Vulnerability-Lookup

CVE-2026-52830 (GCVE-0-2026-52830)

Vulnerability from cvelistv5 – Published: 2026-07-02 20:39 – Updated: 2026-07-06 14:36

Title

fast-mcp-telegram: Bearer token path traversal bypasses reserved Telegram session protection

Summary

fast-mcp-telegram is a Telegram MCP Server. Prior to 0.19.1, fast-mcp-telegram validates HTTP Bearer tokens by joining the raw token string into a session-file path. The verifier rejects the exact reserved token telegram, but it does not reject path separators or normalize the path before checking whether the session file exists. A remote HTTP client can therefore authenticate as the default legacy session with a token such as ../fast-mcp-telegram/telegram when the documented default session file ~/.config/fast-mcp-telegram/telegram.session exists. This bypasses the reserved session name control that is intended to prevent HTTP multi-user sessions from colliding with the default stdio or legacy account. With account-prefixed MCP tools enabled, the attacker still sees and calls the prefixed tools for the default account, so the prefix middleware does not stop the session selection bypass. This vulnerability is fixed in 0.19.1.

Severity

9.4 (Critical)


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

SSVC

Exploitation: poc Automatable: yes Technical Impact: partial

CISA Coordinator · CISA-ADP (v2.0.3)

Decision recorded 2026-07-06 14:35 UTC

CWE

CWE-22 - Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')
CWE-287 - Improper Authentication

Assigner

GitHub_M CNA under the mitre root

CNA scorecard C 72/100 over 7775 records in the last 180 days details

References

2 references

URL	Tags
https://web.archive.org/web/20250926152207/https:…	x_refsource_CONFIRM
https://github.com/advisories/GHSA-rxw2-pc8j-vxwm	x_refsource_MISC

Impacted products

1 product

Vendor	Product	Version	CPE status
leshchenko1979	fast-mcp-telegram	Affected: < 0.19.1	guessed

Show details on NVD website

JSON

To clipboard

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FKIE_CVE-2026-52830

Vulnerability from fkie_nvd - Published: 2026-07-02 21:16 - Updated: 2026-10-25 21:10

Severity

9.4 (Critical) - CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:L

Summary

References

	URL	Tags
	security-advisories@github.com	https://github.com/advisories/GHSA-rxw2-pc8j-vxwm
	security-advisories@github.com	https://web.archive.org/web/20250926152207/https://github.com/leshchenko1979/fast-mcp-telegram
	134c704f-9b21-4f2e-91b3-4a467353bcc0	https://github.com/advisories/GHSA-rxw2-pc8j-vxwm

Impacted products

	Vendor	Product	Version

JSON

To clipboard

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GHSA-RXW2-PC8J-VXWM

Vulnerability from github – Published: 2026-07-02 20:38 – Updated: 2026-08-13 16:57

Summary

fast-mcp-telegram: Bearer token path traversal bypasses reserved Telegram session protection

Details

Summary

fast-mcp-telegram validates HTTP Bearer tokens by joining the raw token string into a session-file path. The verifier rejects the exact reserved token telegram, but it does not reject path separators or normalize the path before checking whether the session file exists. A remote HTTP client can therefore authenticate as the default legacy session with a token such as ../fast-mcp-telegram/telegram when the documented default session file ~/.config/fast-mcp-telegram/telegram.session exists.

This bypasses the reserved session name control that is intended to prevent HTTP multi-user sessions from colliding with the default stdio or legacy account. With account-prefixed MCP tools enabled, the attacker still sees and calls the prefixed tools for the default account, so the prefix middleware does not stop the session selection bypass.

Impact

An unauthenticated network client can access the Telegram account represented by the default telegram.session file without knowing a generated bearer token, if that legacy or default session file is present on a server running HTTP auth. The attacker can then call Telegram MCP tools as that account, including message reading, message sending, MTProto API calls, and attachment-producing tool surfaces available to the session.

Technical details

SessionFileTokenVerifier.verify_token() strips whitespace and rejects exact reserved names:

if token.lower() in RESERVED_SESSION_NAMES:
    return None

It then appends .session to the raw token and checks the resulting path:

session_path = self._session_directory / f"{token}.session"
if not session_path.is_file():
    return None

No check rejects /, \\, .., absolute paths, or resolved paths outside the configured session directory. The session client path is built the same way in src/client/connection.py:

session_path = SESSION_DIR / f"{token}.session"
client = await _build_telegram_client_for_token(session_path, token)

With the default session directory, the token ../fast-mcp-telegram/telegram resolves as follows:

~/.config/fast-mcp-telegram/../fast-mcp-telegram/telegram.session
= ~/.config/fast-mcp-telegram/telegram.session

The exact token telegram is denied, but the traversal alias reaches the same file and is accepted. This is especially important because telegram is the documented default session_name, and the security documentation says reserved names are blocked to prevent conflicts with stdio and HTTP no-auth sessions.

