires no authentication or user interaction. An attacker sends protocol data to any listening stunnel service where that data influences a log message. Common triggers include oversized client identifiers, unusually long SNI values, or malformed protocol fields that stunnel logs verbatim. Any input path that flows into s_vlog and produces a message longer than 1024 bytes is sufficient to reach the vulnerable code. Refer to the Red Hat Bug Report #2462029 for additional context on reproduction conditions.

// No verified proof-of-concept code is published for CVE-2026-70368.
// The vulnerability is triggered by protocol inputs that cause s_vlog
// to format a log message longer than 1024 bytes via vsnprintf.

Detection Methods for CVE-2026-70368

Indicators of Compromise

Unexpected termination or repeated restarts of the stunnel process, particularly following bursts of inbound TLS connections.
Truncated or malformed entries in stunnel logs, including lines where trailing newlines appear to be stripped or replaced.
Inbound connections carrying abnormally long protocol fields such as SNI, client identifiers, or handshake extensions.

Detection Strategies

Monitor stunnel service health and correlate crash events with concurrent inbound traffic patterns.
Inspect TLS handshake telemetry for oversized fields that could produce log lines exceeding 1024 bytes.
Alert on core dumps or segmentation faults originating from stunnel worker processes.

Monitoring Recommendations

Forward stunnel logs and process supervision events into a centralized analytics tier for correlation across hosts.
Track baseline connection volumes and field lengths per stunnel endpoint to surface anomalous protocol traffic.
Enable auditing on the stunnel binary and configuration files to detect tampering following any crash event.

How to Mitigate CVE-2026-70368

Immediate Actions Required

Inventory all stunnel deployments and identify externally reachable listeners.
Apply vendor updates as soon as they are available through your distribution's package manager.
Restrict network exposure of stunnel services to trusted sources until patches are deployed.

Patch Information

Refer to the Red Hat CVE-2026-70368 Advisory for current package status and fix availability. Track upstream stunnel releases and rebuild any downstream packages that embed a vulnerable copy of the s_vlog routine. Confirm that the fix corrects the use of the vsnprintf return value so that buffer indexing cannot exceed the 1024-byte log buffer.

Workarounds

Lower stunnel logging verbosity so that attacker-controlled protocol fields are less likely to reach s_vlog with large payloads.
Place a filtering proxy or network access control in front of stunnel to drop connections with oversized protocol fields.
Enforce firewall rules that restrict stunnel listeners to authenticated peers or known client networks.

bash

# Example: restrict stunnel exposure with a host firewall rule
# Replace 203.0.113.0/24 with your trusted client network
iptables -A INPUT -p tcp --dport 443 -s 203.0.113.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 443 -j DROP

# Example: reduce stunnel log verbosity in /etc/stunnel/stunnel.conf
# debug = 4  # lower than the default to limit logged content