CVE-2025-61730: TLS 1.3 Information Disclosure Flaw

CVE-2025-61730 Overview

CVE-2025-61730 is an SSL/TLS vulnerability affecting the Go programming language's TLS 1.3 handshake implementation. During the TLS 1.3 handshake, if multiple messages are sent in records that span encryption level boundaries (for instance the Client Hello and Encrypted Extensions messages), the subsequent messages may be processed before the encryption level changes. This timing issue can cause minor information disclosure if a network-local attacker can inject messages during the handshake.

Critical Impact

A network-local attacker positioned to inject messages during a TLS 1.3 handshake could potentially extract sensitive information before the encryption level is properly established, compromising the confidentiality of the TLS session.

Affected Products

• Go programming language (crypto/tls package)
• Applications using Go's TLS 1.3 implementation
• Services relying on Go's standard library for TLS connections

Discovery Timeline

• 2026-01-28 - CVE CVE-2025-61730 published to NVD
• 2026-01-29 - Last updated in NVD database

Technical Details for CVE-2025-61730

Vulnerability Analysis

This vulnerability exists in Go's crypto/tls package during TLS 1.3 handshake processing. The issue arises from improper handling of encryption level transitions when multiple handshake messages are batched within TLS records that cross encryption boundaries.

In a standard TLS 1.3 handshake, messages progress through different encryption levels: initial plaintext (Client Hello), handshake traffic secrets, and application traffic secrets. The vulnerability occurs when the implementation processes subsequent messages before properly transitioning to the appropriate encryption level.

The local attack vector requires the attacker to be positioned on the same network segment to inject crafted messages during the handshake window. While the attack complexity is relatively low and requires no privileges or user interaction, the impact is limited to confidentiality exposure without integrity or availability impact.

Root Cause

The root cause stems from the order of operations in Go's TLS 1.3 handshake state machine. When handshake messages span across records with different encryption level requirements, the message processing logic does not adequately verify that the encryption level has been updated before processing the next message. This creates a brief window where messages intended for a higher encryption level may be processed at a lower level.

Attack Vector

An attacker with local network access can exploit this vulnerability by monitoring and injecting traffic during an active TLS 1.3 handshake between a client and server. The attack requires precise timing to inject messages at the moment when encryption levels are transitioning.

The attacker would need to:

1. Position themselves on the local network to intercept TLS handshake traffic
2. Identify handshake sessions using TLS 1.3
3. Inject crafted messages during the encryption level transition window
4. Capture any information disclosed before proper encryption is established

The vulnerability is documented in the Go Issue Tracker and addressed in Go.dev Change Log Entry. For detailed technical information, refer to the Go Vulnerability Advisory #GO-2026-4340.

Detection Methods for CVE-2025-61730

Indicators of Compromise

• Unusual patterns in TLS 1.3 handshake sequences from local network sources
• Anomalous message injection attempts during TLS handshake phases
• Unexpected handshake failures or retries correlating with local network activity

Detection Strategies

• Monitor for malformed or out-of-sequence TLS 1.3 handshake messages
• Implement network-level inspection for unusual traffic patterns during TLS session establishment
• Deploy intrusion detection rules to identify potential message injection attempts during handshake phases
• Audit Go application versions to identify vulnerable deployments

Monitoring Recommendations

• Enable detailed TLS handshake logging in Go applications to capture handshake anomalies
• Implement network monitoring on internal segments where local attackers could position themselves
• Configure alerts for repeated TLS handshake failures that may indicate exploitation attempts

How to Mitigate CVE-2025-61730

Immediate Actions Required

• Identify all applications using Go's crypto/tls package with TLS 1.3 enabled
• Review the Golang Announce Group Post for official guidance
• Plan upgrade to a patched Go version as specified in the advisory
• Assess network segmentation to reduce local attack surface

Patch Information

The Go development team has addressed this vulnerability in the change referenced at Go.dev Change Log Entry. Organizations should upgrade to the patched Go version as detailed in the Go Vulnerability Advisory #GO-2026-4340.

After updating the Go runtime, rebuild all affected applications to incorporate the security fix. Verify the fix by checking the Go version and reviewing TLS handshake behavior in test environments.

Workarounds

• Implement network segmentation to limit local attacker positioning opportunities
• Deploy network-level TLS inspection at trust boundaries to detect anomalous handshake patterns
• Consider using mutual TLS (mTLS) to add additional authentication layers
• Monitor for the official patch and prioritize upgrade scheduling

# Configuration example
# Verify Go version in your environment
go version

# Check for vulnerable dependencies in Go modules
go list -m all | grep -i tls

# Rebuild applications after Go upgrade
go build -v ./...

# Verify TLS configuration in applications
go test -v ./... -run TLS