CVE-2026-92062: Firefox Session Restore Privilege Escalation

CVE-2026-92062 Overview

CVE-2026-92062 is a privilege escalation vulnerability in the Session Restore component of Mozilla Firefox and Thunderbird. The flaw allows an attacker to gain elevated privileges through the browser's session restoration mechanism when a user interacts with malicious content. Mozilla addressed the issue in Firefox 156, Firefox ESR 153.3, Thunderbird 156, and Thunderbird 153.3. The weakness is classified under [CWE-269] Improper Privilege Management.

Critical Impact
An attacker exploiting this flaw can escalate privileges within the browser context, leading to high impact on confidentiality, integrity, and availability of the affected system.

Affected Products

Mozilla Firefox versions prior to 156
Mozilla Firefox ESR versions prior to 153.3
Mozilla Thunderbird versions prior to 156 and 153.3

Discovery Timeline

2026-09-15 - CVE-2026-92062 published to NVD
2026-09-16 - Last updated in NVD database

Technical Details for CVE-2026-92062

Vulnerability Analysis

The vulnerability resides in the Session Restore component, which is responsible for reopening tabs, windows, and browser state after a shutdown or crash. Improper privilege management within this component allows a lower-privileged context to obtain elevated privileges during the restoration workflow. The flaw affects both Firefox and Thunderbird because both products share the underlying platform code. Successful exploitation requires the victim to interact with crafted content, such as visiting a malicious page or restoring a poisoned session.

Root Cause

The root cause is improper privilege management ([CWE-269]) within the Session Restore code path. State data processed during restoration is not correctly constrained to the privilege level of its origin. This allows content-level operations to execute with privileges intended for trusted browser components.

Attack Vector

The attack vector is network-based and requires user interaction. An attacker delivers crafted content that manipulates session state, then relies on the browser restoring that state to trigger the privilege escalation. Refer to Mozilla Security Advisory MFSA 2026-90 and Mozilla Bug Report #2054670 for technical detail. No public proof-of-concept exploit is currently available.

Detection Methods for CVE-2026-92062

Indicators of Compromise

Unexpected creation or modification of Firefox or Thunderbird session store files such as sessionstore.jsonlz4 or entries under sessionstore-backups/.
Browser processes spawning child processes with unusual command lines or writing to sensitive filesystem locations after a session restore event.
Outbound network connections initiated by the browser immediately following startup or crash recovery to previously unseen hosts.

Detection Strategies

Inventory installed Firefox and Thunderbird versions across the estate and flag any hosts running versions below Firefox 156, Firefox ESR 153.3, or Thunderbird 156 / 153.3.
Alert on anomalous file writes to session store directories by non-browser processes, which may indicate session state tampering.
Correlate browser crash events with subsequent privileged operations or unexpected process launches on the same host.

Monitoring Recommendations

Ingest endpoint process, file, and network telemetry into a centralized analytics platform to identify browser-based privilege escalation patterns.
Monitor for exploitation attempts referenced in Mozilla Security Advisory MFSA 2026-93 and related advisories.
Track user reports of unexpected browser restarts or session restore prompts, which may correlate with exploitation attempts.

How to Mitigate CVE-2026-92062

Immediate Actions Required

Upgrade Firefox to version 156 or later on all endpoints where the browser is installed.
Upgrade Firefox ESR to 153.3 or later for environments standardized on the extended support release.
Upgrade Thunderbird to version 156 or 153.3 or later on all mail clients.
Restart affected applications after patching to ensure the vulnerable code paths are unloaded.

Patch Information

Mozilla shipped fixes for CVE-2026-92062 in Firefox 156, Firefox ESR 153.3, Thunderbird 156, and Thunderbird 153.3. See Mozilla Security Advisory MFSA 2026-90, MFSA 2026-93, MFSA 2026-94, and MFSA 2026-96 for release-specific details.

Workarounds

Disable session restore functionality by setting browser.sessionstore.resume_from_crash to false and browser.startup.page to 0 in about:config until patching is complete.
Clear existing session store files before restarting the browser to eliminate potentially tampered state.
Restrict browser usage to trusted sites through enterprise policy where immediate patching is not feasible.

bash

# Enterprise policy example (policies.json) to reduce session restore exposure
{
  "policies": {
    "DisableFormHistory": true,
    "Preferences": {
      "browser.sessionstore.resume_from_crash": {
        "Value": false,
        "Status": "locked"
      },
      "browser.startup.page": {
        "Value": 0,
        "Status": "locked"
      }
    }
  }
}