CVE-2026-53900 Overview
CVE-2026-53900 affects Firefox for iOS and stems from improper handling of cookies during cross-origin HTTP redirects within the TemporaryDocument component. When the browser issued an initial PDF request, it preserved cookies set on that request across cross-origin redirects. A malicious site could exploit this behavior to inject arbitrary cookies into requests directed at an unrelated target domain. Mozilla addressed the flaw in Firefox for iOS 152.0 and tracks the issue under advisory MFSA 2026-56. The weakness maps to [CWE-345] Insufficient Verification of Data Authenticity.
Critical Impact
A malicious site can inject arbitrary cookies into HTTP requests sent to an unrelated target domain, enabling session fixation and cross-site tracking scenarios that require user interaction with a crafted PDF link.
Affected Products
- Mozilla Firefox for iOS versions prior to 152.0
- mozilla:firefox_mobile running on iphone_os
- The TemporaryDocument PDF handling component
Discovery Timeline
- 2026-06-16 - CVE-2026-53900 published to NVD
- 2026-06-17 - Last updated in NVD database
Technical Details for CVE-2026-53900
Vulnerability Analysis
Firefox for iOS uses a TemporaryDocument mechanism to fetch and render PDF content outside the normal page navigation flow. The flaw allowed cookies set on the initial PDF request to persist across cross-origin HTTP redirects. Normally, a browser must isolate cookie state per-origin during a redirect chain to prevent one origin from influencing requests destined for another. The defect violated this isolation boundary for PDF document fetches. Attackers could leverage the behavior to plant cookies that subsequent requests to an unrelated target domain would carry. This category of weakness falls under input authenticity verification failures.
Root Cause
The root cause is the failure of TemporaryDocument to discard or re-evaluate cookies bound to the originating request when an HTTP redirect crossed origins. The redirect handler did not treat the destination origin as a new cookie scope. Consequently, cookies attached to the initial PDF fetch remained attached to follow-up requests at the redirect target.
Attack Vector
Exploitation requires user interaction. A victim must click a link or navigate to attacker-controlled content that triggers a PDF fetch through Firefox for iOS. The attacker hosts an endpoint that returns a Set-Cookie header on the initial PDF response, then issues an HTTP redirect (for example, 302 Found) pointing to an unrelated target domain. Firefox for iOS forwards the attacker-supplied cookie values into the request to that unrelated domain. The result is cookie injection against the target, which can enable session fixation, bypass of cookie-based CSRF tokens, or pollution of analytics and authentication state. No special privileges are required, and the attack is network-reachable. Confidentiality impact is limited; integrity and availability are not directly affected.
Detection Methods for CVE-2026-53900
Indicators of Compromise
- Outbound HTTP requests from iOS Firefox user agents containing unexpected Cookie headers that do not match prior server-issued cookies for the target domain.
- Server-side logs showing Set-Cookie responses on .pdf resource requests followed by 3xx redirects to third-party hosts.
- Authentication or session anomalies tied to mobile Safari/Firefox iOS user agent strings, such as session IDs appearing before login.
Detection Strategies
- Inspect web server logs for redirect chains that originate from PDF endpoints and terminate at unrelated origins, especially when the initial response sets cookies.
- Correlate client User-Agent strings identifying Firefox iOS builds below 152.0 with suspicious cookie values on inbound requests.
- Deploy web application firewall rules that flag cross-origin redirect responses returning Set-Cookie from PDF MIME-type responses.
Monitoring Recommendations
- Monitor mobile endpoint inventories for Firefox for iOS versions prior to 152.0 and track update compliance.
- Alert on session token reuse or token presence preceding any authentication event for sessions originating on iOS Firefox clients.
- Review CDN and load balancer telemetry for atypical Referer patterns chaining PDF URLs into login or API endpoints.
How to Mitigate CVE-2026-53900
Immediate Actions Required
- Update Firefox for iOS to version 152.0 or later through the Apple App Store on all managed devices.
- Enforce mobile device management (MDM) policies that require minimum browser versions before granting access to corporate web resources.
- Audit authentication systems for session fixation resilience by binding sessions to additional client attributes beyond the cookie value.
Patch Information
Mozilla released the fix in Firefox for iOS 152.0. Full details are documented in the Mozilla Security Advisory MFSA 2026-56 and the Mozilla Bug Report #2043204. Administrators should validate that deployed versions on iOS devices meet or exceed the patched build.
Workarounds
- Instruct users to avoid opening PDF links from untrusted sources within Firefox for iOS until the update is applied.
- Use an alternative browser on iOS for PDF viewing during the remediation window.
- Set the SameSite=Strict attribute on sensitive cookies server-side to reduce the impact of injected cookies on authenticated endpoints.
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
