Vulnerability-Lookup

GHSA-76R6-X97P-67VR

Vulnerability from github – Published: 2026-06-11 20:29 – Updated: 2026-06-11 20:29

Summary

Russh: SSH identification parsing accepted non-canonical client banners and did not bound pre-banner input

Details

Summary

russh did not enforce the SSH identification-string rules as deliberately as OpenSSH. In particular, the server-side identification reader used the same permissive path as the client, allowing pre-banner lines from clients, and the reader did not enforce a bounded number of pre-banner lines.

For a library server built on russh, this could allow a remote peer to hold connection setup resources in the cleartext pre-authentication phase with malformed identification input that should have been rejected early.

Details

RFC 4253 section 4.2 defines the SSH protocol version exchange. The identification string is a single line terminated by CR LF, must fit within 255 characters including CR LF, and clients should not send pre-banner lines before their SSH identification string.

Before the fix, russh's identification reader lived in:

russh/src/ssh_read.rs
russh/src/server/mod.rs

The same read_ssh_id() behavior was used for both client and server contexts. That allowed server-side parsing to accept preliminary banner lines from clients, even though RFC 4253 only describes server-side pre-identification text. The reader also discarded preliminary lines without a line-count cap, so a peer could repeatedly send short non-SSH lines and keep the connection in identification parsing until an application-level timeout or external resource limit intervened.

This also creates a remotely observable parser-state oracle inside a single connection. A client can send candidate identification lines one after another: lines not recognized as SSH identification are discarded as pre-banner text, while an accepted identification string terminates banner parsing and advances the connection into key exchange. A strict server would reject the first invalid client pre-banner line and force a reconnect for each probe. This can disclose server-side parser acceptance behavior and make fingerprinting cheaper, though it does not disclose application secrets, credentials, keys, or authenticated user data.

The patch splits the behavior between generic/server-banner-tolerant reading and stricter client-identification reading. It also adds explicit limits for line length and pre-banner line count.

Relevant branch commit:

3de4a68 Harden SSH identification parsing

RFC / OpenSSH Comparison

RFC 4253 section 4.2 says each side sends an identification string of the form SSH-protoversion-softwareversion SP comments CR LF. It allows a server to send other lines before its identification string, but says a client must be able to process such lines. It does not grant the same pre-banner allowance to clients.

OpenSSH portable enforces explicit identification limits:

/home/mjc/projects/openssh-portable/ssh.h: SSH_MAX_BANNER_LEN
/home/mjc/projects/openssh-portable/ssh.h: SSH_MAX_PRE_BANNER_LINES
/home/mjc/projects/openssh-portable/kex.c: client-side handling of server pre-banner lines
/home/mjc/projects/openssh-portable/ssh_api.c: rejects pre-banner lines when acting as a server

I checked /home/mjc/projects/openssh-portable at 45b30e0a5. OpenSSH uses an implementation banner line limit of 8192 bytes and a pre-banner line cap of 1024, which is more permissive than RFC 4253's 255-character identification-string limit. The relevant alignment is the parser shape: OpenSSH permits bounded pre-banner lines when reading a server banner as a client, but rejects pre-banner lines when acting as a server and reading a client identification string.

The russh fix follows that shape: accept bounded pre-banner lines only where the protocol allows them, and reject malformed or excessive identification input early.

PoC

Inline highest-CVSS PoC: unauthenticated remote client pre-banner input to the server identification parser. This demonstrates AV:N/AC:L/PR:N/UI:N.

#[tokio::test]
async fn poc_server_accepts_client_pre_banner_before_ssh_id() {
    use russh::server;
    use tokio::io::{AsyncReadExt, AsyncWriteExt};

    let config = std::sync::Arc::new(server::Config::default());
    let (mut client, server_stream) = tokio::io::duplex(4096);

    let server = tokio::spawn(async move {
        server::run_stream(config, server_stream, NoAuthHandler).await
    });

    let mut server_id = Vec::new();
    client.read_until(b'\n', &mut server_id).await.unwrap();

    client
        .write_all(b"attacker-controlled pre-banner\r\nSSH-2.0-poc\r\n")
        .await
        .unwrap();

    let result = tokio::time::timeout(std::time::Duration::from_millis(250), server).await;

    assert!(
        result.is_err(),
        "vulnerable code keeps processing after accepting a client pre-banner before SSH identification"
    );
}

On vulnerable code, the server-side reader accepts the client pre-banner line and continues instead of rejecting the malformed identification input promptly. The fixed parser rejects client pre-banner lines on the server path.