The vulnerable code is present on current master commit 167ab705f1cd09b21e85c370570471fe75a4f8c9 and in release tag 0.19.0 commit 77bdf6d7e5c6a84d87acc423db613e6c6ba30094.

Reproduction

The following proof uses stub session files and stub Telegram clients, so it does not need real Telegram credentials. It validates the auth decision and the eventual session path used by the client builder.

Run on current master:

git clone https://github.com/leshchenko1979/fast-mcp-telegram.git
cd fast-mcp-telegram
python validation_token_traversal.py

The local proof script created for validation is attached below for reference:

# High-level proof outline
# 1. Create a temporary session directory containing telegram.session and a random token session.
# 2. Instantiate SessionFileTokenVerifier with that directory.
# 3. Verify denied controls: token `telegram` is rejected, and a traversal token to a missing file is rejected.
# 4. Verify allowed control: a normal random token with a matching session file is accepted.
# 5. Verify bypass: token `../fast-mcp-telegram/telegram` is accepted and the client builder receives the default telegram.session path.
# 6. Verify prefix behavior: account-prefixed tools are listed for the traversal-authenticated default account, a prefixed call reaches send_message, and an unprefixed call is still denied.

Key controls from the current-master run:

{
  "reserved_default_token_denied": true,
  "normal_random_token_allowed": true,
  "missing_traversal_token_denied": true,
  "traversal_alias_to_reserved_default_allowed": true,
  "traversal_access_token_value": "../fast-mcp-telegram/telegram",
  "client_builder_used_default_session_file": true,
  "prefixed_tool_listed_for_traversal_token": "defaultalice_send_message",
  "prefixed_tool_call_reached_handler_as": "send_message",
  "unprefixed_tool_call_denied_when_prefix_resolved": true
}

Interpretation:

Denied control: the exact reserved token telegram is rejected.
Allowed control: a normal random session token is accepted when its matching session file exists.
Denied control: a traversal token pointing to a missing file is rejected.
Bypass: ../fast-mcp-telegram/telegram authenticates and the client builder receives the resolved default session path.
Prefix control: once authenticated through the traversal token, account-prefixed tools are listed and a prefixed tools/call reaches the internal send_message handler. An unprefixed call is rejected when the prefix resolves, so the confirmed bug is the session selection and authentication bypass, not a missing-prefix execution bypass.

Why this crosses the auth boundary

A production HTTP auth deployment is expected to require high-entropy per-session bearer tokens. Reserved names are explicitly blocked because common names such as telegram can collide with the default session. The traversal alias turns the public token namespace back into a filesystem namespace and bypasses that reserved-name policy.

The account-prefix middleware is downstream of authentication. It labels tools based on the resolved Telegram account for the token that was accepted. Because the traversal token is accepted as a valid FastMCP AccessToken, the middleware correctly exposes the default account's prefixed tools to the attacker. It cannot recover the lost authentication boundary.

Remediation

Reject bearer tokens that are not strict opaque token identifiers before using them in file paths. Recommended checks:

Accept only a safe token alphabet, for example ^[A-Za-z0-9_-]{32,128}$, matching generated URL-safe base64 tokens.
Reject /, \\, ., .., empty segments, and absolute paths for both header auth and URL auth.
Resolve the final session path and require it to remain directly under the configured session directory:

session_dir = self._session_directory.resolve()
session_path = (session_dir / f"{token}.session").resolve()
if session_path.parent != session_dir:
    return None

Apply the same validation in SessionFileTokenVerifier, URL auth middleware, setup flows, cleanup code, and any code that opens session files by token.
Add regression tests for exact reserved names, traversal aliases such as ../fast-mcp-telegram/telegram, absolute paths, URL-encoded traversal if any route decodes path components, Windows separators, and normal generated tokens.

