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Introduction: A Small Validation Mistake With Massive Security Consequences
Authentication systems are designed to serve as the first and strongest line of defense against unauthorized access. However, even a tiny oversight in file path validation can completely undermine that protection. A recently disclosed vulnerability in fast-mcp-telegram, a widely used Model Context Protocol (MCP) server for Telegram integrations, demonstrates exactly how dangerous improper input validation can become. Instead of breaking encryption or guessing credentials, attackers can exploit a simple directory traversal weakness to completely bypass Bearer token authentication and seize control of existing Telegram sessions.
The flaw highlights an important lesson for developers building AI-powered integrations and authentication systems: security failures often originate from seemingly harmless filesystem operations rather than sophisticated cryptographic attacks.
Vulnerability Summary
Security researchers have disclosed a critical authentication bypass vulnerability affecting fast-mcp-telegram versions 0.19.0 and earlier. The vulnerability is tracked under GHSA-rxw2-pc8j-vxwm, while version 0.19.1 introduces the official security fix.
The weakness exists inside the SessionFileTokenVerifier.verify_token() function, which is responsible for validating Bearer authentication tokens before granting access to Telegram sessions.
Although the application correctly blocks the reserved token name “telegram”, it fails to properly sanitize user-controlled input before constructing filesystem paths. Because directory traversal sequences are never removed or normalized, attackers can craft specially designed authentication tokens that resolve to protected session files.
Instead of requiring possession of a legitimate Bearer token, the vulnerable implementation allows attackers to authenticate by abusing filesystem traversal.
How the Authentication Mechanism Fails
The vulnerable verification logic builds session file locations by directly appending the supplied Bearer token into a filesystem path before adding the .session extension.
Rather than validating the final destination, the software only checks whether the resulting file exists.
For example, while the literal token:
telegram
is intentionally rejected, an attacker can instead provide:
../fast-mcp-telegram/telegram
The operating system resolves this path into the exact same session file:
~/.config/fast-mcp-telegram/telegram.session
Since the application never canonicalizes or validates the path, authentication succeeds despite the original token being prohibited.
The result is a complete authentication bypass.
Directory Traversal Becomes Session Hijacking
The vulnerability extends far beyond simple directory traversal.
Because authentication depends entirely on locating an existing session file, successfully resolving the protected default session automatically authenticates the attacker as the legitimate Telegram account owner.
No password.
No Bearer token.
No brute-force attack.
No cryptographic weakness.
Only filesystem manipulation.
This transforms what might initially appear to be a path traversal bug into a complete authentication bypass capable of compromising production Telegram integrations.
Multiple Components Share the Same Weakness
Researchers discovered that the insecure path handling is not isolated to a single function.
The same unsafe pattern appears inside:
src/client/connection.py
Here, session paths are reconstructed again without validating:
Forward slashes (/)
Backslashes ()
Double-dot traversal (..)
Absolute filesystem paths
Because every authentication path follows the same insecure logic, attackers can consistently escape the intended token namespace.
This greatly increases the reliability of exploitation across different deployment scenarios.
Complete Telegram Account Compromise
If a server enables HTTP authentication while still maintaining the legacy telegram.session file, remote attackers can immediately impersonate that account.
Once authenticated through the traversal alias, attackers gain unrestricted access to every Telegram capability exposed through the MCP server.
Potential impacts include:
Reading private conversations
Sending messages as the victim
Executing arbitrary MTProto API requests
Accessing shared files and attachments
Using every MCP tool associated with the compromised account
Since these actions occur through a legitimate authenticated session, traditional authentication monitoring may never detect the compromise.
Why Account Prefixing Does Not Stop the Attack
Security researcher David Carliez confirmed that
The middleware simply labels tools after authentication has already completed.
For example, authenticated users may receive tool names similar to:
defaultalice_send_message
Because the authentication layer has already been bypassed, the middleware faithfully exposes every tool associated with the hijacked account.
In other words, account prefixes improve organization, not security.
The authentication boundary has already been broken long before the middleware executes.
Researchers Verified the Exploit
Testing confirmed several important behaviors.
The reserved token:
telegram
is correctly rejected.
Traversal strings referencing nonexistent files are also rejected.
However, traversal aliases pointing toward the default session file are accepted successfully.
Once authenticated, the Telegram client initializes normally using the resolved session file, and all associated MCP tools become fully available.
These findings demonstrate that the vulnerability is not merely an issue with tool naming or middleware configuration.
It is a genuine authentication bypass leading directly to session hijacking.
Official Mitigation and Security Recommendations
The security advisory recommends significantly tightening Bearer token validation.
Developers should:
Accept only URL-safe Base64-generated authentication tokens.
Reject any token containing directory separators.
Block forward slashes.
Block backslashes.
Reject double dots.
Reject empty path segments.
Reject absolute filesystem paths.
Canonicalize every generated path before use.
Verify that resolved paths remain inside the configured session directory.
Apply identical validation across every authentication workflow, setup process, cleanup routine, and API endpoint.
Introduce regression tests specifically targeting traversal attacks and encoded path variations.
These defensive measures eliminate filesystem ambiguity before authentication decisions are made.
