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Technical How CVE-2026-39834 Works
The vulnerability resides in the `golang.org/x/crypto/ssh` package, a widely-used library for implementing SSH clients and servers in Go. The core of the issue is an integer overflow in the internal payload size calculation when handling large data writes.
Specifically, when an application performs a single `Write` call on an SSH channel with data exceeding 4 GB, the size of this data is truncated due to the use of a 32-bit integer for the payload size calculation. This overflow causes the write loop to misinterpret the remaining data length, leading to a scenario where the loop spins indefinitely.
In this infinite loop, the system continuously sends empty packets over the network without making any progress on the actual data transfer. This consumes CPU resources and network bandwidth, effectively causing a Denial of Service (DoS). The attack vector is straightforward: a malicious or compromised peer, or even a local process with the ability to write large amounts of data, can trigger this condition.
The fix is simple but critical: the size comparison now uses an `int64` data type, which is large enough to handle data sizes far exceeding 4 GB, thereby preventing the integer truncation that causes the overflow. The vulnerability was discovered and reported by NCC Group Cryptography Services, with the work sponsored by Teleport. It affects all versions of the `golang.org/x/crypto` module prior to v0.52.0.
DailyCVE Form:
Platform: golang.org/x/crypto/ssh
Version: < v0.52.0
Vulnerability : Integer Overflow
Severity: 9.1 Critical
date: 2026-05-22
Prediction: 2026-05-22
What Undercode Say:
The following analytics and commands are relevant for identifying and understanding this vulnerability.
Check your project's dependency version for golang.org/x/crypto go list -m golang.org/x/crypto To see all versions and their details go list -m -versions golang.org/x/crypto Update to the patched version go get golang.org/x/[email protected] Update all dependencies go get -u ./...
The vulnerability is tracked as GO-2026-5020 in the Go vulnerability database. The fix is implemented in commit b61cf85. The issue is also tracked internally by Google as b/502989042.
Exploit:
An attacker can exploit this vulnerability by writing data larger than 4 GB in a single `Write` call on an SSH channel. This can be achieved through various functions that utilize the SSH channel, such as channel.Write, Session.Run, Session.Output, and others. A successful exploit will cause the target process to enter an infinite loop, consuming CPU and network resources, and leading to a Denial of Service.
Protection:
The primary and most effective protection is to update the `golang.org/x/crypto` module to version `v0.52.0` or later. This version contains the fix that changes the size comparison to use int64, preventing the integer overflow.
Impact:
- Denial of Service (DoS): The most direct impact is that an attacker can cause the application to hang indefinitely, making it unresponsive.
- Resource Exhaustion: The infinite loop leads to high CPU usage and the continuous sending of empty network packets, exhausting system resources.
- CVSS Score: The vulnerability has been assigned a CVSS v3.1 score of 9.1 (Critical).
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Sources:
Reported By: github.com
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