Impact

Suggested CVSS v3.1:

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
Score: 5.3

Reasoning:

AV:N: reachable over the SSH transport connection
AC:L: no special race or unusual setup is required
PR:N, UI:N: occurs before authentication and needs no user interaction
C:N, I:N: no confidentiality or integrity impact demonstrated
A:L: malformed identification input can consume connection setup resources until rejected by timeout or external limits

Additional impact investigation did not identify a stronger confidentiality, integrity, downgrade, or code-execution primitive. The accepted client pre-banner line is discarded before key exchange and does not become part of remote_sshid; the final client identification string is what feeds the key-exchange transcript. remote_sshid is otherwise exposed to library handlers and debug formatting, but the discarded pre-banner text does not influence authentication state, strict-kex negotiation, KEX algorithm selection, or later packet framing.

One parser-boundary nuance on vulnerable code is that behavior can depend on read chunking: if a client pre-banner line and the real SSH identification line are delivered in the same read, the old parser can discard the buffered identification line and then wait or disconnect; if delivered separately, the old server path can accept the pre-banner and continue. This supports malformed pre-authentication availability impact, but not a demonstrated confidentiality or integrity impact.

Fix / Patch Direction

Use a stricter server-side client-identification reader, enforce the RFC identification-line length, and cap preliminary banner lines. The server path should reject client pre-banner lines instead of treating them like allowed server pre-identification text.

Severity

5.3 (Medium)