Immediate Actions for Administrators
Organizations running fast-mcp-telegram with HTTP authentication enabled should upgrade immediately to version 0.19.1.
For environments where immediate patching is not possible, administrators should inspect systems for the presence of the default:
telegram.session
file.
Temporarily removing, renaming, or relocating this legacy session file reduces exposure until permanent updates can be applied.
Although this is only a temporary mitigation, it significantly limits the attack surface available to remote adversaries.
Deep Analysis: Detecting and Investigating the Vulnerability
Security teams should investigate whether vulnerable deployments contain default session files and whether authentication logs include suspicious traversal-based Bearer tokens.
Useful Linux commands during incident response include:
Check installed version pip show fast-mcp-telegram
Search for Telegram session files
find ~/.config -name ".session"
Locate legacy default session
find / -name "telegram.session" 2>/dev/null
Search for traversal attempts in logs
grep -R "../" /var/log/
Look for suspicious Bearer tokens
grep -Ri "Authorization: Bearer" /var/log/
Monitor filesystem activity
auditctl -w ~/.config/fast-mcp-telegram -p rwxa
Verify file permissions
ls -la ~/.config/fast-mcp-telegram/
Inspect running Python processes
ps aux | grep python
Identify listening services
ss -tulpn
Review active connections
netstat -antp
Search recent authentication events
journalctl | grep authentication
Check recent file modifications
find ~/.config/fast-mcp-telegram -mtime -7
Generate SHA256 hashes
sha256sum ~/.config/fast-mcp-telegram/.session
Inspect application configuration
cat config.yaml
Update package safely
pip install --upgrade fast-mcp-telegram
Freeze installed packages
pip freeze
Verify package version
python -c "import fast_mcp_telegram; print(fast_mcp_telegram.<strong>version</strong>)"
Monitor logs live
tail -f /var/log/syslog
Detect unexpected network activity
lsof -i
Review cron jobs
crontab -l
These commands assist administrators in identifying exposed deployments, locating legacy session files, monitoring suspicious activity, validating software versions, and strengthening forensic investigations after patching.
What Undercode Say:
The fast-mcp-telegram vulnerability is another reminder that authentication is only as strong as its weakest implementation detail.
The application attempted to prevent collisions by reserving the literal token “telegram.”
Unfortunately, security decisions were based on string comparison rather than filesystem reality.
Operating systems resolve paths differently than applications often assume.
Attackers understand this extremely well.
Directory traversal remains one of the oldest classes of vulnerabilities, yet it continues appearing inside modern AI infrastructure.
The rise of MCP servers has expanded the attack surface considerably.
These servers frequently bridge AI models with messaging platforms, cloud APIs, and enterprise services.
Compromising authentication on such middleware effectively compromises everything connected behind it.
One overlooked normalization step became equivalent to removing authentication entirely.
This incident demonstrates why canonicalization must always occur before authorization.
Security controls cannot rely on blacklist filtering.
Reserved keywords alone are never sufficient.
Every filesystem interaction involving user input should assume malicious intent.
Developers should prefer allowlists over blocklists.
URL-safe Base64 tokens already provide predictable character sets.
Anything outside those expectations should immediately fail validation.
Defense in depth is equally important.
Authentication should never depend solely on filesystem existence.
Multiple independent validation layers reduce catastrophic failures.
Regression testing deserves far more attention.
Every security patch should introduce permanent tests that prevent identical mistakes from returning.
Code reviews must specifically examine path construction.
Static analysis tools can detect many traversal patterns before deployment.
Containerization also offers benefits by restricting filesystem visibility.
Least-privilege access limits damage when vulnerabilities emerge.
Legacy compatibility often introduces hidden security debt.
Default session files intended for convenience may become liabilities years later.
Modern AI infrastructure increasingly interacts with real user identities.
That raises the stakes significantly.
A compromised MCP server is no longer just an application issue.
It becomes an identity compromise.
Organizations adopting AI integrations should audit authentication logic with the same rigor traditionally reserved for financial systems.
Security boundaries must be mathematically predictable.
Filesystem behavior should never influence authentication outcomes.
Simple validation mistakes continue proving that attackers rarely need sophisticated exploits when fundamental trust assumptions are broken.
✅ Confirmed: The vulnerability GHSA-rxw2-pc8j-vxwm affects fast-mcp-telegram versions 0.19.0 and earlier, and version 0.19.1 contains the official fix.
✅ Confirmed: Researchers demonstrated that directory traversal can bypass Bearer token authentication by resolving the protected default Telegram session file instead of requiring a valid authentication token.
✅ Confirmed: Security guidance recommends strict token validation, filesystem path canonicalization, child-directory verification, and immediate upgrades for administrators operating vulnerable HTTP-authenticated deployments.
Prediction
(+1) Future MCP frameworks will likely adopt stricter filesystem validation libraries and standardized authentication middleware, significantly reducing directory traversal risks across AI integrations.
(-1) As MCP servers continue connecting AI systems with external messaging platforms and enterprise services, attackers will increasingly target authentication logic instead of encryption, making implementation flaws one of the fastest-growing attack vectors in AI infrastructure.
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