What is CVE-2020-15658?

CVE-2020-15658 is a medium-severity vulnerability in Mozilla Firefox, classified under Improper Check for Unusual or Exceptional Conditions. CVSS score: 6.5/10. Published 2020-08-10.

How severe is CVE-2020-15658?

Medium severity. CVSS v3 base score is 6.5 out of 10.

Is CVE-2020-15658 known to be exploited?

1 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.

Vulnerability in Mozilla Firefox

CVE-2020-15658

The code for downloading files did not properly take care of special characters, which led to an attacker being able to cut off the file ending at an earlier position, leading to a different file type being downloaded than shown in the dialog. This vulnerability affects Firefox ESR < 78.1, Firefox < 79, and Thunderbird < 78.1.

Published 2020-08-10 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 6.5 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N.

EPSS exploit prediction

EPSS: 0.014 (71.2th percentile), scored 2026-09-29.

Low probability of exploitation in the next 30 days, but still tracked. 71st percentile — 71.2% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.012 · newest: 0.014 · change: +0.002

Affected products

Mozilla Firefox — versions unspecified
Mozilla Firefox Esr — versions unspecified
Mozilla Firefox Esr
Mozilla Thunderbird — versions unspecified
Canonical Ubuntu Linux — versions 16.04, 18.04, 20.04

Weakness classification (CWE)

CWE-754 — Improper Check for Unusual or Exceptional Conditions

Public proof-of-concept exploits

Live-Hack-CVE/CVE-2020-15658

References

www.mozilla.org/security/advisories/mfsa2020-30/ (Vendor Advisory)
www.mozilla.org/security/advisories/mfsa2020-32/ (Vendor Advisory)
www.mozilla.org/security/advisories/mfsa2020-33/ (Vendor Advisory)
bugzilla.mozilla.org/show_bug.cgi (Permissions Required, Issue Tracking, Vendor Advisory)
lists.opensuse.org/opensuse-security-announce/2020-08/msg00025.html (vendor-advisory, Third Party Advisory)
usn.ubuntu.com/4443-1/ (vendor-advisory, Third Party Advisory)

Frequently asked questions

What is CVE-2020-15658?: CVE-2020-15658 is a medium-severity vulnerability in Mozilla Firefox, classified under Improper Check for Unusual or Exceptional Conditions. CVSS score: 6.5/10. Published 2020-08-10.
How severe is CVE-2020-15658?: Medium severity. CVSS v3 base score is 6.5 out of 10.
Is CVE-2020-15658 known to be exploited?: 1 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.