FirefoxWeb browser · Mozilla
CVE-2018-5121
Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.
NVD · uneditedLow descenders on some Tibetan characters in several fonts on OS X are clipped when rendered in the addressbar. When used as part of an Internationalized Domain Name (IDN) this can be used for domain name spoofing attacks. Note: This attack only affects OS X operating systems. Other operating systems are unaffected. This vulnerability affects Firefox < 58.
Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.
dbcve analysis · high confidenceFirefox on OS X clips low descenders on certain Tibetan characters when rendering them in the address bar. This rendering flaw can be exploited in Internationalized Domain Name (IDN) spoofing attacks, where malicious domains appear legitimate due to character display manipulation.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.
NVD · CPE data
FirefoxWeb browser
<= 57.0.4CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.
From the vector- Attack vector
- Network
- Complexity
- Low
- Privileges
- None
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- None
- Integrity
- Low
- Availability
- None
CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.
dbcve checksWork through these to decide whether this CVE applies to you.
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Check your operating systemOpen Terminal and run 'sw_vers' or go to Apple menu > About This Mac to confirm you are running macOS/OS X. This vulnerability only affects Firefox on macOS/OS X, not other platforms.Affected if The system is not running macOS/OS X (the vulnerability does not apply to Windows, Linux, or other platforms)
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Determine installed Firefox versionOpen Firefox, go to Help > About Firefox, or enter 'about:support' in the address bar. Note the version number displayed.Affected if The installed version is 57.0.4 or any version lower than 57.0.4 (e.g., 57.0.3, 57.0, 56.x, etc.)
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Verify IDN domain display in address barVisit a website with an Internationalized Domain Name (IDN) containing Tibetan characters (or non-Latin scripts with descenders) and examine the address bar. The vulnerability causes low descenders on certain Tibetan characters to be clipped, potentially allowing spoofed domains to appear legitimate.Affected if IDN domains are displayed in the address bar and Tibetan characters render with clipped descenders
You are affected if you are running Firefox version 57.0.4 or earlier on macOS/OS X and use IDN domains, as the clipping of Tibetan character descenders could enable IDN spoofing attacks.
Generated from the published advisory. Verify against your own configuration.
Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.
From vendor dataUpgrade Firefox to version 58 or later on all affected OS X systems to obtain the rendering fix.
Fix this in Firefox
Scoped from the published advisory
- Consultation1.0 h
- Implementation1.0 h
- Testing1.0 h
- Review / QA1.0 h
An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $1,200.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2018-5121 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2018-5121 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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- Verified mitigations, workarounds, and config changes
- Version or environment caveats, and links to real fixes
- No weaponised exploit code, or anything meant to cause harm
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