CVE-2021-26858

Exploited in the wild: Yesadded 2021-11-03 · due 2022-05-03 · known ransomware use. An observation, not a forecast.
EPSS · 30-day forecast: 94%Chance of exploitation in the next 30 days, 100th percentile of all CVEs. A forecast; KEV outranks it.
CVSS · NVD: not stated by sourcesNo NVD record yet.
Internet exposure: not stated by sourcesNo exposure census on this CVE's dispatches.

Exploits

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On the wire

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Tied by cited sources to

HAFNIUM
1. “CVE-2021-26858 – A post-authentication arbitrary file write vulnerability in Exchange. If Silk Typhoon could authenticate with the Exchange server, then it could use this vulnerability to write a file to any path on the server.”
  cited as exploiting · source: microsoft.com ↗ · 2026-10-17
2. “CVE-2021-26858 is a post-authentication arbitrary file write vulnerability in Exchange. If HAFNIUM could authenticate with the Exchange server then they could use this vulnerability to write a file to any path on the server.”
  cited as exploiting · source: microsoft.com ↗ · 2026-10-17
3. “Three additional vulnerabilities (CVE-2021-26857, CVE-2021-26858, and CVE-2021-27065) were also identified as part of this activity. When chained together along with CVE-2021-26855 for initial access, the attacker would have complete control over the Exchange server.”
  cited as exploiting · named nearby · source: splunk.com ↗ · 2026-10-17
APT27
1. “Exploitation of recent vulnerabilities ( CVE-2021-26855 , CVE-2021-26857 , CVE-2021-26858 , and CVE-2021-27065 ) in Microsoft Exchange Server”
  attributed by a report · named nearby · source: trendmicro.com ↗ · 2026-10-17
Shadow-Earth-053
1. “We have observed them targeting server-based N-day vulnerabilities, such as the ProxyLogon chain targeting Microsoft Exchange Server ( CVE-2021-26855 , CVE-2021-26857 , CVE-2021-26858 , and CVE-2021-27065 ).”
  cited as exploiting · named nearby · source: trendmicro.com ↗ · 2026-10-17

Each actor is cited by the quoted source, linked under the quote. A citation is that source's claim, not our confirmation.

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