What is CVE-2021-26855?

CVE-2021-26855 is Microsoft Exchange Server Remote Code Execution Vulnerability. The supplied enrichment data classifies it as SSRF mapped to CWE-918 Server-Side Request Forgery (SSRF).

Which versions are affected?

Affected product and version evidence is listed in the product table above. Current public advisories should be checked for exact version scope before patch targeting is finalized.

Is there active exploitation?

Yes: CISA KEV signal present. If exploitation is not confirmed from current sources, teams should document that status and continue monitoring KEV, vendor, and telemetry updates.

Yes. The supplied enrichment data lists CVE-2021-26855 in CISA KEV with required action: Apply updates per vendor instructions.. KEV status should be monitored because it can change after initial disclosure or after exploitation becomes publicly confirmed.

Is there a public exploit?

315 public PoC repositories reported; 1 marked weaponized in current dataset. Public exploit or PoC availability should raise triage priority, especially for exposed or business-critical systems.

How can defenders detect it?

Start with web/proxy/WAF logs, application logs, network IDS/IPS, cloud metadata access logs, egress proxy logs where available. Look for vulnerability-class behavior, product errors, suspicious authentication events, exploit probes, and abnormal process activity based only on evidence that applies to the affected product.

How should teams prioritize it?

Prioritize CVE-2021-26855 using CVSS 7.5, EPSS 100%, KEV status, public PoC status, and asset exposure. Internet-facing, privileged, or business-critical deployments should be handled first.

Are workarounds available?

Workarounds are not confirmed from current sources. Fixed-version evidence is listed in the mitigation section above.

Which threat actors are linked publicly?

Publicly linked actor or ransomware signals in the supplied enrichment data: FIN7 (), WIZARD SPIDER (), Mikroceen (), PKPLUG (), APT42 (), Ransomware use: Known. Do not attribute exploitation to a threat actor unless the source evidence explicitly supports that linkage.

What should be done in the next 24 hours?

Identify exposed assets running affected vendor/product/version combinations. Treat as emergency remediation because CISA KEV status is present. Search available logs for exploit probes, errors, authentication anomalies, or suspicious child processes matching the vulnerability class. Open remediation tickets with affected assets, owners, evidence status, and detection coverage so decisions are traceable.

CISA KEVRansomwarehighSSRFpublic exploit

CVE-2021-26855

CVE-2021-26855 — Microsoft Exchange Server Remote Code Execution Vulnerability

Microsoft Exchange Server Remote Code Execution Vulnerability

Published Mar 3, 2021Updated Aug 19, 2026Sources: NVD, Microsoft (CNA), CISA, FIRST EPSS, GitHub, Exploit-DB, Microsoft MSRC, SOCRadar CTI

Triage

Is it exploited, how likely is exploitation, what does it touch, and how severe do the scoring sources call it.

Exploitation

Confirmed

CISA KEV, listed Nov 3, 2021

EPSS

100%

top of FIRST's scoreset

Affects

Microsoft

48 products listed

CVSS base

7.5

high · 4 sources

CISA SSVC assessment

Three decision points CISA publishes for the CVEs it assesses · SSVC 2.0.3. A stakeholder decision, not a severity score.

CISA

Exploitation

Active

none · proof-of-concept · active

Automatable

Yes

can an attacker script all four kill-chain steps

Technical impact

Total

partial · total control of the vulnerable component

Remediation

The vendor's own words where we have them.

Apply Microsoft security update KB5000871 (2021-Mar release) — http://www.microsoft.com/download/details.aspx?familyid=31211a48-0cef-462e-bb11-c36440f80bb3.

CISA required action

Apply updates per vendor instructions.

Federal deadline May 3, 2022 — passed 1,615 days ago

First 24 hours

Ordered from the record's own fields — exposure first, because you cannot patch what you have not found.

Identify exposed assets running affected vendor/product/version combinations.
Treat as emergency remediation because CISA KEV status is present.
Search available logs for exploit probes, errors, authentication anomalies, or suspicious child processes matching the vulnerability class.

Affected scope

Vendor, product and version as the advisories word them.