Severity

9.4 (Critical)


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

Show details on source website

JSON

To clipboard

{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 0.19.0"
      },
      "package": {
        "ecosystem": "PyPI",
        "name": "fast-mcp-telegram"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "0.19.1"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-52830"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-22",
      "CWE-287"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-07-02T20:38:50Z",
    "nvd_published_at": "2026-07-02T21:16:56Z",
    "severity": "CRITICAL"
  },
  "details": "## Summary\n\nfast-mcp-telegram validates HTTP Bearer tokens by joining the raw token string into a session-file path. The verifier rejects the exact reserved token `telegram`, but it does not reject path separators or normalize the path before checking whether the session file exists. A remote HTTP client can therefore authenticate as the default legacy session with a token such as `../fast-mcp-telegram/telegram` when the documented default session file `~/.config/fast-mcp-telegram/telegram.session` exists.\n\nThis bypasses the reserved session name control that is intended to prevent HTTP multi-user sessions from colliding with the default stdio or legacy account. With account-prefixed MCP tools enabled, the attacker still sees and calls the prefixed tools for the default account, so the prefix middleware does not stop the session selection bypass.\n\n## Impact\n\nAn unauthenticated network client can access the Telegram account represented by the default `telegram.session` file without knowing a generated bearer token, if that legacy or default session file is present on a server running HTTP auth. The attacker can then call Telegram MCP tools as that account, including message reading, message sending, MTProto API calls, and attachment-producing tool surfaces available to the session.\n\n## Technical details\n\n`SessionFileTokenVerifier.verify_token()` strips whitespace and rejects exact reserved names:\n\n```python\nif token.lower() in RESERVED_SESSION_NAMES:\n    return None\n```\n\nIt then appends `.session` to the raw token and checks the resulting path:\n\n```python\nsession_path = self._session_directory / f\"{token}.session\"\nif not session_path.is_file():\n    return None\n```\n\nNo check rejects `/`, `\\\\`, `..`, absolute paths, or resolved paths outside the configured session directory. The session client path is built the same way in `src/client/connection.py`:\n\n```python\nsession_path = SESSION_DIR / f\"{token}.session\"\nclient = await _build_telegram_client_for_token(session_path, token)\n```\n\nWith the default session directory, the token `../fast-mcp-telegram/telegram` resolves as follows:\n\n```text\n~/.config/fast-mcp-telegram/../fast-mcp-telegram/telegram.session\n= ~/.config/fast-mcp-telegram/telegram.session\n```\n\nThe exact token `telegram` is denied, but the traversal alias reaches the same file and is accepted. This is especially important because `telegram` is the documented default `session_name`, and the security documentation says reserved names are blocked to prevent conflicts with stdio and HTTP no-auth sessions.\n\nThe vulnerable code is present on current `master` commit `167ab705f1cd09b21e85c370570471fe75a4f8c9` and in release tag `0.19.0` commit `77bdf6d7e5c6a84d87acc423db613e6c6ba30094`.\n\n## Reproduction\n\nThe following proof uses stub session files and stub Telegram clients, so it does not need real Telegram credentials. It validates the auth decision and the eventual session path used by the client builder.\n\nRun on current master:\n\n```bash\ngit clone https://github.com/leshchenko1979/fast-mcp-telegram.git\ncd fast-mcp-telegram\npython validation_token_traversal.py\n```\n\nThe local proof script created for validation is attached below for reference:\n\n```python\n# High-level proof outline\n# 1. Create a temporary session directory containing telegram.session and a random token session.\n# 2. Instantiate SessionFileTokenVerifier with that directory.\n# 3. Verify denied controls: token `telegram` is rejected, and a traversal token to a missing file is rejected.\n# 4. Verify allowed control: a normal random token with a matching session file is accepted.