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How CVE-2026-39829 Works
The vulnerability resides in the RSA and DSA public key parsers within the `golang.org/x/crypto/ssh` package. Prior to version v0.52.0, these parsers did not enforce any size limits on the parameters of incoming public keys. An attacker, even without prior authentication, can exploit this by presenting a specially crafted SSH public key containing an excessively large RSA modulus or oversized DSA parameters (such as p, q, or g) during the public key authentication phase of an SSH handshake.
When the SSH server attempts to verify the signature of this malicious key, the parser processes the enormous parameters. This leads to an algorithmic complexity attack where the mathematical operations (modular exponentiation, primality testing, or parameter validation) consume an disproportionate amount of CPU resources. In a real-world scenario, a single crafted key can cause several minutes of 100% CPU utilization on the server, effectively rendering the SSH service unresponsive.
This is a pre-authentication attack vector, meaning the server is vulnerable before any username or password is exchanged, making it highly dangerous for exposed SSH services. The root cause is the lack of resource throttling and input validation on untrusted cryptographic material. The fix introduced in v0.52.0 implements two critical mitigations: RSA moduli are now strictly capped at a maximum of 8192 bits, and DSA parameters are rigorously validated against the requirements specified in FIPS 186-2, rejecting any key that does not conform to these standards.
DailyCVE Form:
Platform: golang.org/x/crypto/ssh
Version: < 0.52.0
Vulnerability: Resource Exhaustion DoS
Severity: High (CVSS 7.5)
date: 2026-06-25
Prediction: Patch already available
What Undercode Say: Analytics
The following analytics and commands can be used to detect vulnerable versions and test for the presence of the flaw.
1. Checking Your Go Module Version
To determine if your project is using an affected version of the `crypto/ssh` package, run the following command in your Go module root:
go list -m -u -json golang.org/x/crypto | jq '.Version'
If the output shows a version lower than v0.52.0, your application is vulnerable.
2. Simulating the Vulnerability (Proof of Concept)
While a full exploit requires crafting a specific ASN.1 DER blob for the public key, the following Go code snippet illustrates the vulnerable parsing logic that could be triggered by a malicious ssh.PublicKey:
package main
import (
"crypto/rsa"
"crypto/x509"
"encoding/pem"
"fmt"
"golang.org/x/crypto/ssh"
)
func main() {
// Simulated oversized RSA public key (modulus > 8192 bits)
// In a real attack, this would be sent during authentication.
oversizedKey := `--BEGIN RSA PUBLIC KEY--
MIIBCgKCAQEA... (extremely long modulus) ...IDAQAB
--END RSA PUBLIC KEY--`
block, _ := pem.Decode([]byte(oversizedKey))
if block == nil {
return
}
pub, err := x509.ParsePKIXPublicKey(block.Bytes)
if err != nil {
return
}
rsaPub, ok := pub.(rsa.PublicKey)
if !ok {
return
}
// This conversion triggers the vulnerable parser in older versions
sshKey, err := ssh.NewPublicKey(rsaPub)
if err != nil {
fmt.Println("Error (Expected in fixed version):", err)
return
}
fmt.Println("Key parsed (Vulnerable if no error):", sshKey.Type())
}
In versions prior to v0.52.0, this code would accept the oversized key and potentially cause high CPU usage during subsequent verification steps.
3. Detecting Exploitation Attempts in Logs
Monitoring SSH logs for repeated authentication failures with unusually large key exchange payloads can indicate an ongoing attack. Use the following `grep` command to search for anomalies in auth logs:
sudo grep "Failed publickey" /var/log/auth.log | awk '{print $NF}' | sort | uniq -c | sort -nr
Look for spikes in authentication attempts originating from a single source IP, as this may signal a DoS attempt.
Exploit: Technical Execution
An unauthenticated attacker can exploit this vulnerability by initiating an SSH connection and, during the `SSH_MSG_USERAUTH_REQUEST` message, presenting a public key blob where the `RSA modulus` (or DSA `p` parameter) is excessively large (e.g., 16,384 bits or more). The server, upon receiving this message, will attempt to parse and verify the key, entering a CPU-intensive loop that can last for several minutes.
Attack Vector Details:
- Pre-Authentication: The attack occurs before the user is authenticated, meaning no valid credentials are required.
- Amplification: A single 10 KB malicious key can cause minutes of CPU time, effectively creating a 1:1000 resource amplification ratio.
- Protocol: Exploits the standard SSH public key authentication method (RFC 4252).
Protection: Mitigation Strategies
The primary and most effective protection is to update the `golang.org/x/crypto` module to version v0.52.0 or later.
1. Immediate Patching
Update your Go dependencies using the following commands:
go get golang.org/x/[email protected] go mod tidy
2. Network-Level Mitigations (If Patching Is Not Immediately Possible)
– Rate Limiting: Implement connection rate limiting on port 22 (SSH) using tools like `fail2ban` or `iptables` to slow down repeated authentication attempts.
– Firewall Rules: Restrict SSH access to trusted IP ranges only, reducing the attack surface.
– Disable Public Key Authentication (Temporary): As a last resort, temporarily switch to password-based authentication only (though this introduces other risks), as the vulnerability is triggered specifically during public key processing.
Impact: Business and Technical Consequences
- Service Disruption (Availability): A successful exploit leads to a complete Denial of Service (DoS) of the SSH daemon. The server becomes unresponsive, preventing legitimate administrators from accessing the system via SSH.
- Resource Exhaustion: The attack consumes 100% CPU on the targeted core(s) for extended periods (minutes per request), potentially affecting other services running on the same host due to resource contention.
- Operational Overhead: Recovery requires manual intervention (killing the SSH process or restarting the service), leading to significant operational downtime and incident response costs.
- Supply Chain Risk: As `golang.org/x/crypto/ssh` is a core dependency in many cloud-native and containerized environments (e.g., Kubernetes, OpenShift), a single vulnerable instance can compromise the entire orchestration layer’s management plane.
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Sources:
Reported By: github.com
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