CVE-2026-103418

Executive Summary

CVE-2026-103418 is a code-injection vulnerability in Firefox for iOS in which the browser directly interpolated a web page's HTML lang (language) attribute into translation JavaScript, allowing attacker-controlled markup to execute in Firefox's privileged content world. Because execution occurs in a privileged context, the injected code bypasses the originating page's Content Security Policy (CSP), a security boundary that normally constrains script execution to the web content's own origin. Any Firefox for iOS user who loads a page containing a maliciously crafted language attribute is potentially affected; the issue is fixed in Firefox for iOS 157.1.

Technical Analysis

The core defect is an input-handling failure at a language-attribute trust boundary. Firefox for iOS uses the value of a page's HTML lang attribute to drive translation functionality. Per the NVD description, the browser "directly interpolated the page's HTML language attribute into translation JavaScript" rather than treating it as an inert, validated data value. Interpolation here means string concatenation: the raw attribute value is placed into a JavaScript code context (the translation script) without encoding or neutering characters that carry syntactic meaning in that context. Because the attribute value was not sanitized for the JavaScript-string context, attacker-controlled markup reached an interpreter instead of a data slot.

The second, more significant defect is the execution context. The interpolated code executed "in Firefox's privileged content world" — the browser's internal script environment rather than the ordinary page-content world. This is the crux of the vulnerability: the privileged world is not governed by the visited page's CSP, and it typically exposes internal APIs, cross-origin data, and privileged objects not reachable from ordinary page script. As a result, the second sentence of the description follows directly: attacker-controlled execution in this world "bypass[es] the page's Content Security Policy." That is the CSP bypass path described in the advisory, and it distinguishes this issue from ordinary reflected or stored XSS, where the impact is confined to the page origin.

The attack surface is the browser's own translation rendering path, reachable through any page that supplies an HTML language attribute — i.e., the fundamental content input of the web, the lang attribute of a document. This is a low-complexity, network-reachable surface (AV:N, AC:L in a CVSS estimate) that requires the victim to load a crafted page (UI:R). The practical implications of privileged-world execution span reading cross-origin data, accessing internal browser APIs, and defeating CSP-based isolation controls on the surrounding page or application — which is why impact classifications for this class of bug normally carry a Scope:Changed (S:C) rating.

The remediation logic is straightforward and matches the stated fix in 157.1: treat the language attribute strictly as data (context-appropriate escaping and strict validation before interpolation), and/or execute any translation script in a non-privileged context. The root cause is therefore not an exotic parsing bug but a classic context-improper interpolation — a CWE-79/CWE-94 class issue — amplified by placement of the vulnerable code in a privileged execution world.

No CNA title, CWE assignment, or CVSS vector was supplied with the source data, so all severity figures in this report are analytical estimates and must not be quoted as vendor scores.

Exploitation Scenarios

No public PoC, exploit code, or Nuclei template is known for CVE-2026-103418. The attack chain below is inferred from the vulnerability class and the NVD description; it deliberately avoids inventing function names, endpoints, or file paths that do not appear in the source data.

1. Reconnaissance / target identification. The attacker does not need to identify a specific deployment. The target is the browser itself: any user browsing with an affected Firefox for iOS build (pre-157.1) who visits attacker-influenced content. Because the trigger is content served from the web, standard lure techniques apply — malicious links sent by email/SMS, malicious ad or third-party content on otherwise benign sites, or a controlled page the victim is directed to.

2. Triggering the flaw class. The attacker causes the victim's browser to process a document whose HTML lang attribute value is under attacker control (the page author completely controls this attribute). Per the advisory, Firefox for iOS "directly interpolated" that value into translation JavaScript. The trigger is the loading of maliciously crafted markup in the language attribute: the vulnerable interpolation path reaches the interpreter with attacker-supplied content. No public details describe the exact character sequence or grammar required; the exploitation primitive at the class level is improper neutralization of input placed into a JavaScript execution context.

