CVE-2026-26927: Szafir SDK Web RCE Vulnerability

CVE-2026-26927 Overview

CVE-2026-26927 is an Input Validation Error vulnerability in Szafir SDK Web, a browser plug-in designed to run the SzafirHost application for downloading necessary files when launched. The vulnerability allows an unauthenticated attacker to manipulate the HTTP Origin URL of application calls, enabling the crafting of malicious websites capable of launching the SzafirHost application with arbitrary arguments via the Szafir SDK Web browser addon.

The core issue stems from insufficient validation of the document_base_url parameter. When exploited, the URL address specified in this parameter is displayed in the application confirmation prompt without verifying whether it relates to the actual address of the calling web application. This allows attackers to deceive users into executing applications in the context of a malicious website, potentially downloading harmful files and libraries.

Critical Impact

Attackers can leverage URL origin spoofing to trick users into executing applications that download malicious files from attacker-controlled websites. If victims have previously accepted execution with the "remember" option for a spoofed URL, subsequent attacks require no user interaction.

Affected Products

• Szafir SDK Web (versions prior to 0.0.17.4)
• SzafirHost application (launched via Szafir SDK Web)
• Browser environments with Szafir SDK Web addon installed

Discovery Timeline

• 2026-04-02 - CVE CVE-2026-26927 published to NVD
• 2026-04-02 - Last updated in NVD database

Technical Details for CVE-2026-26927

Vulnerability Analysis

This vulnerability is classified under CWE-348 (Use of Less Trusted Source), which addresses situations where a system improperly relies on data from a less trusted source. In the case of Szafir SDK Web, the browser addon fails to validate whether the document_base_url parameter corresponds to the legitimate origin of the calling web application.

The attack exploits a fundamental trust boundary violation where the application accepts user-controllable input (the document_base_url parameter) and uses it to establish the execution context for the SzafirHost application. This creates a scenario where the application's security decisions are based on attacker-supplied data rather than the actual browser context.

The vulnerability is particularly concerning due to its ability to bypass user confirmation prompts when the "remember" option has been previously selected. This persistence mechanism, intended for user convenience, becomes a security liability when combined with URL origin spoofing.

Root Cause

The root cause of CVE-2026-26927 is the lack of proper validation between the document_base_url parameter and the actual HTTP Origin of the calling web application. The Szafir SDK Web browser addon accepts arbitrary URL values in this parameter without cross-referencing them against the true origin of the request. This design flaw allows attackers to inject malicious URLs that appear legitimate in confirmation dialogs while actually pointing to attacker-controlled resources.

Attack Vector

The attack is network-based and requires user interaction to initiate. An attacker crafts a malicious website containing JavaScript that invokes the Szafir SDK Web addon with a spoofed document_base_url parameter. When a victim visits this malicious site, the SzafirHost application is launched with the attacker's arbitrary arguments.

The confirmation prompt displays the attacker-specified URL, which may appear trustworthy to the user. If the victim accepts execution, the SzafirHost application operates in the context of the malicious URL and may download files from the attacker's server. The attack becomes zero-click if the victim has previously selected "remember" for the spoofed URL, as no confirmation prompt will be displayed for subsequent requests.

Detection Methods for CVE-2026-26927

Indicators of Compromise

• Unexpected SzafirHost application launches from unfamiliar websites or domains
• Network connections from SzafirHost to suspicious or unknown external URLs
• Browser addon activity logs showing document_base_url parameters pointing to non-standard domains
• Downloaded files or libraries from unexpected sources appearing in SzafirHost directories

Detection Strategies

• Monitor browser addon activity for Szafir SDK Web invocations with mismatched origin headers and document_base_url parameters
• Implement network monitoring to detect SzafirHost connections to domains outside expected trusted sources
• Review browser extension logs for patterns of repeated URL origin manipulation attempts
• Deploy endpoint detection rules to identify anomalous SzafirHost execution patterns

Monitoring Recommendations

• Enable detailed logging for Szafir SDK Web addon activity and SzafirHost application launches
• Configure network security tools to alert on SzafirHost connections to newly registered or low-reputation domains
• Implement user behavior analytics to detect unusual patterns of application confirmation prompt interactions
• Monitor for bulk downloads or library fetches from non-standard URLs initiated by SzafirHost

How to Mitigate CVE-2026-26927

Immediate Actions Required

• Update Szafir SDK Web to version 0.0.17.4 or later immediately
• Review and clear any saved "remember" permissions for URL confirmations in the Szafir SDK Web addon
• Audit systems for any indicators of compromise listed above
• Consider temporarily disabling the Szafir SDK Web addon until the update can be applied

Patch Information

The vulnerability has been addressed in Szafir SDK Web version 0.0.17.4. Organizations should update to this version or later to remediate the URL origin validation flaw. For additional details, refer to the CERT Vulnerability Overview and the Elektroniczny Podpis Resource.

Workarounds

• Disable the Szafir SDK Web browser addon in environments where it is not essential
• Implement browser policies to restrict addon execution to trusted domains only
• Train users to carefully verify URLs in SzafirHost confirmation prompts before accepting
• Deploy network egress filtering to block SzafirHost connections to non-approved domains
• Clear saved "remember" permissions periodically to force re-verification of URL origins