Mozilla Firefox Security Updates Released

On May 19, 2026, Mozilla released security advisories addressing vulnerabilities affecting Firefox and Firefox ESR. The affected products include Firefox versions prior to 151, Firefox ESR versions prior to 115.36, and Firefox ESR versions prior to 140.11. These vulnerabilities could potentially be exploited by attackers to compromise systems running vulnerable versions of Firefox. Users and administrators are urged to review the Mozilla security advisories and apply the necessary updates to mitigate the risk.

Attack Chain

1. Attacker identifies a vulnerable Firefox or Firefox ESR version.
2. Attacker crafts a malicious web page or utilizes an existing compromised website.
3. The victim visits the malicious web page through the vulnerable Firefox browser.
4. The malicious web page exploits a vulnerability within Firefox (e.g., memory corruption, use-after-free).
5. Successful exploitation allows the attacker to execute arbitrary code on the victim’s system.
6. The attacker gains control of the Firefox process, potentially escalating privileges.
7. The attacker installs malware, such as a keylogger or remote access trojan (RAT).
8. The attacker uses the compromised system to steal sensitive data or launch further attacks.

Impact

Successful exploitation of these vulnerabilities could allow attackers to execute arbitrary code on a victim’s system. This can lead to data theft, malware installation, and further compromise of the affected system. The severity of the impact depends on the specific vulnerability exploited and the privileges gained by the attacker. Given the widespread use of Firefox, a large number of users are potentially at risk if they do not apply the necessary updates.

Recommendation

• Upgrade Firefox to version 151 or later to address the vulnerabilities outlined in MFSA2026-46.
• Upgrade Firefox ESR to version 115.36 or later to address the vulnerabilities outlined in MFSA2026-47.
• Upgrade Firefox ESR to version 140.11 or later to address the vulnerabilities outlined in MFSA2026-48.
• Deploy the Sigma rule “Detect Exploitation of Firefox Vulnerabilities via HTTP User-Agent” to identify potential exploitation attempts.
• Monitor web server logs for suspicious User-Agent strings indicative of exploit attempts, as covered by the Sigma rule.