CVE-2025-54804

The channel window adjust message of the SSH protocol is used to track the free space in the receive buffer of the other side of a channel. The current implementation takes the value from the message and adds it to an internal state value. This can result in a integer overflow. If the Rust code is compiled with overflow checks, it will panic. A malicious client can crash a server.

Details

According https://datatracker.ietf.org/doc/html/rfc4254#section-5.2, The value must not overflow.
The incorrect handling is done in server/encrypted.rs and client/encrypted.rs in the handling of CHANNEL_WINDOW_ADJUST.

let amount = map_err!(u32::decode(&mut r))?;
...
channel.recipient_window_size += amount;

It could be replaced with something like

  if let Some(ref mut channel) = enc.channels.get_mut(&channel_num) {
                        // rfc 4254: The window MUST NOT be increased above 2^32 - 1 bytes.
                        new_size = channel.recipient_window_size.saturating_add(amount);
                        channel.recipient_window_size = new_size;
                    }
...

PoC

A customized client code would be required to send a message with a big value like u32_max. Not done yet.

This problem seems only critical to a server. One user can crash the server, which might take down the service. A malicious server could also crash a single client, but this seems not very critical.

An arithmetic operation produces a value that exceeds the integer type's maximum, causing it to wrap to an unexpected small value. Typical impact: incorrect size calculations leading to heap overflows or logic errors.

CVE-2025-54804 has a CVSS score of 6.5 (Medium). The vector is network-reachable, low privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (0.54.1); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2025-54804? CVE-2025-54804 is a medium-severity integer overflow or wraparound vulnerability in russh (rust), affecting versions < 0.54.1. It is fixed in 0.54.1. An arithmetic operation produces a value that exceeds the integer type's maximum, causing it to wrap to an unexpected small value.
2. How severe is CVE-2025-54804? CVE-2025-54804 has a CVSS score of 6.5 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of russh are affected by CVE-2025-54804? russh (rust) versions < 0.54.1 is affected.
4. Is there a fix for CVE-2025-54804? Yes. CVE-2025-54804 is fixed in 0.54.1. Upgrade to this version or later.
5. Is CVE-2025-54804 exploitable, and should I be worried? Whether CVE-2025-54804 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2025-54804 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2025-54804? Upgrade russh to 0.54.1 or later.