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ProxyLogon → webshell on Exchange → DA

Unauth SSRF + auth bypass against on-prem Exchange (CAS) — write a webshell as SYSTEM on the Exchange server, dump LSASS for cached domain creds, pivot to DA.

Filed by AD Knowledge Base
§ Kill-chainDrag · zoom · scroll

§ Context

Assumed environment: on-prem Exchange 2013/2016/2019 unpatched for CVE-2021-26855 et al. Internet-exposed OWA or reachable from an internal foothold. Default Exchange machine account has elevated AD rights.

§ Steps

  1. 01
    Exchange machine account → WriteDACL on AdminSDHolderPersistence
    AD-ADMINSDHOLDER— AdminSDHolder Abuse
  2. 02
    DCSync as DACredential Access
    T1003.006— DCSync
  3. 03
    Identify Exchange versionReconnaissance
    W-RECON-FINGERPRINT— Tech Stack Fingerprinting
  4. 04
    Drop webshell as SYSTEM on ExchangePersistence
    W-WEBSHELL— Webshell Deployment
  5. 05
    Dump LSASS for cached credsCredential Access
    W-LSASS-PROCDUMP— LSASS via procdump / comsvcs.dll
  6. 06
    ProxyLogon SSRF + auth bypassInitial Access
    EX-PROXYLOGON— ProxyLogon (CVE-2021-26855)

§ References

§ Frequently asked

What is the "ProxyLogon → webshell on Exchange → DA" attack path?
Unauth SSRF + auth bypass against on-prem Exchange (CAS) — write a webshell as SYSTEM on the Exchange server, dump LSASS for cached domain creds, pivot to DA. It chains 6 steps drawn from real-world offensive-security techniques.
What starting position does this attack require?
The first step is Exchange machine account → WriteDACL on AdminSDHolder (AD-ADMINSDHOLDER) — a persistence primitive. Assumed environment: on-prem Exchange 2013/2016/2019 unpatched for CVE-2021-26855 et al.
What is the final impact of this kill-chain?
The final step lands on ProxyLogon SSRF + auth bypass (EX-PROXYLOGON), which falls under Initial Access. From here, an operator typically pivots into post-exploitation or maintains persistence.
How can defenders detect or prevent this attack?
Detection and prevention vary per step. Refer to each linked MITRE ATT&CK entry under "References" — every technique on that page lists defensive controls, detection telemetry, and known threat-actor usage.

§ Related dossiers