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

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{
  "affected": [
    {
      "package": {
        "ecosystem": "crates.io",
        "name": "russh"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0.34.0-beta.1"
            },
            {
              "fixed": "0.61.0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-48108"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-20"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-06-11T20:29:14Z",
    "nvd_published_at": "2026-06-10T22:17:01Z",
    "severity": "MODERATE"
  },
  "details": "### Summary\n\n`russh` did not enforce the SSH identification-string rules as deliberately as OpenSSH. In particular, the server-side identification reader used the same permissive path as the client, allowing pre-banner lines from clients, and the reader did not enforce a bounded number of pre-banner lines.\n\nFor a library server built on `russh`, this could allow a remote peer to hold connection setup resources in the cleartext pre-authentication phase with malformed identification input that should have been rejected early.\n\n### Details\n\nRFC 4253 section 4.2 defines the SSH protocol version exchange. The identification string is a single line terminated by CR LF, must fit within 255 characters including CR LF, and clients should not send pre-banner lines before their SSH identification string.\n\nBefore the fix, `russh`\u0027s identification reader lived in:\n\n- `russh/src/ssh_read.rs`\n- `russh/src/server/mod.rs`\n\nThe same `read_ssh_id()` behavior was used for both client and server contexts. That allowed server-side parsing to accept preliminary banner lines from clients, even though RFC 4253 only describes server-side pre-identification text. The reader also discarded preliminary lines without a line-count cap, so a peer could repeatedly send short non-SSH lines and keep the connection in identification parsing until an application-level timeout or external resource limit intervened.\n\nThis also creates a remotely observable parser-state oracle inside a single connection. A client can send candidate identification lines one after another: lines not recognized as SSH identification are discarded as pre-banner text, while an accepted identification string terminates banner parsing and advances the connection into key exchange. A strict server would reject the first invalid client pre-banner line and force a reconnect for each probe. This can disclose server-side parser acceptance behavior and make fingerprinting cheaper, though it does not disclose application secrets, credentials, keys, or authenticated user data.\n\nThe patch splits the behavior between generic/server-banner-tolerant reading and stricter client-identification reading. It also adds explicit limits for line length and pre-banner line count.\n\nRelevant branch commit:\n\n- `3de4a68 Harden SSH identification parsing`\n\n### RFC / OpenSSH Comparison\n\nRFC 4253 section 4.2 says each side sends an identification string of the form `SSH-protoversion-softwareversion SP comments CR LF`. It allows a server to send other lines before its identification string, but says a client must be able to process such lines. It does not grant the same pre-banner allowance to clients.\n\nOpenSSH portable enforces explicit identification limits:\n\n- `/home/mjc/projects/openssh-portable/ssh.h`: `SSH_MAX_BANNER_LEN`\n- `/home/mjc/projects/openssh-portable/ssh.h`: `SSH_MAX_PRE_BANNER_LINES`\n- `/home/mjc/projects/openssh-portable/kex.c`: client-side handling of server pre-banner lines\n- `/home/mjc/projects/openssh-portable/ssh_api.c`: rejects pre-banner lines when acting as a server\n\nI checked `/home/mjc/projects/openssh-portable` at `45b30e0a5`. OpenSSH uses an implementation banner line limit of `8192` bytes and a pre-banner line cap of `1024`, which is more permissive than RFC 4253\u0027s 255-character identification-string limit. The relevant alignment is the parser shape: OpenSSH permits bounded pre-banner lines when reading a server banner as a client, but rejects pre-banner lines when acting as a server and reading a client identification string.\n\nThe `russh` fix follows that shape: accept bounded pre-banner lines only where the protocol allows them, and reject malformed or excessive identification input early.\n\n### PoC\n\nInline highest-CVSS PoC: unauthenticated remote client pre-banner input to the server identification parser. This demonstrates `AV:N/AC:L/PR:N/UI:N`.\n\n```rust\n#[tokio::test]\nasync fn poc_server_accepts_client_pre_banner_before_ssh_id() {\n    use russh::server;\n    use tokio::io::{AsyncReadExt, AsyncWriteExt};\n\n    let config = std::sync::Arc::new(server::Config::default());\n    let (mut client, server_stream) = tokio::io::duplex(4096);\n\n    let server = tokio::spawn(async move {\n        server::run_stream(config, server_stream, NoAuthHandler).await\n    });\n\n    let mut server_id = Vec::new();\n    client.read_until(b\u0027\\n\u0027, \u0026mut server_id).await.unwrap();\n\n    client\n        .write_all(b\"attacker-controlled pre-banner\\r\\nSSH-2.0-poc\\r\\n\")\n        .await\n        .unwrap();\n\n    let result = tokio::time::timeout(std::time::Duration::from_millis(250), server).await;\n\n    assert!(\n        result.is_err(),\n        \"vulnerable code keeps processing after accepting a client pre-banner before SSH identification\"\n    );\n}\n```\n\nOn vulnerable code, the server-side reader accepts the client pre-banner line and continues instead of rejecting the malformed identification input promptly. The fixed parser rejects client pre-banner lines on the server path.\n\n### Impact\n\nSuggested CVSS v3.1:\n\n- `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L`\n- Score: `5.3`\n\nReasoning:\n\n- `AV:N`: reachable over the SSH transport connection\n- `AC:L`: no special race or unusual setup is required\n- `PR:N`, `UI:N`: occurs before authentication and needs no user interaction\n- `C:N`, `I:N`: no confidentiality or integrity impact demonstrated\n- `A:L`: malformed identification input can consume connection setup resources until rejected by timeout or external limits\n\nAdditional impact investigation did not identify a stronger confidentiality, integrity, downgrade, or code-execution primitive. The accepted client pre-banner line is discarded before key exchange and does not become part of `remote_sshid`; the final client identification string is what feeds the key-exchange transcript. `remote_sshid` is otherwise exposed to library handlers and debug formatting, but the discarded pre-banner text does not influence authentication state, strict-kex negotiation, KEX algorithm selection, or later packet framing.\n\nOne parser-boundary nuance on vulnerable code is that behavior can depend on read chunking: if a client pre-banner line and the real SSH identification line are delivered in the same read, the old parser can discard the buffered identification line and then wait or disconnect; if delivered separately, the old server path can accept the pre-banner and continue. This supports malformed pre-authentication availability impact, but not a demonstrated confidentiality or integrity impact.\n\n### Fix / Patch Direction\n\nUse a stricter server-side client-identification reader, enforce the RFC identification-line length, and cap preliminary banner lines. The server path should reject client pre-banner lines instead of treating them like allowed server pre-identification text.",
  "id": "GHSA-76r6-x97p-67vr",
  "modified": "2026-06-11T20:29:14Z",
  "published": "2026-06-11T20:29:14Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/Eugeny/russh/security/advisories/GHSA-76r6-x97p-67vr"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-48108"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/Eugeny/russh"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L",
      "type": "CVSS_V3"
    }
  ],
  "summary": "Russh: SSH identification parsing accepted non-canonical client banners and did not bound pre-banner input"
}

CVE-2026-48108 (GCVE-0-2026-48108)

Vulnerability from cvelistv5 – Published: 2026-06-10 20:24 – Updated: 2026-06-11 13:50

Title

Russh: SSH identification parsing accepted non-canonical client banners and did not bound pre-banner input

Summary

Russh is a Rust SSH client & server library. From version 0.34.0-beta.1 to before version 0.61.0, russh did not enforce the SSH identification-string rules as deliberately as OpenSSH. In particular, the server-side identification reader used the same permissive path as the client, allowing pre-banner lines from clients, and the reader did not enforce a bounded number of pre-banner lines. For a library server built on russh, this could allow a remote peer to hold connection setup resources in the cleartext pre-authentication phase with malformed identification input that should have been rejected early. This issue has been patched in version 0.61.0.