\n# 5. Verify bypass: token `../fast-mcp-telegram/telegram` is accepted and the client builder receives the default telegram.session path.\n# 6. Verify prefix behavior: account-prefixed tools are listed for the traversal-authenticated default account, a prefixed call reaches send_message, and an unprefixed call is still denied.\n```\n\nKey controls from the current-master run:\n\n```json\n{\n  \"reserved_default_token_denied\": true,\n  \"normal_random_token_allowed\": true,\n  \"missing_traversal_token_denied\": true,\n  \"traversal_alias_to_reserved_default_allowed\": true,\n  \"traversal_access_token_value\": \"../fast-mcp-telegram/telegram\",\n  \"client_builder_used_default_session_file\": true,\n  \"prefixed_tool_listed_for_traversal_token\": \"defaultalice_send_message\",\n  \"prefixed_tool_call_reached_handler_as\": \"send_message\",\n  \"unprefixed_tool_call_denied_when_prefix_resolved\": true\n}\n```\n\nInterpretation:\n\n1. Denied control: the exact reserved token `telegram` is rejected.\n2. Allowed control: a normal random session token is accepted when its matching session file exists.\n3. Denied control: a traversal token pointing to a missing file is rejected.\n4. Bypass: `../fast-mcp-telegram/telegram` authenticates and the client builder receives the resolved default session path.\n5. Prefix control: once authenticated through the traversal token, account-prefixed tools are listed and a prefixed `tools/call` reaches the internal `send_message` handler. An unprefixed call is rejected when the prefix resolves, so the confirmed bug is the session selection and authentication bypass, not a missing-prefix execution bypass.\n\n## Why this crosses the auth boundary\n\nA production HTTP auth deployment is expected to require high-entropy per-session bearer tokens. Reserved names are explicitly blocked because common names such as `telegram` can collide with the default session. The traversal alias turns the public token namespace back into a filesystem namespace and bypasses that reserved-name policy.\n\nThe account-prefix middleware is downstream of authentication. It labels tools based on the resolved Telegram account for the token that was accepted. Because the traversal token is accepted as a valid FastMCP `AccessToken`, the middleware correctly exposes the default account\u0027s prefixed tools to the attacker. It cannot recover the lost authentication boundary.\n\n## Remediation\n\nReject bearer tokens that are not strict opaque token identifiers before using them in file paths. Recommended checks:\n\n1. Accept only a safe token alphabet, for example `^[A-Za-z0-9_-]{32,128}$`, matching generated URL-safe base64 tokens.\n2. Reject `/`, `\\\\`, `.`, `..`, empty segments, and absolute paths for both header auth and URL auth.\n3. Resolve the final session path and require it to remain directly under the configured session directory:\n\n```python\nsession_dir = self._session_directory.resolve()\nsession_path = (session_dir / f\"{token}.session\").resolve()\nif session_path.parent != session_dir:\n    return None\n```\n\n4. Apply the same validation in `SessionFileTokenVerifier`, URL auth middleware, setup flows, cleanup code, and any code that opens session files by token.\n5. Add regression tests for exact reserved names, traversal aliases such as `../fast-mcp-telegram/telegram`, absolute paths, URL-encoded traversal if any route decodes path components, Windows separators, and normal generated tokens.",
  "id": "GHSA-rxw2-pc8j-vxwm",
  "modified": "2026-08-13T16:57:36Z",
  "published": "2026-07-02T20:38:50Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/leshchenko1979/fast-mcp-telegram/security/advisories/GHSA-rxw2-pc8j-vxwm"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-52830"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/alexeyleshchenko/fast-mcp-telegram"
    },
    {
      "type": "WEB",
      "url": "https://github.com/pypa/advisory-database/tree/main/vulns/fast-mcp-telegram/PYSEC-2026-2473.yaml"
    },
    {
      "type": "WEB",
      "url": "https://pypi.org/project/fast-mcp-telegram"
    },
    {
      "type": "WEB",
      "url": "https://web.archive.org/web/20250926152207/https://github.com/leshchenko1979/fast-mcp-telegram"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:L",
      "type": "CVSS_V3"
    }
  ],
  "summary": "fast-mcp-telegram: Bearer token path traversal bypasses reserved Telegram session protection"
}

