CVE-2026-52830

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.

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.

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:

{
 …

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.

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:

{
 …

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:

{
 …