ALAS2027-2026-221

References: CVE-2026-78663  CVE-2026-78659  CVE-2026-78662  CVE-2026-56855 
FAQs regarding Amazon Linux ALAS/CVE Severity

Issue Overview:

Package updates are available for Amazon Linux 2027 that fix the following vulnerabilities:
CVE-2026-78663:
The HTTP/2 server could refund connection-level flow control twice for the same data: Once when a client resets a stream (refunding data for any sent-but-unread portion of the stream), and again when a request handler reads the buffered data.
A malicious client could exploit this to bypass the configured connection-level flow control limit (MaxReceiveBufferPerConnection). Total buffered data is still limited by the concurrent stream limit and stream-level flow control.
The HTTP/2 server now waits to refund connection-level flow control for reset streams until after the request handler is complete.

NOTE: https://www.openwall.com/lists/oss-security/2026/10/08/9
NOTE: https://github.com/golang/go/issues/81743
NOTE: Fixed by: https://github.com/golang/go/commit/31dc9e2449e394439498142451627af52626d7c4 (go1.27.2)
NOTE: Fixed by: https://github.com/golang/go/commit/6e049521882c944afc1e519a83971ec421dc2974 (go1.26.9)

CVE-2026-78662:
Previously, a channel registered in the mux's chanList is not usable until it is established. A malicious peer was able flood the channel's incomingRequests, deadlocking the entire connection. Now, we add an atomic established state, set when a channel becomes usable. Until such a time, handlePacket drops every packet other than the open confirmation/failure, without blocking and without tearing down the connection.

CVE-2026-78659:
When "Trailer" headers are sent by a client, the HTTP server internally uses the header values to populate the Request.Trailer map passed to the server handler. Because Request.Trailer is a map, each entry incurs memory overhead. For HTTP/2 servers, a malicious client can exploit this by sending a "Trailer" header that declares a large number of fields causing the server to allocate a disproportionate amount of memory while bypassing Server.MaxHeaderValueCount and Server.MaxHeaderBytes limits.
This exploit is not applicable for HTTP/1 servers, which do not support multiplexing a large number of requests over one TCP connection, and whose Server.MaxHeaderBytes are calculated differently.
Server.MaxHeaderValueCount and Server.MaxHeaderBytes limits are now applied towards the trailer fields declared in "Trailer" headers.

NOTE: https://www.openwall.com/lists/oss-security/2026/10/08/9
NOTE: https://github.com/golang/go/issues/81857
NOTE: Fixed by: https://github.com/golang/go/commit/cbb0fb8b51cba866018b8a0fb4b5458f16c5c80b (go1.27.2)
NOTE: Fixed by: https://github.com/golang/go/commit/6e049521882c944afc1e519a83971ec421dc2974 (go1.26.9)

CVE-2026-56855:
Previously, after a channel has been established, a malicious peer could send crafted messages that would deadlock the entire connection. Now, we handle all RFC 4254 channel messages; global requests are handled explicitly. Then, treat all other messages as a protocol error and tear the connection down instead of buffering and blocking.

Issue Correction:
Run dnf update nerdctl or dnf update --advisory ALAS2027-2026-221 to update your system.

New Packages:

aarch64:
    nerdctl-2.3.5-2.amzn2027.0.3.aarch64

src:
    nerdctl-2.3.5-2.amzn2027.0.3.src

x86_64:
    nerdctl-2.3.5-2.amzn2027.0.3.x86_64