CVE-2026-19445
CriticalCVSS 9.2Exploitation Probability (EPSS)
Low risk35th percentile - higher than 35% of all known CVEs
Summary
A remote, unauthenticated TLS client can crash a server or trigger a use-after-free if its sni_callback assigns a different context to SSLSocket.context (the documented way to select a certificate per server name) and nothing else keeps the original ssl.SSLContext alive. Typical cases are servers that create an SSLContext per connection or replace it while connections are open; servers that wrap their listening socket with it are not affected. Mitigation: keep a reference to every SSLContext that sets sni_callback for the lifetime of the server. TLS clients are not affected.
Risk Assessment
The organization may experience server crashes or unpredictable behavior, potentially leading to service disruption or remote code execution. Lack of authentication increases the risk of external attacks.
Recommendation
Ensure all SSLContext instances with sni_callback are kept referenced for the server's lifetime. Apply the vendor's patch or update if available.
Other vulnerabilities in Python ssl module
See all- CVE-2026-115557Critical
Python has added a denylist for known compromised certificates to the "ssl" module to ensure user safety even without certificate revocation configured. Known compromised certificates will automatically be rejected regardless of the SSLContext policy configuration.
- CVE-2026-19553High
ssl.SSLContext.wrap_bio() didn't require the server_hostname argument to not be None if ssl.SSLContext.check_hostname was set. Due to a missing parameter check in SSLObject, if the server_hostname argument isn't supplied then hostname verification would be silently skipped. This defect could lead to programs where certificate hostname verification *appeared* to be succeeding with SSLContext.check_hostname = True and no ValueError being raised due to misconfiguration.
Original NVD description (English source)
A remote, unauthenticated TLS client can make a server crash or call through a freed pointer if its sni_callback assigns a different context to SSLSocket.context (the documented way to select a certificate per server name) and nothing else keeps the original ssl.SSLContext alive. Typical cases are servers that create an SSLContext per connection or replace it while connections are open; servers that wrap their listening socket with it are not affected. Mitigation: keep a reference to every SSLContext that sets sni_callback for the lifetime of the server. TLS clients are not affected.
Vulnerability data from NVD (NIST) · CISA KEV · EPSS