CVE-2021-26858: Microsoft Exchange Server Remote Code Execution Vulnerability

Recommended actionPatch now

CISA confirms exploitation in the wild and lists 2022-05-03 as the remediation due date.

Patch available

What, why and how

Microsoft Exchange Server contains an unspecified vulnerability that allows for remote code execution. This vulnerability is part of the ProxyLogon exploit chain.

What

Microsoft Exchange Server contains an unspecified vulnerability that allows for remote code execution. This vulnerability is part of the ProxyLogon exploit chain.

Why

The current structured CVE record identifies a security weakness, but the root cause requires confirmation in the linked vendor material.

How

An attacker operating through local access may attempt exploitation without authentication after a user interaction. If successful, the issue may execute code or commands in the affected security context.

What

Microsoft Exchange Server contains an unspecified vulnerability that allows for remote code execution. This vulnerability is part of the ProxyLogon exploit chain.

Why

The current structured CVE record identifies a security weakness, but the root cause requires confirmation in the linked vendor material.

How

Exploit reality and attack path

CVSS severity, EPSS forecast probability, public exploit material and CISA-confirmed exploitation are separate signals.

Observed exploitation

Confirmed exploitation and public exploit material are separate signals. Attacks can occur without public proof-of-concept or exploit code.

Confirmed in the wild

CISA added this CVE to its Known Exploited Vulnerabilities catalogue on 2021-11-03. Known ransomware campaign use is recorded.

Public PoC / exploit material

Confirmed exploitation and public exploit material are separate signals. Attacks can occur without public proof-of-concept or exploit code.

None recorded

No exploit-tagged reference or CISA SSVC proof-of-concept state is currently recorded. Research may still exist outside the structured feeds.

Likely attack path: local access → vulnerable operation → execute code or commands in the affected security context
Attack surface: Local
Privileges required: None: unauthenticated exploitation is possible
User interaction: Required interaction required
Attack complexity: Low: no specialised conditions are recorded
Security boundary: Unchanged: impact remains within the vulnerable component's security authority
Weakness ? CWE means Common Weakness Enumeration.: CWE not yet assigned
CVSS vector ? CVSS means Common Vulnerability Scoring System. The vector records the metric values used to calculate technical severity.: CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H/E:F/RL:O/RC:C
Common Vulnerability Scoring System 3.1: the compact vector below is decoded into plain language.
AVLocalAttack vector: The attacker needs local access to the vulnerable system.ACLowAttack complexity: No specialised conditions are required beyond attacker-controlled input.PRNonePrivileges required: The attacker does not need an account or existing privileges.UIRequiredUser interaction: Another user must perform an action for exploitation to succeed.SUnchangedScope: The security impact remains within the vulnerable component's authority.CHighConfidentiality impact: A successful attack can cause a major loss.IHighIntegrity impact: A successful attack can cause a major loss.AHighAvailability impact: A successful attack can cause a major loss.
Post-exploitation / living off the land: After compromise, an attacker may use built-in shells, scripting engines, scheduled tasks and native network utilities for discovery, persistence or movement. This is a plausible LoTL path, not evidence that it has occurred for every attack.

UnauthenticatedRemote code executionCISA KEV

Official authority intelligence

Only matched European and national findings are included. Language selectors and unavailable sources are omitted.

ENISA EUVD · EUVD-2021-13642Known-exploited evidence recorded

Microsoft Exchange Server Remote Code Execution Vulnerability

Official EUVD record ↗

Canadian Centre for Cyber Security · English · AL21-007APT Actors Target U.S. and Allied Networks - update 1

On 15 April 2021, the NSA, CISA, and FBI issued a Joint Cybersecurity Advisory [ 1 ], drawing attention to widespread scanning and exploitation of several vulnerabilities by APT actors. The advisory states that the APT actors are targeting vulnerable systems to obtain authentication credentials and enable further access within networks, including national security and government systems. Examples of recent activity include compromising SolarWinds Orion updates, targeting COVID-19 research facilities, and leveraging a VMWare vulnerability for authentication abuse. The Cybersecurity Advisory highlights the following vulnerabilities being exploited: CVE-2018-13379 Fortinet [ 2 ], [ 3 ] CVE-2019-9670 Zimbra [ 3 ] CVE-2019-11510 Pulse Secure [ 3 ], [ 4 ] CVE-2019-19781 Citrix [ 3 ] CVE-2020-4006 VMware [ 5 ] UPDATE 1 [ 6 ] CVE-2020-5902 F5 Big-IP CVE-2020-14882 Oracle WebLogic CVE-2021-21972 VMWare vSphere CVE-2019-1653 Cisco router CVE-2019-2725 Oracle WebLogic Server CVE-2019-7609 Kibana CVE-2021-26857 Exchange (SOAP payload) CVE-2021-26858 Exchange (Arbitrary files) CVE-2021-27065 Exchange (Arbitrary files) The Cyber Centre is highlighting this advisory, as it provides important information to system owners and operators responsible for defending their systems and networks from cyber threats. There are software updates and mitigations for these vulnerabilities. See past reporting by the Cyber Center and partners for more details. Should activity matching the content of this Alert be discovered, recipients are encouraged to contact the Cyber Centre by email ( contact@cyber.gc.ca ) or by telephone ( 1-833-CYBER-88 or 1-833-292-3788 ).

Microsoft Exchange Server Remote Code Execution Vulnerability

Evidence used

Compensating controls

Verification

EUVD-2021-13642

What, why and how

Exploit reality and attack path

Official authority intelligence

Patch and workaround

Evidence and provenance