CVE-2025-69225: Aiohttp Parser Logic Vulnerability

CVE-2025-69225 Overview

CVE-2025-69225 affects aiohttp, an asynchronous HTTP client/server framework for asyncio and Python. The parser logic in versions 3.13.2 and below accepts non-ASCII decimal digits inside the HTTP Range header. The maintainers report no known impact, but the loose parsing raises the theoretical possibility of an HTTP request smuggling vector [CWE-444]. Version 3.13.3 corrects the regular expression to reject non-ASCII digits.

Critical Impact
The flaw permits non-ASCII decimals in the Range header. No confirmed exploitation path exists, but the inconsistent digit handling could enable request smuggling if paired with a differently behaving upstream proxy.

Affected Products

aiohttp versions ≤ 3.13.2
Python applications using aiohttp server-side request handling
Deployments that place aiohttp behind proxies parsing Range headers differently

Discovery Timeline

2026-01-06 - CVE-2025-69225 published to NVD
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2025-69225

Vulnerability Analysis

The vulnerability sits in the Range header parser inside aiohttp/web_request.py. The parser applied a \d pattern without the re.ASCII flag. Python's default Unicode-aware \d matches decimal digits beyond ASCII 0–9, such as Arabic-Indic or full-width digits. An attacker can submit a Range header containing these Unicode digits and have them accepted as valid numeric bounds. The impact is classified as HTTP Request Smuggling [CWE-444], where request boundary desynchronization between frontend and backend parsers can lead to request queue poisoning.

Root Cause

The regular expression r"^bytes=(\d*)-(\d*)$" was evaluated in Python's default Unicode mode. \d therefore matched any code point in the Unicode Nd category. Because upstream proxies typically enforce strict ASCII digit parsing per RFC 9110, the two ends of the request chain could disagree on what constitutes a valid range value.

Attack Vector

Exploitation requires the attacker to send a crafted HTTP request over the network. No authentication or user interaction is needed. A successful smuggling scenario would additionally require a specific upstream component whose header parsing diverges from aiohttp's, which is why the advisory notes no confirmed impact.

python

# Source: https://github.com/aio-libs/aiohttp/commit/c7b7a044f88c71cefda95ec75cdcfaa4792b3b96
# Patch in aiohttp/web_request.py - restrict digit matching to ASCII
if rng is not None:
    try:
        pattern = r"^bytes=(\d*)-(\d*)$"
-       start, end = re.findall(pattern, rng)[0]
+       start, end = re.findall(pattern, rng, re.ASCII)[0]
    except IndexError:  # pattern was not found in header
        raise ValueError("range not in acceptable format")

The fix adds the re.ASCII flag so \d matches only [0-9], aligning aiohttp with RFC-compliant intermediaries.

Detection Methods for CVE-2025-69225

Indicators of Compromise

HTTP requests with Range headers containing non-ASCII characters in the numeric byte positions
Web server access logs showing Range: bytes= values with Unicode Nd category code points
Discrepancies between proxy access logs and aiohttp application logs for the same request identifier

Detection Strategies

Inspect ingress traffic for Range header values that fail an ASCII-only digit validation check
Compare request framing between upstream reverse proxies and aiohttp origin servers to identify desync patterns
Enumerate installed aiohttp versions across Python environments and flag any release at or below 3.13.2

Monitoring Recommendations

Alert on repeated 4xx or 416 responses tied to malformed Range headers
Track aiohttp package versions via software composition analysis and inventory tooling
Log full raw request headers at the reverse proxy to enable retrospective request smuggling analysis

How to Mitigate CVE-2025-69225

Immediate Actions Required

Upgrade aiohttp to version 3.13.3 or later in all Python environments
Audit dependency manifests such as requirements.txt, pyproject.toml, and lock files for pinned aiohttp versions ≤ 3.13.2
Rebuild and redeploy container images that bundle vulnerable aiohttp releases

Patch Information

The fix is delivered in aiohttp 3.13.3 via commit c7b7a044f88c71cefda95ec75cdcfaa4792b3b96. See the aiohttp GitHub Security Advisory GHSA-mqqc-3gqh-h2x8 and the upstream commit for details.

Workarounds

Enforce strict ASCII validation of the Range header at an upstream reverse proxy or WAF before requests reach aiohttp
Strip or reject Range headers containing non-ASCII code points at the edge
Where patching is delayed, disable byte-range request handling in application logic if the feature is unused

bash

# Upgrade aiohttp to the patched release
pip install --upgrade "aiohttp>=3.13.3"

# Verify installed version
python -c "import aiohttp; print(aiohttp.__version__)"