3. Payload / execution method at the class level. The attacker-supplied content is executed as code in Firefox's privileged content world. This is the defining feature of this CVE: execution is not confined to the page's script context and is not subject to the page's CSP, which the injected code bypasses. At the class level the payload is a script injection whose interpreter and permission context are internal to the browser rather than the visited origin.

4. Post-exploitation impact. With code running in the privileged content world, the consequences flow from that context: the injected code can interact with or read data beyond the isolation that normal page script and CSP enforce, and cross-origin boundaries that depend on the page's CSP to hold can be defeated. This yields potential data exposure (from the viewed page or other origins reachable from the privileged context) and manipulation of browser-internal behavior. The advisory does not enumerate specific privileged APIs reachable from the injected context; any such claims would be speculative, so none are asserted here. No evidence of post-exploitation tooling, persistence, or in-the-wild campaigns is available.

Impact Assessment

Successful exploitation yields script execution in Firefox's privileged content world with the page's Content Security Policy bypassed, per the fixed-version advisory for 157.1. The CIA impact is best characterized as primarily confidentiality and integrity impact: arbitrary code execution with privileged-world capabilities can read data the page's origin and page-load context would normally reveal (or, if the privileged world permits broader reach, cross-origin data) and can alter page or browser-interpreted state. Availability impact is expected to be lower.

In CVSS terms: the network-reachable, no-privileges attack path with a user-interaction requirement (the victim must load the malicious page) and a scope change due to privileged-context execution produces an estimated vector of CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N. No CVSS score was assigned in the source data (CVSS Score: N/A), and no EPSS value exists (not yet scored by FIRST.org); treat all severities here as analyst estimates, not vendor figures.

Business blast radius: the vulnerable component is a mainstream mobile browser, so exposure scales with the mobile fleet. Affected systems are iOS devices running Firefox for iOS versions prior to 157.1 — including BYOD handsets used for corporate webmail, SSO, and SaaS access. A user whose browsing session is compromised in a privileged world can expose session tokens, page content from applications visited in the browser, and any data the privileged context can reach, potentially enabling account takeover or data exfiltration. The scope change also means this vulnerability can undercut CSP-dependent defenses web application owners rely on to isolate sensitive views, so defenders deploying CSP as a control cannot fully rely on it for users of affected Firefox for iOS builds. No CISA KEV listing, no exploit-in-the-wild evidence, and no named threat-actor campaign have been observed for this CVE.

Detection Guidance

Detection combines browser telemetry, endpoint artifacts, and network visibility. Because the trigger is client-side markup interpretation inside Firefox for iOS, network rules must look for marker-attribute anomalies (a lang attribute containing HTML/script syntax) and client rules for translation-path activity with executable payload markers. The rules below are heuristic "suspicious artifact" detections for this vulnerability class; they require tuning to a specific environment and are not confirmed signatures, because no CVE-specific exploit artifacts have been published.

Sigma Rule

title: Suspicious Firefox for iOS Translation Activity with Script-Like Language Attribute
id: c0e2d4a6-6b1f-4d2e-9c7a-103418cve01
status: experimental
description: Detects Firefox for iOS processes where translation functionality is invoked in proximity to events carrying script-like lang attribute content, consistent with CVE-2026-103418 exploitation patterns.
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-103418
author: valtersit.com Threat Intelligence
date: 2026/01/01
level: medium
logsource:
  product: macos
  category: process_creation
detection:
  selection_browser:
    Image|endswith:
      - '/firefox'
      - '/Firefox'
    CommandLine|contains:
      - 'translation'
      - 'lang='
  selection_indicator:
    CommandLine|contains:
      - '<script'
      - 'javascript:'
      - 'onerror='
  condition: selection_browser and selection_indicator
falsepositives:
  - Legitimate developer testing of pages with unusual lang attributes
  - Crash reporter uploads containing page metadata
fields:
  - CommandLine
  - Image
  - User
tags:
  - attack.t1059
  - attack.t1189

Note: iOS endpoint telemetry is limited (no native process_creation for arbitrary app internals); deploy this equivalent on jailbroken/forensic platforms or via MDM app-event feeds, and treat it as an indicator of suspicious translation-path input rather than a definitive CVE signature.