Vendor	Product	Versions	Status
Microsoft	Microsoft Exchange Server 2013 Cumulative Update 21	15.00.0 to < 15.00.1395.012	Vulnerable
Microsoft	Microsoft Exchange Server 2013 Cumulative Update 22	15.00.0 to < 15.00.1473.006	Vulnerable
Microsoft	Microsoft Exchange Server 2013 Cumulative Update 23	15.00.0 to < 15.00.1497.012	Vulnerable
Microsoft	Microsoft Exchange Server 2016 Cumulative Update 10	15.01.0 to < 15.01.1531.012	Vulnerable
Microsoft	Microsoft Exchange Server 2016 Cumulative Update 11	15.01.0 to < 15.01.1591.018	Vulnerable
Microsoft	Microsoft Exchange Server 2016 Cumulative Update 12	15.01.0 to < 15.01.1713.010	Vulnerable
Microsoft	Microsoft Exchange Server 2016 Cumulative Update 13	15.01.0 to < 15.01.1779.008	Vulnerable
Microsoft	Microsoft Exchange Server 2016 Cumulative Update 14	15.01.0 to < 15.01.1847.012	Vulnerable
Microsoft	Microsoft Exchange Server 2016 Cumulative Update 15	15.01.0 to < 15.01.1913.012	Vulnerable
Microsoft	Microsoft Exchange Server 2016 Cumulative Update 16	15.01.0 to < 15.01.1979.008	Vulnerable
Microsoft	Microsoft Exchange Server 2016 Cumulative Update 17	15.01.0 to < 15.01.2044.013	Vulnerable
Microsoft	Microsoft Exchange Server 2016 Cumulative Update 18	15.01.0 to < 15.01.2106.013	Vulnerable

Attack characteristics

The CVSS vector, decoded. It describes the attack, not your exposure to it.

Availability

Confidentiality

Integrity

Attack Complexity

Low

Attack Vector

Network

Every base score collected

Sources score independently and disagree; each row says who scored it and under which version.

Score	Version	Severity	Expl.	Impact	Source
9.1	CVSS 3.1	critical	3.9	5.2	Microsoft (CNA)
9.8	CVSS 3.1	critical	3.9	5.9	nist.gov (CNA)
7.5	CVSS 2.0	high	10	6.4	NVD
9.1	CVSS 3.1	critical	—	—	Microsoft (CNA), Microsoft

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

What this weakness leads to

MITRE's own consequences and mitigations for the weakness class — the authority's wording, not guidance derived from the CVSS vector.

MITRE

CWE-918 · Server-Side Request Forgery (SSRF)

Read Application Data
Execute Unauthorized Code or Commands::

Weakness & attack patterns

CWE-918Server-Side Request Forgery (SSRF)

Inferred from the weakness class — not observed against this CVE.

CAPEC-664 · Server Side Request Forgery

CISA KEV details

CISA lists a CVE here once it has evidence of exploitation against real targets.

CISAThat month →

Listed

Nov 3, 2021

Federal deadline

May 3, 2022

passed 1,615 days ago

Ransomware use

Known

CISA records ransomware use only for campaigns it has confirmed, so “Unknown” means unrecorded, not ruled out. The deadline binds US federal agencies; for everyone else it is a date to argue against.

Public exploit

Capability, not use: code existing is a different claim from anyone running it.

Repositories

253

1 of the 5 sampled are weaponized

Exploit entries

Exploit-DB and CTI exploit indexes

Malware

Families a CTI source ties to this CVE. One named, with the total it tracks.

Families tracked

Named

avos_locker

elf.avoslocker, Avoslocker, win.avos_locker

Social mentions

Posts naming this CVE. Attention, not evidence of exploitation — it spikes on disclosure and decays whether or not anything is exploited.

2 more posts on this CVEShowing 5 of 7 · Who is discussing this CVE, and how loudlyUnlock free

Detection rules

Sigma and YARA rules a source published for this CVE — the only detection content any connected source supplies.

2 more detection rules on this CVEShowing 5 of 7 · Sigma and YARA content you can deploy todayUnlock free

Detection

Read off the CVSS vector and the weakness class. Starting points, not rules we have tested.

