Code execution via arbitrary server file write
Published Mar 2, 2021 · Updated Aug 19, 2026
Arbitrary file write in Microsoft Exchange Server allows remote authenticated users to write attacker-chosen files anywhere on the server. The Offline Address Book generator accepts an attacker-controlled destination and places a temporary download outside its designated OAB Temp directory. Administrative Exchange credentials, or an authentication-bypass chain, are required; writing an ASPX web shell enables code execution.
Summary
What happened
Arbitrary file write in Microsoft Exchange Server allows remote authenticated users to write attacker-chosen files anywhere on the server. The Offline Address Book generator accepts an attacker-controlled destination and places a temporary download outside its designated OAB Temp directory. Administrative Exchange credentials, or an authentication-bypass chain, are required; writing an ASPX web shell enables code execution.
The record
- CVE
- CVE-2021-26858
- Published
- Mar 2, 2021
- Updated
- Aug 19, 2026
- Vendor
- Microsoft
- Product
- Microsoft Exchange Server 2013 Cumulative Update 22
- Classifications
- T1505.003
- Attack vector
- network
- Privileges
- admin
Timeline
How it unfolded
- Mar 2, 2021CVE publishedPublication date reported by the CVE source.
- Aug 19, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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Is a vulnerable build present?
Compare these published version ranges with your installed build and any vendor patches.
- Affected versionversion=15.00.0 <15.00.1473.006
What conditions does exploitation require?
What is affected?
Published CVSS scores
CVSS describes severity. EPSS estimates exploitation probability.
Attacks
What attackers are doing with it
Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.
Weakness, pattern, technique
Public exploit references
- Public ProxyLogon proof-of-concept exploitfunctional · validated
Labels summarize the accepted research assessment. They do not indicate a test against your environment.
Reported exploitation
- HAFNIUM Exchange exploitation campaignthreat actor
- ProxyLogon ransomware exploitationransomware
- CISA Known Exploited Vulnerabilities entryknown exploited catalog
Technologies
Your stack
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Your stack
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