YARA Rule

rule CVE_2026_103418_Firefox_iOS_Lang_Injection
{
    meta:
        description = "Heuristic detection of browser artifacts referencing CVE-2026-103418 lang-attribute injection patterns in Firefox for iOS data/cache files"
        author = "valtersit.com Threat Intelligence"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-103418"
        date = "2026-01-01"
        severity = "medium"
    strings:
        $cve = "CVE-2026-103418" ascii nocase
        $lang = "lang=" ascii nocase
        $script = "<script" ascii nocase
        $js1 = "javascript:" ascii nocase
        $js2 = "onerror=" ascii nocase
        $trans = "translation" ascii nocase
    condition:
        (uint16(0) == 0x4D5A or uint32(0) == 0x504B0304) and
        ($lang and $trans and 1 of ($js*) ) or $cve
}

Operational note: the condition targets Zip/APK-style container headers or plain mobile app cache files that combine a translation reference with script-like lang content; the $cve term also matches any forensic sample referencing the identifier.

Suricata IDS Rule

alert http any any -> any any (msg:"CVE-2026-103418 Firefox for iOS possible lang-attribute script injection (translation)"; flow:established,to_client; http.header; content:"lang="; nocase; pcre:"/(?:<|%3C)\s*script|javascript:|onerror\s*=|onload\s*=/i"; reference:cve,2026-103418; reference:url,nvd.nist.gov/vuln/detail/CVE-2026-103418; classtype:web-application-attack; sid:10341801; rev:1;)

This is a passive observation rule (no drop). It flags responses carrying a lang= marker combined with script-like tokens — the class-level artifact this CVE exposes. Deploy in detect-only mode and baseline false positives from localization-heavy sites.

Elastic Detection

sequence by host.id, process.entity_id with maxspan=30s
  [process where process.name in ("Firefox", "firefox", "com.mozilla.ios.Firefox")
     and process.command_line : "*translation*" or process.command_line : "*lang=*"]
  [process where process.command_line : ("*<script*" or "*javascript:*" or "*onerror=*")]

This EQL sequence targets the translation path invoking on language-attribute content and then encountering script-like payload markers within a 30-second window, on hosts where Firefox for iOS artifacts are visible via MDM/endpoint telemetry.

Splunk SPL Query

index=web OR index=proxy
| eval lang_attr=coalesce(lang, http_lang, cs_uri_query)
| where match(lang_attr, "(?i)(<|%3C)\\s*script|javascript:|onerror\\s*=|onload\\s*=")
| eval firefox_ua=if(match(coalesce(http_user_agent, cs_user_agent), "(?i)FxiOS|Firefox/\\d+.*(iPhone|iPad)"), 1, 0)
| stats count min(_time) as first_seen max(_time) as last_seen values(firefox_ua) as ffiOS by src_ip, dest_host, lang_attr
| sort - count

Escalation logic: high count plus ffiOS=1 (Firefox for iOS user agent present) indicates concentrated delivery of script-like lang attributes to Firefox for iOS clients and should be triaged as a probable exploitation attempt.

Wazuh OSSEC Rule

<group name="web,cve_2026_103418,">
  <rule id="103418" level="10">
    <decoded_as>json</decoded_as>
    <field name="url">lang=</field>
    <description>CVE-2026-103418: possible Firefox for iOS lang-attribute script injection targeting translation.</description>
    <options>no_full_log</options>
    <mitre>
      <id>T1059</id>
      <id>T1189</id>
    </mitre>
    <info type="cve">CVE-2026-103418</info>
    <info type="vendor_link">https://nvd.nist.gov/vuln/detail/CVE-2026-103418</info>
  </rule>

  <rule id="103419" level="12">
    <if_sid>103418</if_sid>
    <field name="url">(&lt;|%3C)script|javascript:|onerror=</field>
    <description>CVE-2026-103418: escalated - lang attribute contains executable script-like payload delivered to Firefox for iOS.</description>
    <mitre>
      <id>T1059</id>
    </mitre>
  </rule>
</group>

Parent/child escalation: rule 103418 matches the lang= marker in URL telemetry (level 10), and 103419 escalates when the same request also contains script-like tokens (level 12). Decode must be mapped to whatever proxy or web-server JSON pipeline carries query strings; non-JSON pipelines should drop the <decoded_as>json</decoded_as> line. All signatures above are heuristic and demand local tuning.

