CVE-2026-25961: SumatraPDF RCE Vulnerability

CVE-2026-25961 Overview

SumatraPDF, a popular multi-format document reader for Windows, contains a critical vulnerability in its update mechanism. The software disables TLS hostname verification by using the INTERNET_FLAG_IGNORE_CERT_CN_INVALID flag and executes downloaded installers without performing signature verification checks. This combination of security flaws allows a network attacker possessing any valid TLS certificate (including freely available certificates from Let's Encrypt) to intercept update check requests, inject a malicious installer URL, and achieve arbitrary code execution on the victim's system.

Critical Impact

A man-in-the-middle attacker can exploit the disabled TLS hostname verification and missing signature checks to execute arbitrary code on systems running vulnerable SumatraPDF versions during the update process.

Affected Products

• SumatraPDF version 3.5.0
• SumatraPDF version 3.5.1
• SumatraPDF version 3.5.2

Discovery Timeline

• 2026-02-09 - CVE-2026-25961 published to NVD
• 2026-02-10 - Last updated in NVD database

Technical Details for CVE-2026-25961

Vulnerability Analysis

This vulnerability (CWE-295: Improper Certificate Validation) represents a fundamental failure in secure software update implementation. The affected SumatraPDF versions disable critical TLS security mechanisms during the update check process by explicitly ignoring certificate Common Name (CN) validation. When combined with the absence of code signing verification for downloaded installers, this creates a complete bypass of the software's update integrity chain.

The attack requires a network position that allows interception of the update traffic, such as a compromised network, rogue access point, or DNS spoofing attack. Once positioned, an attacker can present any valid TLS certificate to establish a trusted connection with the victim's SumatraPDF client, then serve a malicious installer that will be executed without verification.

Root Cause

The root cause is the use of INTERNET_FLAG_IGNORE_CERT_CN_INVALID in the Windows Internet API (WinINet) calls that handle update checks. This flag instructs the system to accept TLS certificates even when the hostname doesn't match the certificate's Common Name or Subject Alternative Names. Additionally, the update mechanism lacks cryptographic signature verification for downloaded installer files, meaning any executable served by the attacker will be executed with the current user's privileges.

Attack Vector

The attack vector is network-based and requires user interaction (the user must trigger or accept an update). An attacker positioned in the network path between the victim and SumatraPDF's update servers can:

1. Intercept the HTTPS update check request
2. Present a valid TLS certificate for any domain (bypassing hostname verification)
3. Respond with a modified update response containing a malicious installer URL
4. Serve a trojanized installer when the victim proceeds with the update
5. Achieve arbitrary code execution when the unsigned installer is executed

The vulnerability is particularly concerning because the attacker only needs any valid TLS certificate, not one specifically issued for SumatraPDF's domains. Free certificate authorities like Let's Encrypt make obtaining such certificates trivial.

Detection Methods for CVE-2026-25961

Indicators of Compromise

• Unexpected network connections from SumatraPDF processes to IP addresses not associated with legitimate update servers
• SumatraPDF update processes connecting to domains with recently issued or unusual TLS certificates
• Downloaded installer files with mismatched or missing digital signatures in the user's temp directories
• Execution of unsigned or unexpectedly signed executables following SumatraPDF update prompts

Detection Strategies

• Monitor TLS certificate validation events and flag connections where hostname verification warnings are suppressed
• Implement network detection rules for SumatraPDF update traffic to non-standard destinations
• Deploy endpoint detection to identify unsigned executables launched by SumatraPDF processes
• Use application whitelisting to prevent execution of untrusted installers

Monitoring Recommendations

• Configure network monitoring to alert on SumatraPDF connections to IP addresses outside expected update infrastructure ranges
• Enable process monitoring for child processes spawned by SumatraPDF, particularly installer or executable launches
• Review Windows Application logs for certificate validation errors associated with SumatraPDF network activity

How to Mitigate CVE-2026-25961

Immediate Actions Required

• Disable automatic updates in SumatraPDF settings until a patched version is available
• Download SumatraPDF updates only from the official website (https://www.sumatrapdfreader.org) over a trusted network
• Manually verify the digital signature of any downloaded SumatraPDF installer before execution
• Consider using network segmentation to limit exposure of systems running vulnerable versions

Patch Information

Users should monitor the GitHub Security Advisory for patch availability and upgrade instructions. Upgrade to version 3.5.3 or later when available, which should implement proper TLS hostname verification and installer signature validation.

Workarounds

• Disable automatic update checks in SumatraPDF: Settings → Options → Automatically check for updates (uncheck)
• Use a trusted VPN when performing manual updates to reduce man-in-the-middle exposure
• Block SumatraPDF from making outbound network connections via Windows Firewall or endpoint security tools
• Implement application control policies to prevent execution of unsigned installers system-wide

# Windows Firewall rule to block SumatraPDF network access (PowerShell as Administrator)
New-NetFirewallRule -DisplayName "Block SumatraPDF Outbound" -Direction Outbound -Program "C:\Program Files\SumatraPDF\SumatraPDF.exe" -Action Block