russh agent server converts constraint records into unbounded never-rolled-back tasks

CISA KEV is observed exploitation evidence. FIRST EPSS is a dated prediction of exploitation in the next 30 days. Missing scores are unknown. This collector requests predictions only for explicit CVE aliases in the public NuGet archive; Microsoft release views retain their existing prediction scope.

No collected risk observation for these identifiers. This does not establish absence of exploitation or a zero probability.

russh is a Rust SSH client and server library that includes an SSH agent protocol implementation. Prior to 0.64.1, the agent server spawns a separate long-lived task for every SSH_AGENT_CONSTRAIN_LIFETIME record in a constrained add-key request. The loop accepts repeated lifetime records, clones the key blob into each task, and does not roll back tasks when later constraint parsing fails. An attacker that can speak the agent protocol locally or through agent forwarding can exhaust memory and tasks until the agent process is starved or terminated, denying SSH agent service. This issue is fixed in version 0.64.1.

Source published

19 Oct 2026 16:30:24 UTC

Source modified

19 Oct 2026 18:29:47 UTC

Observed here

23 Oct 2026 15:48:27 UTC

Source terms

CVE Program Terms of Use

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CVSS 3.1 · 4.3 CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

russh is a Rust SSH client and server library that includes an SSH agent protocol implementation. Prior to 0.64.1, the agent server spawns a separate long-lived task for every SSH_AGENT_CONSTRAIN_LIFETIME record in a constrained add-key request. The loop accepts repeated lifetime records, clones the key blob into each task, and does not roll back tasks when later constraint parsing fails. An attacker that can speak the agent protocol locally or through agent forwarding can exhaust memory and tasks until the agent process is starved or terminated, denying SSH agent service. This issue is fixed in version 0.64.1.

Source published

Oct 19, 2026, 4:30:24 PM (timezone not stated)

Source modified

Oct 19, 2026, 6:29:47 PM (timezone not stated)

Observed here

7 Nov 2026 04:32:34 UTC

Source terms

ENISA EUVD · source terms and attribution apply

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: euvd · ENISA EUVD · source terms and attribution apply. Source text is retained; field grouping and change labels are provided by ChangeIntel.

EUVD CVSS 3.1 · 4.3 CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

russh is a Rust SSH client and server library that includes an SSH agent protocol implementation. Prior to 0.64.1, the agent server spawns a separate long-lived task for every SSH_AGENT_CONSTRAIN_LIFETIME record in a constrained add-key request. The loop accepts repeated lifetime records, clones the key blob into each task, and does not roll back tasks when later constraint parsing fails. An attacker that can speak the agent protocol locally or through agent forwarding can exhaust memory and tasks until the agent process is starved or terminated, denying SSH agent service. This issue is fixed in version 0.64.1.

Source published

19 Oct 2026 17:17:03 UTC

Source modified

19 Oct 2026 19:17:22 UTC

Observed here

25 Oct 2026 05:17:13 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: nvd · NIST NVD usage terms; underlying source attribution retained. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 3.1 · [email protected] · Secondary · 4.3 CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

