high CVSS 7.5 EPSS 0.32%

CVE-2026-54277·Python vulnerability

AIOHTTP is an asynchronous HTTP client/server framework for asyncio and Python. Prior to 3.14.1, it is possible to bypass the max_line_size check in parts of an HTTP request in the C parser. If using the optimised C parser (the default in pre-built wheels), then an attacker may be able to send oversized lines through the HTTP parser and use an excessive amount of memory, potentially leading to DoS. This vulnerability is fixed in 3.14.1.

Severity
high
Software
Python
Fixed in
3.14.1
Published

How likely is exploitation

FIRST's Exploit Prediction Scoring System puts the probability of this CVE being exploited in the next 30 days at 0.32% — higher than 24% of all published CVEs. EPSS is an independent forecast of exploitation, not a severity rating: it says nothing about how much damage a successful attack does, which is what a CVSS score measures.

Read the two together. CVE-2026-54277 scores 7.5 on CVSS, so a successful attack would be serious — but at 0.32%, almost nobody is currently attempting it. That is a different situation from a flaw under mass exploitation, and it is usually the difference between patching on your next maintenance window and patching tonight.

Affected versions

Until: 3.14.1

Fixed in: 3.14.1

How to fix this CVE

Update Python and aiohttp to version 3.14.1 or later to patch the HTTP parser vulnerability that allows oversized request lines to bypass security checks. Organizations using the default C parser implementation should prioritize this update, as attackers could exploit this flaw to trigger memory exhaustion and cause service disruptions. Verify your aiohttp installation version and apply the security patch immediately.

sudo dnf check-update && sudo dnf upgrade python3-aiohttp-3.14.1

Defensia detects this vulnerability

What an exploitation attempt looks like

Sample log line indicative of exploitation attempts:

POST|GET|PUT|DELETE.*HTTP/1.\d\r\n(?:[A-Za-z0-9-]+:.*\r\n){0,}[A-Za-z0-9-]+:[ \t]*(.{2048,})

WAF mitigation (if patching is not yet possible)

Add this rule to your WAF to block exploitation attempts while you schedule the patch. 

Implement a WAF rule to block HTTP requests with individual header lines exceeding 8192 bytes. Configure request size limits at the reverse proxy level (nginx: client_max_body_size, Apache: LimitRequestFieldSize) to reject oversized headers before they reach the aiohttp parser. Apply rate limiting to connections sending abnormally large request headers.

How to check if you are affected

  1. Step 1: Check aiohttp version by running: python3 -m pip show aiohttp | grep Version
  2. Step 2: Verify if your application imports and uses aiohttp by searching source code: grep -r 'import aiohttp' /path/to/app/
  3. Step 3: Review HTTP server logs for unusual patterns of requests with extremely long headers or status lines that exceed normal sizes
  4. Step 4: After patching, confirm the fix by re-running: python3 -m pip show aiohttp | grep Version and verify it shows 3.14.1 or higher

FAQ

What is CVE-2026-54277?

This vulnerability affects the C parser in aiohttp versions before 3.14.1, where the max_line_size validation can be bypassed on certain HTTP request components, allowing an attacker to send excessively long header lines that consume excessive memory and trigger denial of service.

Is CVE-2026-54277 being actively exploited?

No, this vulnerability is not currently listed on the CISA Known Exploited Vulnerabilities (KEV) catalog, and no public exploit code is available. However, the high CVSS score warrants proactive patching.

What versions of Python are affected by CVE-2026-54277?

The vulnerability exists in aiohttp versions prior to 3.14.1. All Python installations using affected aiohttp versions are at risk, particularly those relying on the optimized C parser implementation in pre-built wheel distributions.

How do I check if my server is vulnerable to CVE-2026-54277?

Run: python3 -m pip list | grep aiohttp and check if the version is below 3.14.1. Additionally, verify the parser type with: python3 -c 'import aiohttp._http_parser; print(aiohttp._http_parser.CPARSER)' — if it returns True, you are using the vulnerable C parser.

Does Defensia detect CVE-2026-54277?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If aiohttp is installed on a monitored server, CVE-2026-54277 will appear in your dashboard with remediation steps.

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References

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