PYSEC-2026-2473

Vulnerability from pysec - Published: 2026-07-13 15:46 - Updated: 2026-07-13 16:04

Details

Summary

Impact

Technical details

SessionFileTokenVerifier.verify_token() strips whitespace and rejects exact reserved names:

if token.lower() in RESERVED_SESSION_NAMES:
    return None

It then appends .session to the raw token and checks the resulting path:

session_path = self._session_directory / f"{token}.session"
if not session_path.is_file():
    return None

No check rejects /, \\, .., absolute paths, or resolved paths outside the configured session directory. The session client path is built the same way in src/client/connection.py:

session_path = SESSION_DIR / f"{token}.session"
client = await _build_telegram_client_for_token(session_path, token)

With the default session directory, the token ../fast-mcp-telegram/telegram resolves as follows:

~/.config/fast-mcp-telegram/../fast-mcp-telegram/telegram.session
= ~/.config/fast-mcp-telegram/telegram.session

The vulnerable code is present on current master commit 167ab705f1cd09b21e85c370570471fe75a4f8c9 and in release tag 0.19.0 commit 77bdf6d7e5c6a84d87acc423db613e6c6ba30094.

Reproduction

Run on current master:

git clone https://github.com/leshchenko1979/fast-mcp-telegram.git
cd fast-mcp-telegram
python validation_token_traversal.py

The local proof script created for validation is attached below for reference:

# High-level proof outline
# 1. Create a temporary session directory containing telegram.session and a random token session.
# 2. Instantiate SessionFileTokenVerifier with that directory.
# 3. Verify denied controls: token `telegram` is rejected, and a traversal token to a missing file is rejected.
# 4. Verify allowed control: a normal random token with a matching session file is accepted.
# 5. Verify bypass: token `../fast-mcp-telegram/telegram` is accepted and the client builder receives the default telegram.session path.
# 6. Verify prefix behavior: account-prefixed tools are listed for the traversal-authenticated default account, a prefixed call reaches send_message, and an unprefixed call is still denied.

Key controls from the current-master run:

{
  "reserved_default_token_denied": true,
  "normal_random_token_allowed": true,
  "missing_traversal_token_denied": true,
  "traversal_alias_to_reserved_default_allowed": true,
  "traversal_access_token_value": "../fast-mcp-telegram/telegram",
  "client_builder_used_default_session_file": true,
  "prefixed_tool_listed_for_traversal_token": "defaultalice_send_message",
  "prefixed_tool_call_reached_handler_as": "send_message",
  "unprefixed_tool_call_denied_when_prefix_resolved": true
}

Interpretation:

Denied control: the exact reserved token telegram is rejected.
Allowed control: a normal random session token is accepted when its matching session file exists.
Denied control: a traversal token pointing to a missing file is rejected.
Bypass: ../fast-mcp-telegram/telegram authenticates and the client builder receives the resolved default session path.
Prefix control: once authenticated through the traversal token, account-prefixed tools are listed and a prefixed tools/call reaches the internal send_message handler. An unprefixed call is rejected when the prefix resolves, so the confirmed bug is the session selection and authentication bypass, not a missing-prefix execution bypass.

Why this crosses the auth boundary

Remediation

Reject bearer tokens that are not strict opaque token identifiers before using them in file paths. Recommended checks:

Accept only a safe token alphabet, for example ^[A-Za-z0-9_-]{32,128}$, matching generated URL-safe base64 tokens.
Reject /, \\, ., .., empty segments, and absolute paths for both header auth and URL auth.
Resolve the final session path and require it to remain directly under the configured session directory:

session_dir = self._session_directory.resolve()
session_path = (session_dir / f"{token}.session").resolve()
if session_path.parent != session_dir:
    return None

Apply the same validation in SessionFileTokenVerifier, URL auth middleware, setup flows, cleanup code, and any code that opens session files by token.
Add regression tests for exact reserved names, traversal aliases such as ../fast-mcp-telegram/telegram, absolute paths, URL-encoded traversal if any route decodes path components, Windows separators, and normal generated tokens.

Severity

9.4 (Critical)


