CVE-2025-5020
Mozilla Firefox ≤ 139.0
Raw vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:NSummary
CVE-2025-5020 is a medium-severity Improper Authorization in Handler for Custom URL Scheme (CWE-939) vulnerability in Mozilla Firefox. Its CVSS base score is 4.3 (Medium).
Operationally, ranked at the 11th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.
The strongest mitigations our analysis identified map to AC-3 (Access Enforcement) and AC-6 (Least Privilege) — see the control section below for these in your framework.
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2025-16036
Vulnerability Data
Opening maliciously-crafted URLs in Firefox from other apps such as Safari could have allowed attackers to spoof website addresses if the URLs utilized non-HTTP schemes used internally by the Firefox iOS client. This vulnerability was fixed in Firefox for iOS…
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139.
- CWE(s)
Related Threats
CVEs Like This One
Affected Assets
Mitigating Controls
Mitigating Controls (NIST 800-53 r5) AI
Access enforcement directly stops unauthorized actors from invoking the custom URL handler by requiring approved authorizations for that entry point.
Least privilege limits the authorizations granted to the handler, reducing the chance that any actor can invoke it without restriction.
Mitigating Controls (NIST CSF 2.0) AI
Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→CSF cross-walk (authority under review) — links open the control.
Defining, enforcing, and reviewing access permissions directly addresses the missing authorization checks for custom URL scheme handlers.
Mitigating Controls (ISO/IEC 27001:2022 Annex A) AI
Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→ISO cross-walk (authority under review) — links open the control.
Security testing can discover missing authorization checks, but does not itself implement the required controls.
Enforces explicit authorization checks on which actors may invoke handlers, directly addressing the missing restriction on custom-URL-scheme access.
High-level access-control policy provides governance context but does not prescribe the technical authorization logic needed for URL-scheme handlers.
Requires security requirements that define which callers are permitted to trigger custom URL handlers, mitigating the authorization gap.
Mandates secure coding practices that include proper authorization checks before executing actions triggered by custom URL schemes.