CVE-2021-43848 is a vulnerability identified in the open source HTTP server h2o, specifically affecting its HTTP/3 server-side implementation in versions prior to commit 8c0eca3. The flaw arises from the use of uninitialized memory when processing QUIC frames received in a particular sequence. This leads to the server potentially interpreting uninitialized memory as valid HTTP/3 frames. The critical aspect of this vulnerability is that when h2o is deployed as a reverse proxy, an attacker can exploit this flaw to leak internal server state information to backend servers under their control or to third parties. Additionally, if there exists an HTTP endpoint that reflects client traffic, attackers can leverage this to extract sensitive internal state data from h2o. The leaked internal state includes unencrypted traffic from other connections and TLS session tickets, which could allow attackers to decrypt or hijack sessions, compromising confidentiality and integrity. This vulnerability affects h2o versions between commits 93af138 and d1f0f65, but importantly, none of the officially released versions are impacted. No known workarounds exist, and users running unreleased versions with HTTP/3 support are strongly advised to upgrade immediately to a fixed commit. There are no known exploits in the wild at this time.

For European organizations, the impact of this vulnerability can be significant if they use unreleased or development versions of h2o with HTTP/3 enabled, particularly in reverse proxy configurations. The exposure of internal server state, including unencrypted traffic and TLS session tickets, can lead to severe confidentiality breaches, allowing attackers to intercept or manipulate sensitive data. This could affect sectors handling critical or sensitive information such as finance, healthcare, government, and telecommunications. The integrity of communications could also be compromised, enabling session hijacking or man-in-the-middle attacks. Availability impact is less direct but could arise if attackers leverage leaked information to further exploit the infrastructure. Since HTTP/3 adoption is growing, especially in modern web services, organizations using h2o in production environments with HTTP/3 support may face increased risk. The absence of known exploits reduces immediate threat but does not eliminate the risk of future exploitation.

Organizations should first verify if they are running any unreleased or development versions of h2o with HTTP/3 support between the specified commits. Immediate upgrade to the fixed commit 8c0eca3 or later is essential to eliminate the vulnerability. Since no workarounds exist, disabling HTTP/3 support temporarily in h2o until an upgrade is possible can reduce exposure. Network segmentation should be employed to limit backend server exposure to potentially malicious traffic. Monitoring and logging of HTTP/3 traffic patterns may help detect anomalous QUIC frame sequences indicative of exploitation attempts. Additionally, organizations should audit their reverse proxy configurations to ensure that backend servers do not inadvertently receive sensitive internal state information. Employing TLS session ticket rotation and strict session management policies can mitigate risks associated with leaked session tickets. Finally, organizations should maintain up-to-date threat intelligence feeds to respond promptly if exploits emerge.