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

Impacted products

Name	purl
fast-mcp-telegram	pkg:pypi/fast-mcp-telegram

Aliases

JSON

To clipboard

{
  "affected": [
    {
      "package": {
        "ecosystem": "PyPI",
        "name": "fast-mcp-telegram",
        "purl": "pkg:pypi/fast-mcp-telegram"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "0.19.1"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ],
      "versions": [
        "0.0.1",
        "0.1.1",
        "0.1.2",
        "0.1.3",
        "0.1.4",
        "0.10.0",
        "0.10.1",
        "0.10.2",
        "0.11.0",
        "0.11.1",
        "0.12.0",
        "0.12.1",
        "0.12.2",
        "0.12.3",
        "0.13.0",
        "0.13.1",
        "0.14.0",
        "0.14.1",
        "0.14.2",
        "0.15.0",
        "0.16.0",
        "0.16.1",
        "0.16.2",
        "0.16.3",
        "0.16.4",
        "0.16.5",
        "0.16.6",
        "0.17.0",
        "0.17.1",
        "0.17.2",
        "0.17.3",
        "0.18.0",
        "0.18.1",
        "0.19.0",
        "0.2.0",
        "0.2.1",
        "0.3.0",
        "0.4.0",
        "0.4.1",
        "0.4.2",
        "0.4.3",
        "0.4.4",
        "0.4.5",
        "0.5.0",
        "0.6.0",
        "0.6.1",
        "0.7.0",
        "0.7.1",
        "0.8.0",
        "0.8.1",
        "0.8.2",
        "0.8.3",
        "0.8.4",
        "0.9.0",
        "0.9.1"
      ]
    }
  ],
  "aliases": [
    "CVE-2026-52830",
    "GHSA-rxw2-pc8j-vxwm"
  ],
  "details": "## Summary\n\nfast-mcp-telegram validates HTTP Bearer tokens by joining the raw token string into a session-file path. The verifier rejects the exact reserved token `telegram`, but it does not reject path separators or normalize the path before checking whether the session file exists. A remote HTTP client can therefore authenticate as the default legacy session with a token such as `../fast-mcp-telegram/telegram` when the documented default session file `~/.config/fast-mcp-telegram/telegram.session` exists.\n\nThis bypasses the reserved session name control that is intended to prevent HTTP multi-user sessions from colliding with the default stdio or legacy account. With account-prefixed MCP tools enabled, the attacker still sees and calls the prefixed tools for the default account, so the prefix middleware does not stop the session selection bypass.\n\n## Impact\n\nAn unauthenticated network client can access the Telegram account represented by the default `telegram.session` file without knowing a generated bearer token, if that legacy or default session file is present on a server running HTTP auth. The attacker can then call Telegram MCP tools as that account, including message reading, message sending, MTProto API calls, and attachment-producing tool surfaces available to the session.\n\n## Technical details\n\n`SessionFileTokenVerifier.verify_token()` strips whitespace and rejects exact reserved names:\n\n```python\nif token.lower() in RESERVED_SESSION_NAMES:\n    return None\n```\n\nIt then appends `.session` to the raw token and checks the resulting path:\n\n```python\nsession_path = self._session_directory / f\"{token}.session\"\nif not session_path.is_file():\n    return None\n```\n\nNo check rejects `/`, `\\\\`, `..`, absolute paths, or resolved paths outside the configured session directory. The session client path is built the same way in `src/client/connection.py`:\n\n```python\nsession_path = SESSION_DIR / f\"{token}.session\"\nclient = await _build_telegram_client_for_token(session_path, token)\n```\n\nWith the default session directory, the token `../fast-mcp-telegram/telegram` resolves as follows:\n\n```text\n~/.config/fast-mcp-telegram/../fast-mcp-telegram/telegram.session\n= ~/.config/fast-mcp-telegram/telegram.session\n```\n\nThe exact token `telegram` is denied, but the traversal alias reaches the same file and is accepted. This is especially important because `telegram` is the documented default `session_name`, and the security documentation says reserved names are blocked to prevent conflicts with stdio and HTTP no-auth sessions.\n\nThe vulnerable code is present on current `master` commit `167ab705f1cd09b21e85c370570471fe75a4f8c9` and in release tag `0.19.0` commit `77bdf6d7e5c6a84d87acc423db613e6c6ba30094`.\n\n## Reproduction\n\nThe following proof uses stub session files and stub Telegram clients, so it does not need real Telegram credentials. It validates the auth decision and the eventual session path used by the client builder.\n\nRun on current master:\n\n```bash\ngit clone https://github.com/leshchenko1979/fast-mcp-telegram.git\ncd fast-mcp-telegram\npython validation_token_traversal.py\n```\n\nThe local proof script created for validation is attached below for reference:\n\n```python\n# High-level proof outline\n# 1. Create a temporary session directory containing telegram.session and a random token session.\n# 2. Instantiate SessionFileTokenVerifier with that directory.\n# 3. Verify denied controls: token `telegram` is rejected, and a traversal token to a missing file is rejected.\n# 4. Verify allowed control: a normal random token with a matching session file is accepted.\n# 5. Verify bypass: token `../fast-mcp-telegram/telegram` is accepted and the client builder receives the default telegram.session path.