CVE-2025-66199
📋 TL;DR
A TLS 1.3 vulnerability in OpenSSL allows attackers to force large memory allocations (up to 22 MiB per connection) via certificate compression, potentially causing denial of service through resource exhaustion. This affects OpenSSL 3.3-3.6 builds with TLS 1.3 certificate compression enabled and compression algorithms available. Only systems using TLS 1.3 with certificate compression negotiation are vulnerable.
💻 Affected Systems
- OpenSSL
📦 What is this software?
Openssl by Openssl
OpenSSL is a robust, commercial-grade toolkit for the Transport Layer Security (TLS) and Secure Sockets Layer (SSL) protocols. It provides cryptographic functions and is one of the most widely used libraries for implementing secure communications in applications worldwide.
Learn more about Openssl →Openssl by Openssl
OpenSSL is a robust, commercial-grade toolkit for the Transport Layer Security (TLS) and Secure Sockets Layer (SSL) protocols. It provides cryptographic functions and is one of the most widely used libraries for implementing secure communications in applications worldwide.
Learn more about Openssl →Openssl by Openssl
OpenSSL is a robust, commercial-grade toolkit for the Transport Layer Security (TLS) and Secure Sockets Layer (SSL) protocols. It provides cryptographic functions and is one of the most widely used libraries for implementing secure communications in applications worldwide.
Learn more about Openssl →Openssl by Openssl
OpenSSL is a robust, commercial-grade toolkit for the Transport Layer Security (TLS) and Secure Sockets Layer (SSL) protocols. It provides cryptographic functions and is one of the most widely used libraries for implementing secure communications in applications worldwide.
Learn more about Openssl →⚠️ Risk & Real-World Impact
Worst Case
Resource exhaustion leading to complete service degradation or crash of TLS services handling multiple connections
Likely Case
Service performance degradation under targeted attack, increased memory usage and CPU consumption
If Mitigated
Minimal impact with proper controls; memory allocations bounded by max_cert_list setting
🎯 Exploit Status
Attack requires establishing TLS 1.3 connections with certificate compression. No memory corruption or information disclosure occurs.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Patches available via OpenSSL security updates; check specific version for your distribution
Vendor Advisory: https://openssl-library.org/news/secadv/20260127.txt
Restart Required: Yes
Instructions:
1. Update OpenSSL to patched version from your distribution's repositories. 2. Restart all services using OpenSSL. 3. Verify the fix with version check and testing.
🔧 Temporary Workarounds
Disable certificate compression reception
allSet SSL_OP_NO_RX_CERTIFICATE_COMPRESSION option to disable receiving compressed certificates
In application code: SSL_CTX_set_options(ctx, SSL_OP_NO_RX_CERTIFICATE_COMPRESSION)
🧯 If You Can't Patch
- Disable TLS 1.3 certificate compression in build configuration
- Implement rate limiting on TLS connections to mitigate DoS impact
🔍 How to Verify
Check if Vulnerable:
Check OpenSSL version and build configuration: openssl version -a | grep 'OpenSSL 3.[3-6]' and verify if certificate compression is enabledCheck Version:
openssl versionVerify Fix Applied:
Verify OpenSSL version is updated and test TLS 1.3 connections with certificate compression📡 Detection & Monitoring
Log Indicators:
- Unusually large memory allocations during TLS handshakes
- Multiple TLS handshake failures with large certificate sizes
- Increased memory usage in TLS services
Network Indicators:
- TLS 1.3 connections with certificate compression extension
- Abnormally large CompressedCertificate messages
SIEM Query:
Search for TLS handshake failures with error codes related to certificate processing or memory allocation errors🔗 References
- https://github.com/openssl/openssl/commit/3ed1f75249932b155eef993a8e66a99cb98bfef4
- https://github.com/openssl/openssl/commit/6184a4fb08ee6d7bca570d931a4e8bef40b64451
- https://github.com/openssl/openssl/commit/895150b5e021d16b52fb32b97e1dd12f20448be5
- https://github.com/openssl/openssl/commit/966a2478046c311ed7dae50c457d0db4cafbf7e4
- https://openssl-library.org/news/secadv/20260127.txt