### Summary `russh_keys::agent::server::add_key` (`russh/src/keys/agent/server.rs:271-319`) spawns a dedicated tokio task for every 5-byte `SSH_AGENT_CONSTRAIN_LIFETIME` record attached to an added key, each cloning the key blob and sleeping for the declared lifetime (up to ~136 years). The loop has no per-request task budget, and spawned tasks are not rolled back when the overall request fails. Repeating one constrained add-key message over an agent connection converts wire bytes into resident tasks and heap at roughly 141x amplification: in our measurement 10.2 MiB of protocol traffic produced ~2,096,120 live tasks and +1,447,856 KiB RSS, while the same traffic sent as unconstrained add-key requests cost +268 KiB. An attacker reaching the agent socket (a local process, or a remote SSH server the user connects to with agent forwarding, since russh clients accept the `[email protected]` channel by default at `russh/src/client/mod.rs:2686`) can exhaust the agent process's memory and task capacity with a renewable, cheap request stream. Verified on russh HEAD efa9a03 (v0.63.3-10, 2026-10-01). ### Details ```rust // russh/src/keys/agent/server.rs:271-319 (add_key, constraint loop) while let Ok(t) = u8::decode(r) { match t { SSH_AGENT_CONSTRAIN_LIFETIME => { let lifetime = u32::decode(r)?; ... tokio::spawn({ let key = key.clone; async move { tokio::time::sleep(...lifetime...).await; /* remove key */ } }); } ... ``` Each lifetime record clones the key blob into a new task regardless of how many records the same request already spawned, and a failure later in the request leaves earlier tasks alive. Data flow: agent connection -> SSH_AGENTC_ADD_IDENTITY_CONSTRAINED -> constraint decode loop -> unbounded `tokio::spawn` per 5 bytes. The control measurement (identical wire volume as unconstrained adds) isolates the constraint path as the amplifier. ### Proof of concept Prerequisites: Linux host with cargo and git. The archive's deploy script provisions russh at the pinned revision and the PoC crate (rewriting the path dependency); the PoC runs a real russh agent server on loopback and speaks the agent protocol to it. ```bash $ POC_HOME=$HOME/russh-poc RUSSH_REPO=/home/ubuntu/HuntTargets/russh ./deployment/deploy.sh russh at efa9a03 -> /home/ubuntu/russh-poc/poc $ POC_HOME=$HOME/russh-poc ./poc/poc.sh 40 [*] round 29: reply code=6 RSS=1090272 KiB (delta +1087580 KiB) [*] round 39: reply code=6 RSS=1450548 KiB (delta +1447856 KiB) [*] final RSS = 1450548 KiB (delta +1447856 KiB, x141 amplification by wire bytes) [*] ~2096120 tasks now sleep for ~136 years holding their blob + timer. [*] fds = 19 ``` The broker acks every request (code 6 = SUCCESS) while RSS grows to +1.45 GB from ~10 MiB of input. The harness's control mode (same wire volume, unconstrained adds) measured +268 KiB. The automated archive run reproduces exactly this trace. #### Manual reproduction, step by step (no archive required) 1. Provision russh at the pinned revision and build the PoC crate (it hosts a real russh agent server in-process): ```bash git clone https://github.com/Eugeny/russh && cd russh && git checkout efa9a03e541b69505d5e6ec0d9213d011c9f6310 cargo build --release # in the PoC crate directory; the path dependency points at the clone ``` 2. Run the PoC: it starts the agent on loopback, then sends repeated `SSH_AGENTC_ADD_IDENTITY_CONSTRAINED` messages whose constraint sections contain ~52,000 five-byte `SSH_AGENT_CONSTRAIN_LIFETIME` records each: ```bash ./target/release/poc-agent-addkey 40 # [*] round 29: reply code=6 RSS=1090272 KiB (delta +1087580 KiB) # [*] round 39: reply code=6 RSS=1450548 KiB (delta +1447856 KiB) # [*] final RSS = 1450548 KiB (delta +1447856 KiB, x141 amplification by wire bytes) # [*] ~2096120 tasks now sleep for ~136 years holding their blob + timer. ``` 3. Control: the same wire volume sent as UNCONSTRAINED add-key requests measures +268 KiB (`MODE=control`), isolating the constraint-record task spawn as the amplifier. 4. Reachability: any local process with access to the agent socket, or a remote SSH server the user connects to with agent forwarding (russh clients accept `[email protected]` channels by default, `russh/src/client/mod.rs:2686`), can drive this. Each spawned task clones the key blob and sleeps for the declared lifetime (up to ~136 years); tasks are never rolled back when a request fails. ### Impact An attacker able to speak the agent protocol to a russh-based agent (local process on the socket, or a remote SSH server the user connects to with agent forwarding enabled) can drive gigabyte-scale memory growth and millions of leaked tasks per session with megabytes of traffic, renewable until the agent process is starved or OOM-killed, taking the user's SSH key agent down. The trust boundary is the agent protocol peer versus the agent process's resources. No confidentiality or integrity impact.

Source published

19 Oct 2026 17:06:46 UTC

Source modified

19 Oct 2026 17:15:04 UTC

Observed here

24 Oct 2026 13:19:54 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V3 · CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

Publisher rating · MODERATE