Prioritize edge telemetry for network-reachable Microsoft Exchange Server 2013 Cumulative Update 21, Microsoft Exchange Server 2013 Cumulative Update 22, Microsoft Exchange Server 2013 Cumulative Update 23, Microsoft Exchange Server 2016 Cumulative Update 10, Microsoft Exchange Server 2016 Cumulative Update 11, Microsoft Exchange Server 2016 Cumulative Update 12, Microsoft Exchange Server 2016 Cumulative Update 13, Microsoft Exchange Server 2016 Cumulative Update 14, Microsoft Exchange Server 2016 Cumulative Update 15, Microsoft Exchange Server 2016 Cumulative Update 16, Microsoft Exchange Server 2016 Cumulative Update 17, Microsoft Exchange Server 2016 Cumulative Update 18, Microsoft Exchange Server 2016 Cumulative Update 19, Microsoft Exchange Server 2016 Cumulative Update 8, Microsoft Exchange Server 2016 Cumulative Update 9, Microsoft Exchange Server 2019, Microsoft Exchange Server 2019 Cumulative Update 1, Microsoft Exchange Server 2019 Cumulative Update 2, Microsoft Exchange Server 2019 Cumulative Update 3, Microsoft Exchange Server 2019 Cumulative Update 4, Microsoft Exchange Server 2019 Cumulative Update 5, Microsoft Exchange Server 2019 Cumulative Update 6, Microsoft Exchange Server 2019 Cumulative Update 7, Microsoft Exchange Server 2019 Cumulative Update 8.
Monitor for scanner or exploit-pattern traffic after 315 public PoC repositories were reported.

web/proxy/WAF logsapplication logsnetwork IDS/IPScloud metadata access logsegress proxy logs

Exposure queries

Search-engine queries that surface exposed instances. Run them against your own estate first.

vuln:CVE-2021-26855

Groups associated with this CVE

Groups reported in connection with this vulnerability. A reported association, not a confirmed observation of that group using it — and not scoped to any sector or campaign.

FIN7WIZARD SPIDERMikroceenPKPLUGAPT42

56 more threat groups on this CVEShowing 5 of 61 · Attribution, campaigns and the malware each one runsUnlock free

ip138.197.191.8773
CTU-AIPP-BlackList-4 · Mar 11, 2024
ip206.81.24.22730
CINSscore · Jun 26, 2024
ip143.244.168.16114
CINSscore · Jun 26, 2024
ip79.124.40.17464
AbuseIPDB Hourly Update · Mar 13, 2024
hashc77fd97410068dd961eda80318b5b03ea94f29cb2d64bc608c3283a638bcf05238
AlienVault Ransomware-Firehol · Mar 17, 2021

5 more indicators on this CVEShowing 5 of 10 · Hashes, addresses and domains from a live threat exchangeUnlock free

Timeline

What happened to this CVE, newest first — with the readings a source repeats on a schedule counted underneath rather than listed.

2026
Added · CVSS 3.1 7.5 (AV:N)
Aug 19, 2026 · NVD
2021
Added to CISA KEV catalog
Nov 3, 2021 · CISA
Added · webapps entry: Microsoft Exchange 2019 - Unauthenticated Email Download
May 18, 2021 · Exploit-DB
Added · remote entry: Microsoft Exchange 2019 - Server-Side Request Forgery
Mar 14, 2021 · Exploit-DB
Added · webapps entry: Microsoft Exchange 2019 - Server-Side Request Forgery (Proxylogon) (PoC)
Mar 11, 2021 · Exploit-DB
Initial · CVE published
Mar 3, 2021 · NVD

Source activity

Readings a source repeats on a schedule, counted rather than listed.

