CVE-2026-102825

Russh is a Rust SSH client and server library. Prior to 0.62.6, the USERAUTH_REQUEST path reached from server::run_stream in russh/src/server/encrypted.rs increments self.common.auth_attempts but never compares it with server::Config.max_auth_attempts. An unauthenticated remote client can continue submitting authentication requests on one connection beyond the configured cap, bypassing the deployment's attempt-limiting policy and increasing online guessing opportunity and backend authentication workload. This issue is fixed in version 0.62.6.

Russh is a Rust SSH client and server library. Prior to 0.62.6, the USERAUTH_REQUEST path reached from server::run_stream in russh/src/server/encrypted.rs increments self.common.auth_attempts but never compares it with server::Config.max_auth_attempts. An unauthenticated remote client can continue submitting authentication requests on one connection beyond the configured cap, bypassing the deployment's attempt-limiting policy and increasing online guessing opportunity and backend authentication workload. This issue is fixed in version 0.62.6.

Details

Affected versions and vulnerable location

• Confirmed present on default branch main at HEAD 0089c89c94753bebbec12b956c07a1cd38740379.
• Crate version at HEAD: 0.62.4.
• Vulnerable locations on current default branch:
  • russh/src/server/mod.rs:91 (pub max_auth_attempts: usize)
  • russh/src/server/mod.rs:121 (default max_auth_attempts: 10)
  • russh/src/server/encrypted.rs:89 (USERAUTH_REQUEST dispatch into auth handler path)
  • russh/src/server/encrypted.rs:98 (self.common.auth_attempts += 1)
  • russh/src/server/encrypted.rs:53 (only runtime read of auth_attempts, used for initial reject timing, not attempt limiting)
• Default-branch history check did not show a newer merged commit adding enforcement against config.max_auth_attempts.

Reachability trace verified

1. Entry point: exported server API server::run_stream in russh/src/server/mod.rs:1049.
2. Session run loop in russh/src/server/session.rs processes incoming packets and calls reply(...) (server/session.rs:725).
3. reply forwards encrypted packets to session.server_read_encrypted(...) (server/mod.rs:1221).
4. server_read_encrypted routes USERAUTH_REQUEST to enc.server_read_auth_request(...) (server/encrypted.rs:89).
5. On each request, self.common.auth_attempts += 1 executes (server/encrypted.rs:98).
6. No comparison against self.common.config.max_auth_attempts is present in this runtime flow.

PoC

Reproduction steps and observed output

I did not run a full server process in this environment because Rust tooling is unavailable. I verified the issue from source and command output on the audited tree.

1. Show where max_auth_attempts appears:

rtk rg -n "max_auth_attempts" .scratch/russh/russh/src/server/mod.rs .scratch/russh/russh/src/server/encrypted.rs .scratch/russh/russh/src/server/session.rs

Observed:

.scratch/russh/russh/src/server/mod.rs:91:    pub max_auth_attempts: usize,
.scratch/russh/russh/src/server/mod.rs:121:            max_auth_attempts: 10,
.scratch/russh/russh/src/server/mod.rs:148:            .field("max_auth_attempts", &self.max_auth_attempts)

2. Show runtime auth-attempt handling:

rtk rg -n "auth_attempts == 0|auth_attempts \\+= 1" .scratch/russh/russh/src/server/encrypted.rs

Observed:

53:        let initial_none_rejection_wait_until = if self.common.auth_attempts == 0 {
98:                self.common.auth_attempts += 1;

3. Show production entrypoint-to-auth path references:

rtk rg -n "pub async fn run_stream|match reply\\(|server_read_encrypted\\(|server_read_auth_request\\(" .scratch/russh/russh/src/server/mod.rs .scratch/russh/russh/src/server/session.rs .scratch/russh/russh/src/server/encrypted.rs

Observed:

.scratch/russh/russh/src/server/encrypted.rs:89:                enc.server_read_auth_request(
.scratch/russh/russh/src/server/session.rs:725:                            match reply(&mut self, &mut handler, &mut pkt).await {
.scratch/russh/russh/src/server/mod.rs:1049:pub async fn run_stream<H, R>(
.scratch/russh/russh/src/server/mod.rs:1221:    session.server_read_encrypted(handler, pkt).await

4. Toolchain check:

cargo --version

Observed:

/bin/bash: line 1: cargo: command not found

Impact

Attacker model

• Attacker: unauthenticated remote client with TCP reachability to a russh-backed SSH service.
• Preconditions: deployer expects server::Config.max_auth_attempts to cap attempts.
• Impact: repeated USERAUTH_REQUEST attempts continue for a single connection beyond configured limit, increasing online guessing opportunity and backend auth workload.

Suggested fix

Enforce max_auth_attempts in the USERAUTH_REQUEST branch before invoking auth-method handlers, and fail closed once threshold is reached.

Concrete patch direction in russh/src/server/encrypted.rs:

if self.common.config.max_auth_attempts > 0
  …

Russh is a Rust SSH client and server library. Prior to 0.62.6, the USERAUTH_REQUEST path reached from server::run_stream in russh/src/server/encrypted.rs increments self.common.auth_attempts but never compares it with server::Config.max_auth_attempts. An unauthenticated remote client can continue submitting authentication requests on one connection beyond the configured cap, bypassing the deployment's attempt-limiting policy and increasing online guessing opportunity and backend authentication workload. This issue is fixed in version 0.62.6.

