CVE-2026-34838

CriticalRCE

Published: 02 April 2026

Published

02 April 2026

Modified

15 April 2026

KEV Added

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Patch

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CVSS Score 9.9 CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H

EPSS Score 0.0078 74.0th percentile

Risk Priority 20 60% EPSS · 20% KEV · 20% CVSS

Summary

CVE-2026-34838 is a critical-severity Deserialization of Untrusted Data (CWE-502) vulnerability in Intermesh Group-Office. Its CVSS base score is 9.9 (Critical).

Operationally, exploitation aligns with the MITRE ATT&CK technique Exploit Public-Facing Application (T1190); ranked in the top 26.0% of CVEs by exploit likelihood; it is not currently listed in the CISA KEV catalog.

The strongest mitigations our analysis identified are NIST 800-53 SI-10 (Information Input Validation) and SI-2 (Flaw Remediation).

Threat & Defense at a Glance

What attackers do: exploitation maps to Exploit Public-Facing Application (T1190). What defenders deploy: see the NIST 800-53 controls recommended below.

Threat & Defense Details

Mitigating Controls (NIST 800-53 r5)AI

SI-2 Flaw Remediation good match

prevent

Directly mitigates the insecure deserialization vulnerability by requiring timely remediation through vendor patches in versions 6.8.156, 25.0.90, and 26.0.12.

SI-10 Information Input Validation good match

prevent

Prevents exploitation by validating and sanitizing setting strings to block injection of malicious serialized objects like FileCookieJar before deserialization.

SI-7 Software, Firmware, and Information Integrity partial match

detect

Detects unauthorized file writes and integrity violations resulting from successful deserialization exploits leading to RCE.

MITRE ATT&CK Enterprise TechniquesAI

T1190 Exploit Public-Facing Application Initial Access

Adversaries may attempt to exploit a weakness in an Internet-facing host or system to initially access a network.

attack.mitre.org →

Why these techniques?

Insecure deserialization in Group-Office web application enables remote exploitation of a public-facing application for arbitrary file write and RCE.

Confidence: HIGH · MITRE ATT&CK Enterprise v19.0

NVD Description

Group-Office is an enterprise customer relationship management and groupware tool. Prior to versions 6.8.156, 25.0.90, and 26.0.12, a vulnerability in the AbstractSettingsCollection model leads to insecure deserialization when these settings are loaded. By injecting a serialized FileCookieJar object into a…

setting string, an authenticated attacker can achieve Arbitrary File Write, leading directly to Remote Code Execution (RCE) on the server. This issue has been patched in versions 6.8.156, 25.0.90, and 26.0.12.

Deeper analysisAI

CVE-2026-34838 is a critical insecure deserialization vulnerability (CWE-502, CVSS 9.9) affecting Group-Office, an enterprise customer relationship management and groupware tool. Versions prior to 6.8.156, 25.0.90, and 26.0.12 are vulnerable due to a flaw in the AbstractSettingsCollection model, which insecurely deserializes settings loaded from strings. Attackers can exploit this by injecting a serialized FileCookieJar object into a setting string.

An authenticated attacker with low privileges (PR:L) can exploit the vulnerability remotely (AV:N) with low attack complexity (AC:L) and no user interaction (UI:N), achieving a scope change (S:C) with high confidentiality, integrity, and availability impact (C:H/I:H/A:H). Successful exploitation enables arbitrary file write on the server, directly resulting in remote code execution (RCE).

The vulnerability is patched in Group-Office versions 6.8.156, 25.0.90, and 26.0.12. Security practitioners should upgrade immediately and review the official security advisory at https://github.com/Intermesh/groupoffice/security/advisories/GHSA-h22j-frrf-5vxq along with release notes at https://github.com/Intermesh/groupoffice/releases/tag/v6.8.156, https://github.com/Intermesh/groupoffice/releases/tag/v25.0.90, and https://github.com/Intermesh/groupoffice/releases/tag/v26.0.12 for mitigation details.