CVE-2023-25728: Mozilla Firefox Information Disclosure Flaw

CVE-2023-25728 Overview

CVE-2023-25728 is an information leakage vulnerability affecting Mozilla Firefox, Firefox ESR, and Thunderbird. The Content-Security-Policy-Report-Only header could allow an attacker to leak a child iframe's unredacted URI when interaction with that iframe triggers a redirect. This vulnerability exploits the CSP reporting mechanism to extract sensitive URL information that should otherwise be protected.

Critical Impact

Attackers can leverage this vulnerability to leak sensitive information contained within iframe URIs, potentially exposing session tokens, authentication parameters, or other confidential data embedded in URLs during user interactions with embedded content.

Affected Products

• Mozilla Firefox versions prior to 110
• Mozilla Firefox ESR versions prior to 102.8
• Mozilla Thunderbird versions prior to 102.8

Discovery Timeline

• 2023-06-02 - CVE-2023-25728 published to NVD
• 2025-01-10 - Last updated in NVD database

Technical Details for CVE-2023-25728

Vulnerability Analysis

This vulnerability is classified under CWE-203 (Observable Discrepancy), which relates to information exposure through timing or behavioral differences. The flaw exists in how Mozilla products handle the Content-Security-Policy-Report-Only HTTP header when child iframes perform redirects during user interaction.

Under normal circumstances, when a Content Security Policy violation occurs, the browser sends a report to the specified endpoint. However, this implementation contained a flaw where the full, unredacted URI of a child iframe could be leaked through CSP violation reports when that iframe triggered a redirect. This creates an observable side channel that attackers can exploit to extract sensitive URI components.

Root Cause

The root cause lies in the improper handling of URI redaction in CSP violation reports. When a child iframe performs a redirect during user interaction, the browser should redact sensitive portions of the URI before including it in the CSP report. However, the affected Mozilla products failed to properly sanitize this information, allowing the complete URI—including potentially sensitive query parameters and fragments—to be transmitted to the attacker-controlled reporting endpoint.

Attack Vector

The attack requires network access and user interaction. An attacker would need to:

1. Host a malicious webpage that includes a target iframe
2. Configure a Content-Security-Policy-Report-Only header pointing to an attacker-controlled endpoint
3. Wait for user interaction with the iframe that triggers a redirect
4. Capture the CSP violation report containing the unredacted URI

The attack scenario involves embedding a cross-origin iframe within an attacker-controlled page. When a user interacts with the embedded content and this triggers a navigation or redirect, the CSP reporting mechanism inadvertently leaks the full destination URI to the attacker's reporting endpoint. This could expose sensitive information such as session identifiers, CSRF tokens, or other URL-embedded secrets. For detailed technical analysis, refer to the Mozilla Bug Report #1790345.

Detection Methods for CVE-2023-25728

Indicators of Compromise

• Unusual CSP violation reports being generated and sent to external endpoints
• Webpages with Content-Security-Policy-Report-Only headers pointing to suspicious domains
• Unexpected iframe embedding patterns in web traffic analysis

Detection Strategies

• Monitor network traffic for CSP violation reports being sent to untrusted external endpoints
• Implement web proxy rules to flag suspicious Content-Security-Policy-Report-Only header configurations
• Deploy browser version auditing to identify vulnerable Firefox, Firefox ESR, and Thunderbird installations

Monitoring Recommendations

• Establish baseline CSP reporting patterns for legitimate applications and alert on anomalies
• Configure endpoint detection to track browser version information and flag unpatched installations
• Review DNS queries for connections to known malicious CSP reporting endpoints

How to Mitigate CVE-2023-25728

Immediate Actions Required

• Update Mozilla Firefox to version 110 or later
• Update Mozilla Firefox ESR to version 102.8 or later
• Update Mozilla Thunderbird to version 102.8 or later
• Implement enterprise browser version management policies to ensure timely updates

Patch Information

Mozilla has released security patches addressing this vulnerability. Users should update to the fixed versions immediately:

• Firefox: Version 110+ - See Mozilla Security Advisory MFSA-2023-06
• Firefox ESR: Version 102.8+ - See Mozilla Security Advisory MFSA-2023-05
• Thunderbird: Version 102.8+ - See Mozilla Security Advisory MFSA-2023-07

Workarounds

• If immediate patching is not possible, consider disabling CSP reporting functionality at the network level
• Implement network-level filtering to block CSP reports to untrusted external domains
• Use enterprise browser policies to restrict iframe embedding from untrusted sources
• Monitor for and block connections to known malicious CSP reporting endpoints

# Example: Block CSP reports to external domains via proxy configuration
# Add to squid.conf or similar proxy configuration
acl csp_report_paths urlpath_regex .*csp-report.*
acl trusted_csp_endpoints dstdomain .yourdomain.com
http_access deny csp_report_paths !trusted_csp_endpoints