\n# 6. Verify prefix behavior: account-prefixed tools are listed for the traversal-authenticated default account, a prefixed call reaches send_message, and an unprefixed call is still denied.\n```\n\nKey controls from the current-master run:\n\n```json\n{\n  \"reserved_default_token_denied\": true,\n  \"normal_random_token_allowed\": true,\n  \"missing_traversal_token_denied\": true,\n  \"traversal_alias_to_reserved_default_allowed\": true,\n  \"traversal_access_token_value\": \"../fast-mcp-telegram/telegram\",\n  \"client_builder_used_default_session_file\": true,\n  \"prefixed_tool_listed_for_traversal_token\": \"defaultalice_send_message\",\n  \"prefixed_tool_call_reached_handler_as\": \"send_message\",\n  \"unprefixed_tool_call_denied_when_prefix_resolved\": true\n}\n```\n\nInterpretation:\n\n1. Denied control: the exact reserved token `telegram` is rejected.\n2. Allowed control: a normal random session token is accepted when its matching session file exists.\n3. Denied control: a traversal token pointing to a missing file is rejected.\n4. Bypass: `../fast-mcp-telegram/telegram` authenticates and the client builder receives the resolved default session path.\n5. Prefix control: once authenticated through the traversal token, account-prefixed tools are listed and a prefixed `tools/call` reaches the internal `send_message` handler. An unprefixed call is rejected when the prefix resolves, so the confirmed bug is the session selection and authentication bypass, not a missing-prefix execution bypass.\n\n## Why this crosses the auth boundary\n\nA production HTTP auth deployment is expected to require high-entropy per-session bearer tokens. Reserved names are explicitly blocked because common names such as `telegram` can collide with the default session. The traversal alias turns the public token namespace back into a filesystem namespace and bypasses that reserved-name policy.\n\nThe account-prefix middleware is downstream of authentication. It labels tools based on the resolved Telegram account for the token that was accepted. Because the traversal token is accepted as a valid FastMCP `AccessToken`, the middleware correctly exposes the default account\u0027s prefixed tools to the attacker. It cannot recover the lost authentication boundary.\n\n## Remediation\n\nReject bearer tokens that are not strict opaque token identifiers before using them in file paths. Recommended checks:\n\n1. Accept only a safe token alphabet, for example `^[A-Za-z0-9_-]{32,128}$`, matching generated URL-safe base64 tokens.\n2. Reject `/`, `\\\\`, `.`, `..`, empty segments, and absolute paths for both header auth and URL auth.\n3. Resolve the final session path and require it to remain directly under the configured session directory:\n\n```python\nsession_dir = self._session_directory.resolve()\nsession_path = (session_dir / f\"{token}.session\").resolve()\nif session_path.parent != session_dir:\n    return None\n```\n\n4. Apply the same validation in `SessionFileTokenVerifier`, URL auth middleware, setup flows, cleanup code, and any code that opens session files by token.\n5. Add regression tests for exact reserved names, traversal aliases such as `../fast-mcp-telegram/telegram`, absolute paths, URL-encoded traversal if any route decodes path components, Windows separators, and normal generated tokens.",
  "id": "PYSEC-2026-2473",
  "modified": "2026-07-13T16:04:01.595842Z",
  "published": "2026-07-13T15:46:27.565228Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/leshchenko1979/fast-mcp-telegram/security/advisories/GHSA-rxw2-pc8j-vxwm"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/leshchenko1979/fast-mcp-telegram"
    },
    {
      "type": "PACKAGE",
      "url": "https://pypi.org/project/fast-mcp-telegram"
    },
    {
      "type": "ADVISORY",
      "url": "https://github.com/advisories/GHSA-rxw2-pc8j-vxwm"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-52830"
    }
  ],
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:L",
      "type": "CVSS_V3"
    }
  ],
  "summary": "fast-mcp-telegram: Bearer token path traversal bypasses reserved Telegram session protection"
}

Sightings

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Following the VTEM framework (Black Hat USA 2015), time to patch starts when the vendor or a coordinator was informed, total time to patch at discovery, and disclosure to patch at public disclosure; vendor response time runs from the report to its acknowledgement. Time to exploit and time unpatched start at the earliest discovery or report, falling back to public disclosure. Unpatched time ends at the first recorded patch, or the calculation time when no patch is recorded. Exploitation to patch shows the interval between those two events; a published proof of concept is a public exploit, not exploitation.

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The MITRE ATT&CK techniques below are AI-generated suggestions, inferred from the description of the vulnerability by the CIRCL/vulnerability-attack-technique-classification-roberta-base model, served locally by ML-Gateway. They have not been verified by an analyst and are provided for guidance only.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.
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CVE-2026-52830 (GCVE-0-2026-52830)

FKIE_CVE-2026-52830

GHSA-RXW2-PC8J-VXWM

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PYSEC-2026-2473

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