Threats / Microsoft / CVE-2021-31207
CVE-2021-31207
· EUVD no mirror located
· GCVE no mirror located
Verified 2026-06-22
Microsoft Exchange Server vulnerability
Microsoft Exchange Server vulnerability allowing security feature bypass through improper input validation and file upload handling. Actively exploited in ransomware campaigns.
Verdict
Today item, not a backlog item.
Critical remote vulnerability in Exchange Server enabling attackers to bypass security controls and gain unauthorized access. High exploitation prevalence with confirmed ransomware deployment in active campaigns.
CISA KEV Yes · 2021-11-033Ransomware use Flagged3EPSS 0.99782 (verify live)4Exploit Weaponized · public PoC5
01
Is it exploitable?
— the evidence, ranked above the scoreExploit available
Fully weaponized — public exploit code is cataloged for this vulnerability.We link the existence of the exploit; we do not host or redistribute payloads.
Reported exploitation
99 independent public reports of in-the-wild exploitation are cataloged.Distinct reporting sources (vendor, incident response, government); open them for the underlying claims.
Exploited in the wild
Listed in the CISA Known Exploited Vulnerabilities catalog (added 2021-11-03), flagged for known ransomware use.
Probability (EPSS)
EPSS 0.99782 — modeled likelihood of exploitation activity.EPSS is a daily-changing model output — open the source for today's value.
Severity / affected
Affected: Microsoft, Exchange Server. Confirm exact fixed builds in the vendor advisory.
Weakness (CWE)
Mapped to CWE-20 Improper Input Validation, CWE-434 Unrestricted File Upload — weakness family: Path traversal / file.CWE assignment from the public NVD record; the weakness class drives how the flaw is exploited.
02
Who’s exploiting it?
— attribution turns risk into urgencyAPT40 State-sponsored (PRC)
The CISA-led joint advisory AA24-190A names APT40 (tracked in ATT&CK as Leviathan) as a PRC Ministry of State Security group that rapidly weaponizes newly public vulnerabilities, naming the ProxyShell Exchange chain, Log4Shell, and Atlassian Confluence CVEs alongside the group.14
03
Why it matters
— the attack path, told twice: adversary, then board1
Front door — unauthenticated access narrative 1
Attacker
Craft malicious input or upload files that bypass server-side validation checks
Business
Security controls designed to protect email infrastructure are rendered ineffective
2
Keys to the kingdom — privilege/identity takeover narrative 2
Attacker
Exploit the bypass to execute unauthorized actions on the Exchange Server
Business
Attackers gain foothold within email systems and corporate network perimeter
3
Lateral reach — past segmentation narrative 3
Attacker
Deploy ransomware payloads or establish persistent access mechanisms
Business
Email services disrupted, data encrypted or exfiltrated, operational continuity compromised
04
What to do
— defensible action- Remediate per the vendor advisory — confirm the fixed build for your version and verify exposure.1
Say it to the boardA vulnerability with this evidence profile is a defensible budget line, not a backlog ticket — fund the change against the proof above.
05