The Fix
Changes the default Content-Type to application/octet-stream when the header is malformed, addressing issue #10889.
Based on closed aio-libs/aiohttp issue #10889 · PR/commit linked
Production note: Most teams hit this during upgrades or environment changes. Roll out with a canary and smoke critical endpoints (health, OpenAPI/docs) before 100%.
@@ -0,0 +1,4 @@
@@ -0,0 +1,4 @@
+Updated ``Content-Type`` header parsing to return ``application/octet-stream`` when header contains invalid syntax.
+See :rfc:`9110#section-8.3-5`.
+
Option A — Apply the official fix\nChanges the default Content-Type to application/octet-stream when the header is malformed, addressing issue #10889.\nWhen NOT to use: This fix should not be applied if the application relies on the previous behavior of returning text/plain.\n\n
Why This Fix Works in Production
- Trigger: As a result, the behaviour when the Content-Type is malformed is to set it to text/plain, which doesn't really make sense (and may break things) in a general…
- Mechanism: The Content-Type header parser defaults to text/plain for malformed headers instead of a more appropriate type
- If left unfixed, the same config can fail only in production (env differences), causing startup failures or partial feature outages.
Why This Breaks in Prod
- The Content-Type header parser defaults to text/plain for malformed headers instead of a more appropriate type
- Production symptom (often without a traceback): As a result, the behaviour when the Content-Type is malformed is to set it to text/plain, which doesn't really make sense (and may break things) in a general HTTP application.
Proof / Evidence
- GitHub issue: #10889
- Fix PR: https://github.com/aio-libs/aiohttp/pull/11580
- Reproduced locally: No (not executed)
- Last verified: 2026-02-09
- Confidence: 0.70
- Did this fix it?: Yes (upstream fix exists)
- Own content ratio: 0.79
Discussion
High-signal excerpts from the issue thread (symptoms, repros, edge-cases).
“We're using email.parser.HeaderParser to parse our headers, which uses some behaviour defined in email-specific draft RFCs. As a result, the behaviour when the Content-Type is malformed is to set it to text/plain, which doesn't really make”
Failure Signature (Search String)
- As a result, the behaviour when the Content-Type is malformed is to set it to text/plain, which doesn't really make sense (and may break things) in a general HTTP application.
Copy-friendly signature
Failure Signature
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As a result, the behaviour when the Content-Type is malformed is to set it to text/plain, which doesn't really make sense (and may break things) in a general HTTP application.
Error Message
Signature-only (no traceback captured)
Error Message
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As a result, the behaviour when the Content-Type is malformed is to set it to text/plain, which doesn't really make sense (and may break things) in a general HTTP application.
What Broke
Malformed Content-Type headers lead to incorrect content type being used, potentially causing application issues.
Why It Broke
The Content-Type header parser defaults to text/plain for malformed headers instead of a more appropriate type
Fix Options (Details)
Option A — Apply the official fix
Changes the default Content-Type to application/octet-stream when the header is malformed, addressing issue #10889.
Fix reference: https://github.com/aio-libs/aiohttp/pull/11580
Last verified: 2026-02-09. Validate in your environment.
When NOT to Use This Fix
- This fix should not be applied if the application relies on the previous behavior of returning text/plain.
Did This Fix Work in Your Case?
Quick signal helps us prioritize which fixes to verify and improve.
Prevention
- Capture the exact failing error string in logs and tests so you can reproduce via a minimal script.
- Pin production dependencies and upgrade only with a reproducible test that hits the failing path.
Related Issues
No related fixes found.
Sources
We don’t republish the full GitHub discussion text. Use the links above for context.