DLL registered under the AppCertDLLs registry key

Siemphony@siemphonyhighT1546.009unverified

Matches a write beneath the AppCertDLLs key under `HKLM\System\CurrentControlSet\Control\Session Manager\`, whose contents are loaded into every process that calls the CreateProcess family or WinExec — the registry half of AN1029. AppCertDLLs is a key, not a value: the DLL path an operator installs is an arbitrarily named value one level beneath it, so a real write produces a TargetObject ending in `\Session Manager\AppCertDLLs\<ValueName>`. The match is therefore a contains on `\Session Manager\AppCertDLLs\` including the trailing separator, which catches the write whatever the operator names the value; an endswith on the key name itself would only match a value literally called AppCertDLLs sitting directly under Session Manager, which is not how this mechanism works. Unlike the AppInit_DLLs value used by the sibling T1546.010 rule, Session Manager lives under HKLM\SYSTEM, which WOW64 does not redirect, so there is no Wow6432Node twin to also cover. The two techniques are siblings but not the same shape: AppInit_DLLs is loaded by user32.dll into GUI processes, while AppCertDLLs is loaded by the kernel32 process-creation path itself, so it reaches console and service processes that AppInit never touches, and this rule targets that key specifically rather than reusing AppInit's selector. The brief maps this from Security EventID 4657 onto Sigma's registry_set category, which is Sysmon-shaped (TargetObject); that is the vocabulary chosen here, and a deployment relying on 4657 alone — which needs Audit Registry auditing and a SACL on this key, neither on by default — would need ObjectName/ObjectValueName mapped onto TargetObject first. The analytic's DLL-load correlation (linking the write to a subsequent unexpected module load) needs a cross-event join Sigma cannot express and is not attempted. Sysmon registry monitoring is an allowlist of paths; confirm this key is included before reading silence as a clean result. UNVERIFIED AS AUTHORED — derived from MITRE ATT&CK DET0362, not hand-written and not tested by its author. Any lab result is on this rule's own page.Full description

The 4 SIEM queries below are previews produced by Siemphony’s own translator, not pySigma, and none has been executed against a real backend — review before you deploy.

detection.yml

Sentinel · KQL

Run this as a search.

DeviceRegistryEvents| where RegistryKey contains "\\Session Manager\\AppCertDLLs\\"

Splunk · SPL

Run this as a search.

index=* TargetObject="*\\Session Manager\\AppCertDLLs\\*"

Elastic · ES|QL

Run this as a search.

FROM logs-*| WHERE TO_LOWER(registry.path) LIKE "*\\\\session manager\\\\appcertdlls\\\\*"

Wazuh · XML rule

Deploy to your manager — this is a rule, not a search.

<group name="sigma,windows,registry_set,">  <!-- Rule ids must be unique on your manager; 100000+ is the user range. -->  <rule id="100000" level="12">    <!-- Set <if_sid> to the decoder/base rule for windows so this only evaluates relevant events. -->    <field name="TargetObject" type="pcre2">(?i)\\Session Manager\\AppCertDLLs\\</field>    <description>DLL registered under the AppCertDLLs registry key</description>    <mitre>      <id>T1546.009</id>    </mitre>  </rule></group>

At a glance

Logsource: registry_set / windows
Level: high
Category: registry_persistence
Sigma status: experimental
Backends: 4 of 4 rendered
Updated: 30 Aug 2026
Rule ID: ecf2604e-06b6-4ed7-b24f-fdc0a7d88f5a

ATT&CKT1546.009

T1546.009AppCert DLLs

ATT&CK® is a registered trademark of The MITRE Corporation. These mappings are the rule author’s, and are neither reviewed nor endorsed by MITRE.

Lessonregistry persistence

registry persistence

01 · What this rule detects

Registry values written so that attacker code is launched by Windows itself — Run and RunOnce keys, Image File Execution Options debuggers, LSA security and notification packages, COM server hijacks, AppInit_DLLs, Winlogon shell and userinit entries, and the accessibility shims that hang off IFEO. Each of these is a different key with the same purpose: something the operating system reads and executes without anyone asking it to.

