CVE-2026-11623: Use-After-Free in tmux 3.6a Image Handling – MEDIUM Severity

Technical summary

The vulnerability is a use-after-free (CWE-416) condition in the image_free function within image.c of tmux. The flaw allows an attacker with local system access to reference memory that has already been freed, potentially leading to information disclosure or denial of service. The CVSS 3.1 score of 4.5 (MEDIUM) reflects the requirement for local access and high attack complexity; the vulnerability vector indicates local attack surface, low privilege requirement, no user interaction needed, and confined impact scope. The CWE-119 classification also indicates potential buffer boundary issues. Public disclosure of exploitation techniques has occurred, but practical weaponization remains difficult due to the prerequisites and complexity involved.

Business impact

For organizations running tmux—commonly used by system administrators, developers, and DevOps teams for terminal multiplexing—this vulnerability poses a localized privilege escalation or information leakage risk. An authenticated user on a shared system could potentially extract sensitive information from tmux sessions or cause a denial of service affecting system administration workflows. The business impact is generally contained because the attack requires local system access, meaning it primarily threatens multi-tenant or shared-access environments. Security teams should assess whether tmux instances contain sensitive data or are used in privileged administrative contexts.

Affected systems

The vulnerability affects tmux versions up to and including 3.6a. No vendor or product data is provided in the source advisory, but tmux is deployed across Linux servers, development workstations, and container environments. Any system running an affected tmux version is potentially vulnerable if local user access is available. Users of tmux on shared systems, containerized environments with inter-user isolation, or systems with privileged administrative sessions should prioritize assessment.

Exploitability

Although a public exploit has been disclosed, practical exploitation is difficult and requires multiple preconditions: the attacker must have local user-level access to the system, must understand the memory layout of the running tmux process, and must successfully trigger the use-after-free condition through deliberate code manipulation. The high attack complexity and requirement for local access significantly reduce the real-world risk compared to remote or network-based vulnerabilities. The public disclosure does not materially change the threat model for most organizations, as opportunistic exploitation remains unlikely.

Remediation

Upgrade tmux to version 3.7-rc or later to resolve the issue. Organizations unable to immediately upgrade should verify whether tmux processes handle sensitive data or administrative functions on shared systems and consider additional access controls if necessary. The patch commit fc6d94a9f8a593bd8b7031650802084385d4ee03 is the exact fix; verify this commit is present in your deployed version.

Patch guidance

Obtain and deploy tmux version 3.7-rc or newer from the official tmux project repository. Verify that the installed version includes commit fc6d94a9f8a593bd8b7031650802084385d4ee03 using 'tmux -V' to confirm the version number and review release notes. For Linux distributions, check whether your package manager offers an updated package; if not, build from source or consider filing a request with your distribution vendor. Test the upgrade in a staging environment first, as terminal multiplexer updates can affect existing sessions and administrative workflows.

Detection guidance

Monitor system logs for tmux process crashes or abnormal terminations, particularly following unusual local user activity. Memory analysis tools (e.g., valgrind, AddressSanitizer) can detect use-after-free conditions in tmux binaries during testing. Check running tmux version across systems using 'tmux -V' and maintain an inventory of deployed versions. Intrusion detection systems should monitor for exploitation attempts, though signature-based detection is limited due to the memory-corruption nature of the flaw; behavioral anomaly detection of tmux crashes may be more effective.

Why prioritize this

Although publicly disclosed, this vulnerability merits a measured response rather than emergency action. The MEDIUM severity rating, requirement for local access, high attack complexity, and contained impact scope all indicate lower priority relative to remote code execution or privilege escalation flaws. However, organizations with multi-tenant systems, shared hosting environments, or systems where tmux is used in security-sensitive contexts should prioritize remediation within standard maintenance windows. Single-user workstations with restricted local access face minimal risk.

Risk score, explained

The CVSS 3.1 score of 4.5 reflects a constellation of limiting factors: local-only attack vector (AV:L), high attack complexity (AC:H), requirement for low user privilege (PR:L), and confined impact limited to confidentiality, integrity, and availability of a single component (S:U with C:L/I:L/A:L). The score does not qualify for HIGH or CRITICAL severity, preserving a realistic threat characterization. The public disclosure is noted but does not automatically elevate the score, as exploitation remains difficult.

Frequently asked questions