CVE-2009-3555: TLS/SSL renegotiation flaw allows plaintext injection into sessions

Automated analysis

Generated by VULONE's analysis model from the NVD record, CISA KEV and EPSS data on 18 September 2026. Confidence: high.

high priorityThe flaw is a protocol-level integrity break with a critical CVSS score and very high EPSS, but it requires a man-in-the-middle position and is not listed in KEV, so it is high rather than critical for most defenders.

What it is

The TLS protocol and SSL 3.0 do not properly bind renegotiation handshakes to the existing connection, so a man-in-the-middle can inject data that the server processes as if it came from the authenticated client. This affects widely deployed implementations including IIS 7.0, Apache mod_ssl, OpenSSL, GnuTLS, NSS, and multiple Cisco products, making it a broad protocol-level issue rather than a single-vendor bug.

Impact

An attacker positioned on the network path can inject arbitrary plaintext into an HTTPS or TLS/SSL-protected session, potentially performing actions as the victim or poisoning application data. The flaw undermines the integrity of the protected channel, though confidentiality of the underlying session is not directly broken by the injection itself.

Attack surface

Reachable over the network by a man-in-the-middle who can intercept and modify traffic between client and server; no authentication or user interaction is required per the CVSS vector (AV:N/AC:L/PR:N/UI:N). The attacker must already be positioned to intercept the connection.

Exploitation

Not listed in CISA KEV and no ransomware associations are documented, but EPSS is very high (0.87264, 99.7th percentile) and at least one reference is tagged as an exploit, indicating mature public exploitation techniques exist.

What to do

• Patch affected TLS/SSL implementations to versions that support RFC 5746 secure renegotiation (e.g., OpenSSL 0.9.8l or later, updated Apache mod_ssl, GnuTLS, NSS, and vendor builds).
• Enable and enforce RFC 5746 secure renegotiation on servers and clients where supported.
• Disable or restrict TLS/SSL renegotiation on servers that do not require it.
• Where feasible, require client certificates or application-layer authentication so injected requests cannot be processed as the victim.
• Retire end-of-life TLS/SSL stacks that cannot be updated to support secure renegotiation.

Detection

• Monitor for unexpected TLS renegotiation events in server and load balancer logs, especially renegotiations that immediately precede state-changing requests.
• Inspect network traffic for TLS handshake patterns inconsistent with the established session, such as renegotiation without proper binding.
• Correlate application logs for requests that appear to originate from an authenticated session but lack matching client-side activity.
• Track TLS library versions in use and alert on hosts still running implementations without RFC 5746 support.

This assessment is produced automatically and is not human-reviewed. Verify against the vendor advisory before acting on it.