02 · Telemetry you need

Sysmon EventID 12, 13 and 14 (registry key create/delete, value set, key rename) with TargetObject, Details and Image — Sysmon logs no registry activity unless the config explicitly includes these paths, so an empty result usually means the config, not the environment
Security 4657 (registry value modified) — requires Audit Registry and a SACL on each key you care about; without the SACL it produces nothing at all
Process creation for the writers: reg.exe, regedit.exe, PowerShell Set-ItemProperty and New-ItemProperty, and WMI StdRegProv
Sysmon EventID 11 (FileCreate) for the executable or DLL the value points at, which is usually written moments before
Autoruns-style periodic inventory of the same keys, as a backstop for hosts where registry auditing is not collected

03 · How the logic works

Every rule here is two halves: the key path says which persistence mechanism is in play, the value data says what will run. Matching only the path gives you every installer in the estate; matching the data — an executable under a user-writable directory such as AppData, ProgramData or Temp, a path with no directory at all, a scripting host, a base64 blob — is what makes it an alert. Two coverage traps are worth memorising. First, persistence exists in both hives: a rule scoped to HKLM misses every per-user Run key, and Sysmon reports the user hive as HKU\<SID>\... rather than HKCU, so a rule written with HKCU matches nothing. Second, the 32-bit view under Wow6432Node is a separate path. For the narrower mechanisms — an IFEO Debugger value, an appended LSA package, an AppInit DLL — the existence of the write is itself the signal and no data condition is needed, which makes them the quietest and best rules in the category.

04 · Common false positives

Application installers and updaters registering themselves at logon — Chrome and Edge update, Adobe, Dropbox, OneDrive and similar
Enterprise management and Group Policy preferences writing Run values across the fleet
Password-filter and credential-provider products that legitimately register LSA notification packages
Backup, EDR and disk-encryption agents registering COM servers and Winlogon components during install
Developer tooling setting IFEO values for debugging, and imaging or provisioning workflows that rewrite these keys wholesale

05 · Detection maturity

Tier 1 — starting point: Matches writes to the Run and RunOnce keys.
Tier 2 — tuned: Covers the full set of autostart keys across both hives and the 32-bit view, conditioned on the value data pointing somewhere user-writable, with known publishers excluded.
Tier 3 — correlated: Correlates the write with the creation of the file it points at, the process that performed the write, and how rare that value is across the fleet.

Tuning & false positivesexperimental · 3 declared

experimental (current stage)
test
stable

Written and reasoned about, but not yet run against real telemetry. Expect to tune it before trusting it.

Tuning tips

Alert on where the value points, not on the key being touched — the same Run key is written by every installer you own
Treat IFEO Debugger, GlobalFlag with silent-exit monitoring, AppInit_DLLs and LSA package changes as near-zero-volume rules and give them their own higher severity
Cover HKLM, the per-user hive as your telemetry actually spells it, and the Wow6432Node view in the same rule, then check the rule fires for all three
Exclude by the signature or full path of the writing process where the event carries it, rather than by the value name
Baseline what exists across the fleet — persistence that appears on one host and nowhere else is the finding

Declared by the author

Long-lived enterprise images still carrying a legacy antivirus or host intrusion-prevention suite from a vendor generation (pre-Windows 10 era McAfee/Symantec builds are the well-known examples) that hooked process creation through AppCertDLLs before AppInit and AMSI became the preferred mechanism; the value is set once at install and rarely touched again.
Application-compatibility or license-enforcement shims from older enterprise packaging tools that register a process-creation hook here to monitor launches for compatibility or license-compliance checks — uncommon, but a real and documented legitimate use of this exact key.
Endpoint management, asset-inventory and EDR agents writing or clearing their own value under this key during install, upgrade or repair, which produces the same event as an operator installing one and is distinguishable only by the DLL path and the writing process.

Noise check0 of 6 fired

Fires on none of the 6 ordinary behaviours the baseline holds for this log type. That is not a precision guarantee: it means only not caught by these 6 behaviours, and the next ordinary behaviour nobody wrote down may still match.

Run live against 65 hand-written events of ordinary corporate activity — backup agents, software distribution, patching, helpdesk scripts, EDR. It says nothing about how often the rule would fire in production, which is the number that actually exhausts an analyst.

Verification1 run

Not verified

Siemphony benign baseline
completed
Siemphony IR · 3 Sept 2026
No events matched, so this run proves nothing either way.

Test it yourselfin your browser

Runs here in your browser against public attack recordings. A preview — not the verification badge.

Recordings of T1546.009, evaluated against their registry_set events.

Share this rule

/u/siemphony/ecf2604e-06b6-4ed7-b24f-fdc0a7d88f5a

More from this repertoire

All compositions

Verdicts · reactions · comments

Community

Verdicts from engineers who actually deployed it, and the conversation around it.

Not yet reported on

Nobody has run this in a real environment and said what happened.

Verdicts from engineers who deployed it

0 cast

No verdicts reported yet. The first one is worth more than the tenth — say what you ran it against.

No comments yet. Someone who deploys this will have something to say about it.

DLL registered under the AppCertDLLs registry key

The detection

detection.yml

Sentinel · KQL

Splunk · SPL

Elastic · ES|QL

Wazuh · XML rule

Community

Verdicts from engineers who deployed it