Severity

5.3 (Medium)


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

SSVC

Exploitation: poc Automatable: yes Technical Impact: partial

CISA Coordinator (v2.0.3)

CWE

CWE-20 - Improper Input Validation

Assigner

GitHub_M

References

1 reference

URL	Tags
https://github.com/Eugeny/russh/security/advisori…	x_refsource_CONFIRM

Impacted products

1 product

Vendor	Product	Version
Eugeny	russh	Affected: >= 0.34.0-beta.1, < 0.61.0

Show details on NVD website

JSON

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{
  "containers": {
    "adp": [
      {
        "metrics": [
          {
            "other": {
              "content": {
                "id": "CVE-2026-48108",
                "options": [
                  {
                    "Exploitation": "poc"
                  },
                  {
                    "Automatable": "yes"
                  },
                  {
                    "Technical Impact": "partial"
                  }
                ],
                "role": "CISA Coordinator",
                "timestamp": "2026-06-11T13:50:18.550639Z",
                "version": "2.0.3"
              },
              "type": "ssvc"
            }
          }
        ],
        "providerMetadata": {
          "dateUpdated": "2026-06-11T13:50:36.699Z",
          "orgId": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
          "shortName": "CISA-ADP"
        },
        "references": [
          {
            "tags": [
              "exploit"
            ],
            "url": "https://github.com/Eugeny/russh/security/advisories/GHSA-76r6-x97p-67vr"
          }
        ],
        "title": "CISA ADP Vulnrichment"
      }
    ],
    "cna": {
      "affected": [
        {
          "product": "russh",
          "vendor": "Eugeny",
          "versions": [
            {
              "status": "affected",
              "version": "\u003e= 0.34.0-beta.1, \u003c 0.61.0"
            }
          ]
        }
      ],
      "descriptions": [
        {
          "lang": "en",
          "value": "Russh is a Rust SSH client \u0026 server library. From version 0.34.0-beta.1 to before version 0.61.0, russh did not enforce the SSH identification-string rules as deliberately as OpenSSH. In particular, the server-side identification reader used the same permissive path as the client, allowing pre-banner lines from clients, and the reader did not enforce a bounded number of pre-banner lines. For a library server built on russh, this could allow a remote peer to hold connection setup resources in the cleartext pre-authentication phase with malformed identification input that should have been rejected early. This issue has been patched in version 0.61.0."
        }
      ],
      "metrics": [
        {
          "cvssV3_1": {
            "attackComplexity": "LOW",
            "attackVector": "NETWORK",
            "availabilityImpact": "LOW",
            "baseScore": 5.3,
            "baseSeverity": "MEDIUM",
            "confidentialityImpact": "NONE",
            "integrityImpact": "NONE",
            "privilegesRequired": "NONE",
            "scope": "UNCHANGED",
            "userInteraction": "NONE",
            "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L",
            "version": "3.1"
          }
        }
      ],
      "problemTypes": [
        {
          "descriptions": [
            {
              "cweId": "CWE-20",
              "description": "CWE-20: Improper Input Validation",
              "lang": "en",
              "type": "CWE"
            }
          ]
        }
      ],
      "providerMetadata": {
        "dateUpdated": "2026-06-10T20:24:56.666Z",
        "orgId": "a0819718-46f1-4df5-94e2-005712e83aaa",
        "shortName": "GitHub_M"
      },
      "references": [
        {
          "name": "https://github.com/Eugeny/russh/security/advisories/GHSA-76r6-x97p-67vr",
          "tags": [
            "x_refsource_CONFIRM"
          ],
          "url": "https://github.com/Eugeny/russh/security/advisories/GHSA-76r6-x97p-67vr"
        }
      ],
      "source": {
        "advisory": "GHSA-76r6-x97p-67vr",
        "discovery": "UNKNOWN"
      },
      "title": "Russh: SSH identification parsing accepted non-canonical client banners and did not bound pre-banner input"
    }
  },
  "cveMetadata": {
    "assignerOrgId": "a0819718-46f1-4df5-94e2-005712e83aaa",
    "assignerShortName": "GitHub_M",
    "cveId": "CVE-2026-48108",
    "datePublished": "2026-06-10T20:24:56.666Z",
    "dateReserved": "2026-05-20T18:46:58.287Z",
    "dateUpdated": "2026-06-11T13:50:36.699Z",
    "state": "PUBLISHED"
  },
  "dataType": "CVE_RECORD",
  "dataVersion": "5.2"
}

Sightings

Author	Source	Type	Date	Other

Nomenclature

Seen: The vulnerability was mentioned, discussed, or observed by the user.
Confirmed: The vulnerability has been validated from an analyst's perspective.
Published Proof of Concept: A public proof of concept is available for this vulnerability.
Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
Not confirmed: The user expressed doubt about the validity of the vulnerability.
Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.

Detection rules are retrieved from Rulezet.

Action not permitted

GHSA-76R6-X97P-67VR

Summary

Details

RFC / OpenSSH Comparison

PoC

Impact

Fix / Patch Direction

CVE-2026-48108 (GCVE-0-2026-48108)

Tags

Sightings

Nomenclature