Mitigation & Patch Guidance

Patch confidence: 0/4 (UNPATCHED — no patch evidence from any tracked source). The NVD description states the issue "was fixed in Firefox for iOS 157.1"; however, no patch sources, commit hashes, or vendor advisory links were supplied with the source data, so patch availability cannot be independently verified and the 0/4 confidence rating stands.

1) Immediate mitigations (while patch verification is pending — no vendor workarounds were provided):

• Restrict browsing on affected devices: use an updated alternative browser for high-risk activities (webmail, SSO sessions, SaaS dashboards) until Firefox for iOS 157.1 or later is confirmed and deployed.
• Do not open links from untrusted sources in Firefox for iOS on unpatched devices, and avoid browsing third-party or ad-heavy content in the affected browser; the trigger is page markup (the lang attribute), so any untrusted page is a potential carrier.
• Reduce the attack window where feasible: enforce MDM-managed session timeouts, block or proxy risky site categories on managed iOS fleets, and avoid relying on the page-level CSP as a security boundary for Firefox for iOS users (the advisory confirms injected privileged-world code bypasses CSP).
• Enable available browser hardening that is independent of this issue (e.g., managed bookmarks/allowlists, third-party content restrictions) via MDM, understanding these do not remediate the root cause.

2) Patch / upgrade steps:

• Upgrade Firefox for iOS to 157.1 or later as indicated by the advisory (Verify the installed version: Settings → Firefox → version, or confirm the deployed iOS bundle version via MDM inventory).
• Push the update through MDM, App Store managed distribution, or the organization's app catalog; verify version post-deploy with fleet telemetry and re-check the actual fix presence because no tracked patch source has been verified.
• If an update is not yet deployable, keep the immediate mitigations above in place and track the vendor advisory for confirmation of the fixed release.

3) Long-term hardening:

• Enforce a minimum-version/mainimum-age policy for browsers in mobile app deployment policies and monitor fleet browser versions continuously; extend to all browsers, not just Firefox.
• Treat client-side language-attribute handling, and other "browser interprets page-controlled metadata" paths, as a privileged-input class in any in-house WebView or browser-integrated code: strictly validate and context-encode attributes before interpolation, and never run translation scripts in a privileged world.
• Maintain a defensive posture that does not rely solely on CSP or origin isolation for high-value data: keep server-side monitoring of session anomalies and re-authentication checks for webmail and SaaS, and alert on suspicious CSP-bypass-style behavior.
• Track vendor advisories and CISA/KEV status; should this CVE be observed in the wild, move to rapid emergency upgrade and retroactive log review of sessions on affected devices.

References

• NVD — CVE-2026-103418: https://nvd.nist.gov/vuln/detail/CVE-2026-103418 (primary source for the description and the "fixed in Firefox for iOS 157.1" statement)
• Mozilla Security Advisories (mozilla.org/security/advisories): check for the Firefox for iOS 157.1 advisory entry covering this CVE
• MITRE ATT&CK — T1059 Command and Scripting Interpreter: https://attack.mitre.org/techniques/T1059/ ; T1189 Drive-by Compromise: https://attack.mitre.org/techniques/T1189/
• CWE-79 Improper Neutralization of Input During Web Page Generation (Cross-site Scripting): https://cwe.mitre.org/data/definitions/79.html ; related injection class CWE-94: https://cwe.mitre.org/data/definitions/94.html
• CISA Known Exploited Vulnerabilities catalog: https://www.cisa.gov/known-exploited-vulnerabilities-catalog — no listing for CVE-2026-103418 as of this writing

Evidence status: no public PoC, exploit code, or Nuclei template; not listed in CISA KEV; no observed or reported threat-actor campaign or exploit-in-the-wild activity at time of writing.