Detections
Explore public detection logic contributed by the community across SIEM and rule languages.
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The following analytic detects the addition of a servicePrincipalName (SPN) containing invisible Unicode characters to an Active Directory computer object via Windows Security Event 5136.
This is the key indicator of the KerberLoss attack (CVE-2026-25177), where an attacker with write access to a machine account's SPN attribute injects a collision SPN that contains a Zero Width or other invisible Unicode character (e.g. U+200C Zero Width Non-Joiner).
LDAP's string preparation rules (RFC 4518) cause these characters to be ignored during uniqueness checking, allowing the write to succeed even when an identical SPN already exists on another machine account.
The Kerberos KDC, however, does not apply the same normalisation — it finds two accounts with matching SPNs and returns KDC_ERR_S_PRINCIPAL_UNKNOWN, causing a Kerberos denial-of-service or forcing clients to fall back to NTLM downgrade.
This is the key indicator of the KerberLoss attack (CVE-2026-25177), where an attacker with write access to a machine account's SPN attribute injects a collision SPN that contains a Zero Width or other invisible Unicode character (e.g. U+200C Zero Width Non-Joiner).
LDAP's string preparation rules (RFC 4518) cause these characters to be ignored during uniqueness checking, allowing the write to succeed even when an identical SPN already exists on another machine account.
The Kerberos KDC, however, does not apply the same normalisation — it finds two accounts with matching SPNs and returns KDC_ERR_S_PRINCIPAL_UNKNOWN, causing a Kerberos denial-of-service or forcing clients to fall back to NTLM downgrade.
The following analytic detects a user account (non-machine account) adding or removing a servicePrincipalName (SPN) on a computer object in Active Directory, via Windows Security Event 5136.
In normal AD operations, SPN values on computer objects are managed exclusively by the computer account itself (during domain join or name change), by Domain Controllers during replication, or by the SYSTEM/NETWORK SERVICE context — all of which appear as accounts ending in the dollar sign ($) convention.
A named user account writing to the servicePrincipalName attribute of a computer object is anomalous and may indicate an attacker with delegated WriteProperty rights over computer accounts performing SPN manipulation.
In normal AD operations, SPN values on computer objects are managed exclusively by the computer account itself (during domain join or name change), by Domain Controllers during replication, or by the SYSTEM/NETWORK SERVICE context — all of which appear as accounts ending in the dollar sign ($) convention.
A named user account writing to the servicePrincipalName attribute of a computer object is anomalous and may indicate an attacker with delegated WriteProperty rights over computer accounts performing SPN manipulation.
This rule identifies Windows Server devices, specifically potential Domain Controllers, that are flagged as vulnerable to CVE-2026-27912 (ResetNightmare) and CVE-2026-25177 (KerberLoss). It checks for the absence of the March/April 2026 cumulative updates that address critical Kerberos Change Password and Service Principal Name (SPN) logic flaws.
