CVE-2019-25729: PDF Signer 3.0 Server-Side Template Injection RCE (CRITICAL)

Technical summary

PDF Signer 3.0 is vulnerable to server-side template injection (SSTI) via the CSRF-TOKEN cookie parameter. The application improperly processes untrusted cookie values without sanitization or validation, allowing PHP template expressions to be interpreted and executed on the server. The vulnerability is unauthenticated and requires no user interaction, making it trivially accessible over the network. By injecting PHP functions like shell_exec() into the cookie, attackers achieve remote code execution (RCE) with the privileges of the web server process. The flaw is classified under CWE-352 (Cross-Site Request Forgery), though the actual exploit mechanism is template injection rather than traditional CSRF.

Business impact

Exploitation of this vulnerability poses an existential threat to any organization running PDF Signer 3.0. An attacker can achieve complete system compromise, including data theft, service disruption, malware installation, and lateral movement into the internal network. Because no authentication or user interaction is required, the attack surface is maximized—any internet-connected instance is vulnerable. Breach scenarios could involve exfiltration of documents processed by the application, credentials stored on the server, or customer data. For enterprises relying on PDF Signer in document workflows, this vulnerability demands immediate remediation to prevent operational and compliance disasters.

Affected systems

PDF Signer version 3.0 is affected. Organizations using this version in production environments are at immediate risk. The vulnerability is network-accessible without authentication, meaning both internet-facing and internal deployments accessible from compromised workstations are vulnerable. No patch version information is provided in available advisories; verify the latest security updates directly from the vendor.

Exploitability

This vulnerability is highly exploitable. It requires only network access and can be triggered by sending a specially crafted HTTP request with a malicious CSRF-TOKEN cookie—no credentials, no user interaction, no complex attack chain. The barrier to exploitation is extremely low; proof-of-concept exploitation would be straightforward for any attacker with basic web security knowledge. The CVSS 3.1 score of 9.8 (CRITICAL) reflects this reality: perfect network accessibility (AV:N), low attack complexity (AC:L), no privileges required (PR:N), and no user interaction needed (UI:N).

Remediation

Immediately assess your environment for PDF Signer 3.0 deployments and treat this as a critical priority. Contact the vendor urgently to obtain the latest security patch. If a patch is unavailable or delayed, consider taking the application offline until remediation is available, or implement strict network access controls limiting connectivity to trusted administrative systems only. After patching, conduct a forensic review of server logs and file integrity to detect any unauthorized access or code execution that may have occurred during the exposure window.

Patch guidance

Verify the availability of security updates from the PDF Signer vendor's official advisory channels. The vendor should provide specific patch version numbers and installation instructions. Apply patches to all instances of PDF Signer 3.0 in your environment, including development, staging, and production systems. Test patches in a controlled environment before production deployment to ensure compatibility with your document workflows. Document the patching timeline and maintain records of all systems patched for compliance auditing.

Detection guidance

Monitor for suspicious CSRF-TOKEN cookie values in web server access logs and WAF logs. Look for patterns containing PHP code, shell_exec(), or other executable syntax within cookie parameters. Implement real-time alerting for requests with unusually long or malformed CSRF-TOKEN values. Review application logs for evidence of unexpected command execution or system activity. Consider deploying a Web Application Firewall (WAF) rule to block requests containing obvious PHP template injection patterns. If you suspect exploitation has occurred, examine process execution logs, file modification timestamps, and network connection logs for anomalous behavior correlated with the attack timeline.

Why prioritize this

This is a CRITICAL vulnerability requiring immediate action. The combination of zero authentication requirements, zero user interaction, network accessibility, and remote code execution capability makes it a top-tier security threat. The CVSS 9.8 score reflects maximum severity. Any attacker capable of reaching the application can compromise the entire server within minutes. Delay in patching materially increases risk of breach, data loss, and operational disruption.

Risk score, explained

The CVSS 3.1 score of 9.8 (CRITICAL) is justified by: Attack Vector Network (AV:N)—the flaw is exploitable remotely; Attack Complexity Low (AC:L)—no special conditions or timing required; Privileges Required None (PR:N)—unauthenticated access suffices; User Interaction None (UI:N)—no social engineering or user action needed; Scope Unchanged (S:U)—impact is confined to the vulnerable service; Confidentiality High (C:H)—sensitive data can be read; Integrity High (I:H)—system state can be modified; Availability High (A:H)—service can be disrupted. This is the highest-severity class of vulnerability.

Frequently asked questions