15×TweetsMar 4, 2026 – Jun 26, 2026 · SOCRadar CTI
12×SVRS was changedMay 4, 2026 – May 31, 2026 · SOCRadar CTI
1×CVE modified by MITRE: CVSS metrics updated.Aug 12, 2026 · SOCRadar CTI
1×CVE appeared in the Cve Trends.May 2, 2026 · SOCRadar CTI

References

39 on the record

msrc.microsoft.com/update-guide/vulnerability/CVE-2021-26855
Patch, Vendor Advisory, Mitre, NIST
packetstormsecurity.com/files/161846/Microsoft-Exchange-2019-SSRF-Arbitrary-File-Write.html
Exploit, Third Party Advisory, VDB Entry, NIST
packetstormsecurity.com/files/161938/Microsoft-Exchange-ProxyLogon-Remote-Code-Execution.html
Exploit, Third Party Advisory, VDB Entry, NIST
packetstormsecurity.com/files/162610/Microsoft-Exchange-2019-Unauthenticated-Email-Download.html
Exploit, Third Party Advisory, VDB Entry, NIST
packetstormsecurity.com/files/162736/Microsoft-Exchange-ProxyLogon-Collector.html
Exploit, Third Party Advisory, VDB Entry, NIST
portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-26855
Patch, Vendor Advisory, NIST, MS-BULLETIN, MITRE_REFERENCES
www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-26855
US Government Resource, NIST
www.exploit-db.com/exploits/49663
Vendor Advisory

Coverage

20 stories from the feeds we poll

15 more reports on this CVEShowing 5 of 20 · Vendor research and security press coverageUnlock free

Elsewhere on this site

Microsoftevery CVE for this vendor
November 2021what else CISA listed
SSRFsame class
CWE-918other pages naming this weakness

Not in any source we poll

Listed rather than left blank: an empty field and an unmeasured one look identical on screen, and only one is a reason to look elsewhere.

Only the 5 indicator(s) reported for this CVE are available; no packet-level or host-forensic artifacts supplied.
No organization-specific asset inventory, compensating-control status, or patch deployment evidence supplied.
No exploit packet captures, log samples, or incident case IDs supplied.

Answered from this record1

What should defenders know first?

CVE-2021-26855 is Microsoft Exchange Server Remote Code Execution Vulnerability, a high vulnerability affecting Microsoft Exchange Server 2013 Cumulative Update 21, Microsoft Exchange Server 2013 Cumulative Update 22, Microsoft Exchange Server 2013 Cumulative Update 23, Microsoft Exchange Server 2016 Cumulative Update 10, Microsoft Exchange Server 2016 Cumulative Update 11, Microsoft Exchange Server 2016 Cumulative Update 12, Microsoft Exchange Server 2016 Cumulative Update 13, Microsoft Exchange Server 2016 Cumulative Update 14, Microsoft Exchange Server 2016 Cumulative Update 15, Microsoft Exchange Server 2016 Cumulative Update 16, Microsoft Exchange Server 2016 Cumulative Update 17, Microsoft Exchange Server 2016 Cumulative Update 18, Microsoft Exchange Server 2016 Cumulative Update 19, Microsoft Exchange Server 2016 Cumulative Update 8, Microsoft Exchange Server 2016 Cumulative Update 9, Microsoft Exchange Server 2019, Microsoft Exchange Server 2019 Cumulative Update 1, Microsoft Exchange Server 2019 Cumulative Update 2, Microsoft Exchange Server 2019 Cumulative Update 3, Microsoft Exchange Server 2019 Cumulative Update 4, Microsoft Exchange Server 2019 Cumulative Update 5, Microsoft Exchange Server 2019 Cumulative Update 6, Microsoft Exchange Server 2019 Cumulative Update 7, Microsoft Exchange Server 2019 Cumulative Update 8 from Microsoft. The current evidence lists it in CISA KEV, and the exploit status is: Yes: CISA KEV signal present. Public exploit evidence is: 315 public PoC repositories reported; 1 marked weaponized in current dataset. The affected-version evidence is listed in the key facts and affected products tables. Defenders should first verify whether exposed or business-critical assets run those versions, then apply vendor patches or mitigations, restrict reachable attack surface, and preserve logs for detection review. CVSS 7.5 describes technical severity, while EPSS 100% helps estimate near-term exploit likelihood; neither replaces asset context. Unknown fields should remain explicit in tickets, and threat actor, IOC, victimology, or payload claims should not be added unless a cited source supports them. Monitor CISA KEV, vendor advisories, NVD changes, public PoC repositories, and internal telemetry for update triggers.