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The vulnerability exists within the GSSAPI authentication parsing logic of an SSH server using the affected `golang.org/x/crypto/ssh` library. When a client initiates a GSSAPI authentication request, it sends a message containing a list of supported authentication mechanisms. This list is prefixed by a field specifying the number of mechanisms. The vulnerable code fails to properly validate this number before proceeding to allocate memory for the subsequent mechanism identifiers. An attacker can craft a malicious SSH packet with an excessively large value in this field. This causes the server’s parsing function to enter a loop that attempts to allocate memory for a massive number of non-existent mechanism entries, leading to a Denial of Service (DoS) condition through exhaustive memory consumption and subsequent server panic.
Platform: Golang
Version: x/crypto/ssh
Vulnerability: DoS
Severity: Moderate
date: 2025-11-19
Prediction: 2025-12-10
What Undercode Say:
Simulating a malicious GSSAPI payload with an oversized mechanism count echo "00000100" | xxd -r -p > malicious_payload.bin
// Code snippet highlighting the vulnerable loop (conceptual)
func parseGSSAPIMechs(buf []byte) ([]string, error) {
mechCount, buf := parseUint32(buf) // Not validated
for i := 0; i < mechCount; i++ { // Loop with attacker-controlled count
// Allocate memory for each mechanism...
}
}
How Exploit:
Craft a malformed SSH user authentication request (GSSAPI) containing an impossibly large number of mechanisms, causing the server to exhaust memory and crash.
Protection from this CVE
Update the `golang.org/x/crypto/ssh` library to the patched version once released. Implement network-level controls to limit connection attempts from single sources.
Impact:
Denial of Service (DoS) via resource exhaustion, leading to SSH server unavailability.
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Sources:
Reported By: github.com
Extra Source Hub:
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