CVE-2025-69224: Aiohttp Auth Bypass Vulnerability

CVE-2025-69224 Overview

CVE-2025-69224 is an HTTP request smuggling vulnerability [CWE-444] in the AIOHTTP asynchronous HTTP client/server framework for Python. The flaw affects versions 3.13.2 and below when the pure Python HTTP parser is used, either through installation without C extensions or when AIOHTTP_NO_EXTENSIONS is enabled. Non-ASCII characters in HTTP headers are not properly rejected during parsing. Attackers can craft requests that pass through upstream proxies or firewalls with one interpretation while AIOHTTP parses them differently. The maintainers fixed the issue in version 3.13.3.

Critical Impact
Attackers can smuggle HTTP requests past proxy and firewall protections, bypassing access controls and potentially reaching restricted backend endpoints.

Affected Products

AIOHTTP versions 3.13.2 and earlier (pure Python parser)
Deployments with AIOHTTP_NO_EXTENSIONS environment variable enabled
Python applications using AIOHTTP as a server framework behind reverse proxies

Discovery Timeline

2026-01-05 - CVE-2025-69224 published to NVD
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2025-69224

Vulnerability Analysis

The vulnerability resides in the AIOHTTP HTTP parser implementation used when C extensions are unavailable. The parser accepts non-ASCII characters in header values such as the Upgrade header. Python's str.lower() method applies Unicode case folding, meaning non-ASCII characters can transform into ASCII strings that match protocol-sensitive values like websocket or tcp. This desynchronizes AIOHTTP from upstream proxies that either reject or interpret those characters differently.

Request smuggling attacks exploit this parsing mismatch to inject a second HTTP request within the body of a first request. The front-end proxy sees one boundary while AIOHTTP sees another. Successful exploitation allows attackers to bypass proxy-level authentication, poison shared caches, or reach internal endpoints that should not be exposed.

Root Cause

The underlying weakness is improper input validation of header content [CWE-444]. The pure Python parser and its Cython counterpart in aiohttp/_http_parser.pyx normalized header values using .lower() before checking allowed upgrade values, without first asserting that the input contained only ASCII characters. Unicode case-folding transformations allowed protocol-sensitive comparisons to succeed on inputs proxies would treat as invalid.

Attack Vector

The vulnerability is exploited over the network with no authentication required. An attacker sends a crafted HTTP request containing non-ASCII bytes in headers such as Upgrade. When AIOHTTP sits behind a proxy or firewall performing HTTP inspection, the differing interpretations enable request smuggling and firewall bypass. Attack complexity requires the presence of specific deployment conditions, namely the pure Python parser build.

python

# Security patch in aiohttp/http_parser.py - reject non-ASCII in Upgrade header
def _is_supported_upgrade(headers: CIMultiDictProxy[str]) -> bool:
    """Check if the upgrade header is supported."""
    u = headers.get(hdrs.UPGRADE, "")
    # .lower() can transform non-ascii characters.
    return u.isascii() and u.lower() in {"tcp", "websocket"}
# Source: https://github.com/aio-libs/aiohttp/commit/32677f2adfd907420c078dda6b79225c6f4ebce0

text

# Security patch in aiohttp/_http_parser.pyx - matching Cython-level guard
         headers = CIMultiDictProxy(CIMultiDict(self._headers))
 
         if self._cparser.type == cparser.HTTP_REQUEST:
-            allowed = upgrade and headers.get("upgrade", "").lower() in ALLOWED_UPGRADES
+            h_upg = headers.get("upgrade", "")
+            allowed = upgrade and h_upg.isascii() and h_upg.lower() in ALLOWED_UPGRADES
             if allowed or self._cparser.method == cparser.HTTP_CONNECT:
                 self._upgraded = True
         else:
# Source: https://github.com/aio-libs/aiohttp/commit/32677f2adfd907420c078dda6b79225c6f4ebce0

Detection Methods for CVE-2025-69224

Indicators of Compromise

Inbound HTTP requests containing non-ASCII byte sequences in Upgrade, Transfer-Encoding, or Connection header values.
Discrepancies between proxy access logs and AIOHTTP application logs, such as differing request counts or paths for the same connection.
Unexpected WebSocket or protocol-upgrade handshakes originating from clients that do not normally issue them.

Detection Strategies

Inspect AIOHTTP deployments for the pure Python installation path or the AIOHTTP_NO_EXTENSIONS environment variable, both of which activate the vulnerable parser.
Deploy web application firewall rules that reject any HTTP header containing bytes outside the printable ASCII range.
Correlate front-end proxy and backend application logs to identify request-boundary mismatches indicative of smuggling.

Monitoring Recommendations

Alert on requests where header values fail an isascii() equivalent check at the proxy tier.
Track version metadata of Python dependencies in production images and flag aiohttp<3.13.3.
Monitor for anomalous spikes in 101 Switching Protocols responses that do not align with expected WebSocket clients.

How to Mitigate CVE-2025-69224

Immediate Actions Required

Upgrade AIOHTTP to version 3.13.3 or later across all Python environments.
Verify whether deployments run the pure Python parser and reinstall AIOHTTP so C extensions build successfully.
Unset the AIOHTTP_NO_EXTENSIONS environment variable unless there is a documented operational reason to keep it enabled.

Patch Information

The fix is committed in aio-libs/aiohttp commit 32677f2 and shipped in AIOHTTP 3.13.3. Both aiohttp/http_parser.py and aiohttp/_http_parser.pyx now call isascii() before performing case-insensitive matches on the Upgrade header. Additional context is available in the GitHub Security Advisory GHSA-69f9-5gxw-wvc2.

Workarounds

Ensure AIOHTTP is installed with its native C extensions, which are not affected by the parsing flaw.
Terminate HTTP at a hardened reverse proxy that strictly rejects headers containing non-ASCII bytes before traffic reaches AIOHTTP.
Add middleware that validates incoming header values with isascii() and returns 400 Bad Request on failure until the upgrade is completed.

bash

# Configuration example: upgrade AIOHTTP and confirm C extensions are active
pip install --upgrade "aiohttp>=3.13.3"
unset AIOHTTP_NO_EXTENSIONS
python -c "import aiohttp, aiohttp.http_parser as p; print(aiohttp.__version__, p.HttpRequestParser)"