An attacker could have caused 4 bytes of HMAC output to be written past the end of a buffer stored on the stack. This could be used by an attacker to execute arbitrary code or more likely lead to a crash. This vulnerability affects Firefox < 70, Thunderbird < 68.2, and Firefox ESR < 68.2.

References

• https://nvd.nist.gov/vuln/detail/CVE-2019-11759
• https://bugzilla.mozilla.org/show_bug.cgi?id=1577953
• https://security.gentoo.org/glsa/202003-10
• https://www.mozilla.org/security/advisories/mfsa2019-33/
• https://www.mozilla.org/security/advisories/mfsa2019-34/
• https://www.mozilla.org/security/advisories/mfsa2019-35/
• https://usn.ubuntu.com/4335-1/