russh: negotiating a MAC-requiring block cipher (CTR/CBC) with mac=none causes a slice-index-out-of-range panic

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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. Prior to 0.63.1, a connection configured to permit mac=none can negotiate it with a MAC-requiring CTR or CBC block cipher because the selection logic validates needs_mac() only when MAC selection fails. A remote peer can then send a packet with a decrypted length of zero, causing russh/src/cipher/mod.rs to shrink the previously read block before indexing buffer.buffer[16..], which panics and terminates the connection task. This issue is fixed in version 0.63.1.

Source published

29 Sep 2026 18:23:28 UTC

Source modified

29 Sep 2026 19:53:30 UTC

Observed here

23 Oct 2026 15:48:27 UTC

Source terms

CVE Program Terms of Use

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CVSS 3.1 · 3.7 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L

Russh is a Rust SSH client and server library. Prior to 0.63.1, a connection configured to permit mac=none can negotiate it with a MAC-requiring CTR or CBC block cipher because the selection logic validates needs_mac() only when MAC selection fails. A remote peer can then send a packet with a decrypted length of zero, causing russh/src/cipher/mod.rs to shrink the previously read block before indexing buffer.buffer[16..], which panics and terminates the connection task. This issue is fixed in version 0.63.1.

Source published

Sep 29, 2026, 6:23:28 PM (timezone not stated)

Source modified

Sep 29, 2026, 7:53:30 PM (timezone not stated)

Observed here

11 Nov 2026 05:29:27 UTC

Source terms

ENISA EUVD · source terms and attribution apply

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EUVD CVSS 3.1 · 3.7 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L

Russh is a Rust SSH client and server library. Prior to 0.63.1, a connection configured to permit mac=none can negotiate it with a MAC-requiring CTR or CBC block cipher because the selection logic validates needs_mac() only when MAC selection fails. A remote peer can then send a packet with a decrypted length of zero, causing russh/src/cipher/mod.rs to shrink the previously read block before indexing buffer.buffer[16..], which panics and terminates the connection task. This issue is fixed in version 0.63.1.

Source published

29 Sep 2026 19:17:24 UTC

Source modified

29 Sep 2026 20:17:17 UTC

Observed here

25 Oct 2026 04:17:26 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 · 3.7 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L

### Summary `SshBlockCipher` implementations (AES-CBC, AES-CTR, 3DES-CBC, etc.) report `needs_mac() == true`, meaning they are documented/intended to always be paired with a separate integrity MAC. However, key-exchange negotiation only checks `needs_mac()` inside the *fallback* branch of MAC algorithm selection (used when no common MAC algorithm exists). If both peers' preferred MAC lists simply contain `none` and it is successfully negotiated through the normal selection path, nothing rejects pairing `none` with a cipher that requires a MAC. Once negotiated, a single crafted packet from either peer causes `cipher::read()` to shrink an already-allocated buffer below the number of bytes it is about to index, causing a Rust slice-index-out-of-range panic and killing that connection's task. ### Details In `russh/src/cipher/mod.rs`, `read()` for a block cipher: 1. Reads `packet_length_to_read_for_block_length()` bytes up front (16 bytes for any `SshBlockCipher`) into `buffer.buffer`. 2. Decrypts the first block to recover the plaintext packet-length field `len`. 3. Computes `buffer.len = len + cipher.tag_len()`. 4. Calls `buffer.buffer.resize(buffer.len + 4, 0)`. 5. Immediately indexes `buffer.buffer[16..]` (via the constant used for the first block read) to continue decrypting/reading the rest of the packet. When the negotiated MAC is `none`, `tag_len() == 0`. If the attacker (or a MITM holding the session key, or simply the accepting peer testing a hostile client) sends a packet whose *decrypted* length field is `0`, then `buffer.len = 0` and `resize(0 + 4, 0)` **shrinks** the buffer that was already grown to 16 bytes in step 1 down to 4 bytes. The subsequent slice operation `buffer.buffer[16..]` then panics with `range start index 16 out of range for slice of length 4`. The file already defines a `MINIMUM_PACKET_LEN` constant, but it is only consulted on the *write*/padding side, never on the read path — so nothing prevents an incoming packet from declaring a length shorter than the bytes already buffered. Negotiation gap: `negotiation.rs`'s `Select` MAC-selection logic only special-cases `needs_mac()` when negotiation would otherwise *fail* (no common MAC), substituting `none` only if the cipher does not need one. It never re-validates the case where `none` is a common/successfully-negotiated MAC on both sides regardless of what the chosen cipher requires. So an application (or a malicious peer, since negotiation is attacker-influenced on one side) that includes `none` in its own preferred MAC list — while still allowing the default CTR/CBC cipher suite — ends up with an invalid, panic-inducing combination that the library itself should refuse. ### PoC 1. Configure one side's `Preferred` config to include `mac::NONE` in the MAC list (this is a supported, non-default configuration exposed by the crate's public `Preferred` API — used e.g. for legacy/interop compatibility), while leaving the default cipher list (which includes `aes256-ctr`/`aes256-cbc`) untouched. 2. Complete a normal key exchange; negotiation lands on `{cipher: aes*-ctr (or -cbc), mac: none}` because `none` is present and preferred/common on both sides, and nothing during negotiation rejects this pairing. 3. From the peer, send one transport packet whose decrypted packet-length field is `0` (trivial to construct once the session keys are known to that peer, or for the peer that legitimately owns the connection to simply hand-craft, e.g. a modified client for testing). 4. `cipher::read()` on the receiving side panics: `range start index 16 out of range for slice of length 4`. 5. Because each connection is handled in its own `tokio::spawn`'ed task (see the per-connection `select!` loop that calls into `cipher::read()`), the panic unwinds only that task by default, but it unconditionally terminates that SSH connection/session — a working, currently-unauthenticated, already-established connection is killed with no attacker interaction beyond the one crafted packet, and the check runs on every inbound packet including pre-auth ones. ### Impact Denial of service: a remote peer that can influence MAC preference negotiation (or a MITM in possession of the session key) can crash any individual SSH connection/session that ends up negotiating a block cipher together with `mac=none`, with a single crafted packet, pre-authentication. This does not affect the process as a whole (panic is scoped to that connection's task under `panic=unwind`), and does require a non-default configuration that permits `none` as a preferred MAC — hence Low severity. ### Suggested fix In the MAC-selection logic in `negotiation.rs`, reject (or force substitution of) `mac::NONE` whenever the negotiated cipher's `needs_mac()` is `true`, regardless of *how* `none` came to be selected — not only in the "no common MAC" fallback branch. Defensively, `cipher::read()` could also refuse to shrink a buffer below the number of bytes already consumed for the length-field block, returning a protocol error instead of resizing blindly. For credit/changelog purposes, please use: Yazan Balawneh, Cystack.ps

Source published

30 Sep 2026 23:27:30 UTC

Source modified

30 Sep 2026 23:45:04 UTC

Observed here

24 Oct 2026 13:19:54 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

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

Publisher rating · LOW