Russh is a Rust SSH client and server library. Prior to 0.62.6, the USERAUTH_REQUEST path reached from server::run_stream in russh/src/server/encrypted.rs increments self.common.auth_attempts but never compares it with server::Config.max_auth_attempts. An unauthenticated remote client can continue submitting authentication requests on one connection beyond the configured cap, bypassing the deployment's attempt-limiting policy and increasing online guessing opportunity and backend authentication workload. This issue is fixed in version 0.62.6.

Russh is a Rust SSH client and server library. Prior to 0.62.6, the USERAUTH_REQUEST path reached from server::run_stream in russh/src/server/encrypted.rs increments self.common.auth_attempts but never compares it with server::Config.max_auth_attempts. An unauthenticated remote client can continue submitting authentication requests on one connection beyond the configured cap, bypassing the deployment's attempt-limiting policy and increasing online guessing opportunity and backend authentication workload. This issue is fixed in version 0.62.6.

Details

Affected versions and vulnerable location

• Confirmed present on default branch main at HEAD 0089c89c94753bebbec12b956c07a1cd38740379.
• Crate version at HEAD: 0.62.4.
• Vulnerable locations on current default branch:
  • russh/src/server/mod.rs:91 (pub max_auth_attempts: usize)
  • russh/src/server/mod.rs:121 (default max_auth_attempts: 10)
  • russh/src/server/encrypted.rs:89 (USERAUTH_REQUEST dispatch into auth handler path)
  • russh/src/server/encrypted.rs:98 (self.common.auth_attempts += 1)
  • russh/src/server/encrypted.rs:53 (only runtime read of auth_attempts, used for initial reject timing, not attempt limiting)
• Default-branch history check did not show a newer merged commit adding enforcement against config.max_auth_attempts.

Reachability trace verified

1. Entry point: exported server API server::run_stream in russh/src/server/mod.rs:1049.
2. Session run loop in russh/src/server/session.rs processes incoming packets and calls reply(...) (server/session.rs:725).
3. reply forwards encrypted packets to session.server_read_encrypted(...) (server/mod.rs:1221).
4. server_read_encrypted routes USERAUTH_REQUEST to enc.server_read_auth_request(...) (server/encrypted.rs:89).
5. On each request, self.common.auth_attempts += 1 executes (server/encrypted.rs:98).
6. No comparison against self.common.config.max_auth_attempts is present in this runtime flow.

PoC

Reproduction steps and observed output

I did not run a full server process in this environment because Rust tooling is unavailable. I verified the issue from source and command output on the audited tree.

1. Show where max_auth_attempts appears:

rtk rg -n "max_auth_attempts" .scratch/russh/russh/src/server/mod.rs .scratch/russh/russh/src/server/encrypted.rs .scratch/russh/russh/src/server/session.rs

Observed:

.scratch/russh/russh/src/server/mod.rs:91:    pub max_auth_attempts: usize,
.scratch/russh/russh/src/server/mod.rs:121:            max_auth_attempts: 10,
.scratch/russh/russh/src/server/mod.rs:148:            .field("max_auth_attempts", &self.max_auth_attempts)

2. Show runtime auth-attempt handling:

rtk rg -n "auth_attempts == 0|auth_attempts \\+= 1" .scratch/russh/russh/src/server/encrypted.rs

Observed:

53:        let initial_none_rejection_wait_until = if self.common.auth_attempts == 0 {
98:                self.common.auth_attempts += 1;

3. Show production entrypoint-to-auth path references:

rtk rg -n "pub async fn run_stream|match reply\\(|server_read_encrypted\\(|server_read_auth_request\\(" .scratch/russh/russh/src/server/mod.rs .scratch/russh/russh/src/server/session.rs .scratch/russh/russh/src/server/encrypted.rs

Observed:

.scratch/russh/russh/src/server/encrypted.rs:89:                enc.server_read_auth_request(
.scratch/russh/russh/src/server/session.rs:725:                            match reply(&mut self, &mut handler, &mut pkt).await {
.scratch/russh/russh/src/server/mod.rs:1049:pub async fn run_stream<H, R>(
.scratch/russh/russh/src/server/mod.rs:1221:    session.server_read_encrypted(handler, pkt).await

4. Toolchain check:

cargo --version

Observed:

/bin/bash: line 1: cargo: command not found

Impact

Attacker model

• Attacker: unauthenticated remote client with TCP reachability to a russh-backed SSH service.
• Preconditions: deployer expects server::Config.max_auth_attempts to cap attempts.
• Impact: repeated USERAUTH_REQUEST attempts continue for a single connection beyond configured limit, increasing online guessing opportunity and backend auth workload.

Suggested fix

Enforce max_auth_attempts in the USERAUTH_REQUEST branch before invoking auth-method handlers, and fail closed once threshold is reached.

Concrete patch direction in russh/src/server/encrypted.rs:

if self.common.config.max_auth_attempts > 0
  …

Russh is a Rust SSH client and server library. Prior to 0.62.6, the USERAUTH_REQUEST path reached from server::run_stream in russh/src/server/encrypted.rs increments self.common.auth_attempts but never compares it with server::Config.max_auth_attempts. An unauthenticated remote client can continue submitting authentication requests on one connection beyond the configured cap, bypassing the deployment's attempt-limiting policy and increasing online guessing opportunity and backend authentication workload. This issue is fixed in version 0.62.6.