[Bugfix][Security] Bound the validation-error response body by lzhan011 · Pull Request #54684 · vllm-project/vllm

lzhan011 · 2026-09-01T03:10:25Z

Purpose

validation_exception_handler renders every entry of exc.errors() into the
400 body:

message = f"{count} validation {label}:\n"
message += "".join(f"  {err}\n" for err in errors)

Pydantic puts the whole offending value under input in each error entry, so a
request whose input fails per-element echoes that input once per error. The
response grows with request_size × error_count while the request stays small.

Measured on main by validating a malformed /v1/responses body with the real
ResponsesRequest model and passing the resulting errors to the real handler:

`input` items	request bytes	errors	response bytes	amplification	handler time
1	114	241	473,222	4,151x	0.011 s
10	915	2,401	4,729,926	5,169x	0.091 s
50	4,475	12,001	23,658,207	5,287x	0.488 s
200	17,825	48,001	94,663,257	5,311x	1.986 s
800	71,225	192,001	378,731,458	5,317x	7.024 s

The amplification factor is flat at ~5,300x, so it scales linearly with request
size: whatever body limit sits in front of the server, the 400 is ~5,000 times
larger. The reporter observed 12,047 errors and a ~114 MB body in production,
which stalled the proxy in front of vLLM.

There is a second cost on the server itself. Building the string is O(request × errors), and sanitize_message then runs several regexes over the whole thing —
7 seconds of event-loop time at 800 items, for a request that is only 71 KB.

The fix

Bound both factors in the handler:

report at most _MAX_REPORTED_ERRORS (10) entries, then ...and N more errors
— the true count still leads the message, so nothing is hidden
replace each entry's input with a bounded summary before rendering
cap each rendered entry at _MAX_ERROR_CHARS (1000) as a backstop, since
loc tuples for union-typed fields are long on their own

_summarize_error_input describes containers (<list of 300 items>) instead of
rendering them. That is deliberate: repr() of a large list materializes the
whole string first, which is exactly the cost being avoided. Strings are sliced
before being rendered, and str() on a scalar is wrapped because it can raise —
an integer past sys.get_int_max_str_digits() would otherwise turn the error
handler itself into a 500.

A union-typed field is also spelled out branch-by-branch in loc — ~800
characters of type names per entry on /v1/responses, which dominated what was
left. clean_loc_for_param already exists in this module and is already applied
to param for exactly that reason, so the rendered loc now goes through it
too. That is what turns the entry from noise into something a caller can act on.

After the fix the same measurement is flat:

`input` items	request bytes	errors	response bytes	handler time
1	114	241	2,386	0.002 s
50	4,475	12,001	2,391	0.003 s
800	71,225	192,001	2,394	0.022 s

Small input values are still echoed verbatim, so a client debugging a genuine
mistake loses nothing.

What the error body actually looks like

Requested by @DarkLight1337. Produced by validating a malformed body with the
real ResponsesRequest model and passing the result to the real handler; only
line-wrapping and the <...> elisions are mine.

Case A — the shape from #49239: /v1/responses with input = 50
message-shaped objects whose content[].text is an int instead of a string.
Request body is 4,475 bytes and produces 12,001 validation errors.

Before — 23,658,207 bytes:

12001 validation errors:
  {'type': 'string_type', 'loc': ('input', 'str'), 'msg': 'Input should be a valid string', 'input': [{'type': 'message', 'role': 'user', 'content': [{'type': 'input_text', 'text': 12345}]}, {'type': 'message', 'role': ' <...4389 more chars on this line: the whole input, again...>
  {'type': 'string_type', 'loc': ('input', 'list[union[EasyInputMessageParam,Message,ResponseOutputMessageParam,ResponseFileSearchToolCallParam,ResponseComputerToolCallParam,ComputerCallOutput,ResponseFunctionWebSearchPa <...1379 more chars...>
  {'type': 'string_type', 'loc': ('input', 'list[union[EasyInputMessageParam,Message,ResponseOutputMessageParam,ResponseFileSearchToolCallParam,ResponseComputerToolCallParam,ComputerCallOutput,ResponseFunctionWebSearchPa <...1461 more chars...>
  <...11,997 more lines...>
  {'type': 'missing', 'loc': ('input', 'list[union[EasyInputMessageParam,Message,ResponseOutputMessageParam,... <...>

After — 2,391 bytes:

12001 validation errors:
  {'type': 'string_type', 'loc': 'input', 'msg': 'Input should be a valid string', 'input': '<list of 50 items>', 'url': 'https://errors.pydantic.dev/2.13/v/string_type'}
  {'type': 'string_type', 'loc': 'input.0.EasyInputMessageParam.content', 'msg': 'Input should be a valid string', 'input': '<list of 1 items>', 'url': 'https://errors.pydantic.dev/2.13/v/string_type'}
  {'type': 'string_type', 'loc': 'input.0.EasyInputMessageParam.content.0.ResponseInputTextParam.text', 'msg': 'Input should be a valid string', 'input': '12345', 'url': 'https://errors.pydantic.dev/2.13/v/string_type'}
  <...7 more entries...>
  ...and 11991 more errors

The third entry is the actual mistake, and it now names the field and shows the
offending value.

Case B — an ordinary small mistake: {"model": "m", "input": 123}.

Before — 1,769 bytes:

2 validation errors:
  {'type': 'string_type', 'loc': ('input', 'str'), 'msg': 'Input should be a valid string', 'input': 123, 'url': 'https://errors.pydantic.dev/2.13/v/string_type'}
  {'type': 'list_type', 'loc': ('input', 'list[union[EasyInputMessageParam,Message,ResponseOutputMessageParam,ResponseFileSearchToolCallParam,ResponseComputerToolCallParam,ComputerCallOutput,ResponseFunctionWebSearchPara <...1291 more chars...>

After — 400 bytes:

2 validation errors:
  {'type': 'string_type', 'loc': 'input', 'msg': 'Input should be a valid string', 'input': '123', 'url': 'https://errors.pydantic.dev/2.13/v/string_type'}
  {'type': 'list_type', 'loc': 'input', 'msg': 'Input should be a valid list', 'input': '123', 'url': 'https://errors.pydantic.dev/2.13/v/list_type'}

The value is still there; nothing a caller needs was dropped.

This handler is registered for RequestValidationError app-wide, so the bound
applies to every endpoint, not just /v1/responses.

I checked for overlap: #52120 also touches validation-error handling but changes
sanitize_message's regex in vllm/entrypoints/serve/utils/api_utils.py, a
different file and a different problem (ReDoS in the sanitizer). The two are
complementary — bounding the message before it reaches the sanitizer reduces the
input that PR has to sanitize. #45390 adds request input-size bounds in
sampling_params.py/protocol.py, also disjoint.

Test Plan

pytest tests/entrypoints/serve/exception_handling/ -v

Four tests added to the existing file as TestValidationErrorBodyIsBounded:

test_many_errors_are_capped — 5,000 errors each carrying a 500-item input;
the whole body must stay under 32 KB and still report the real count
test_container_input_is_described_not_echoed — a 300-item list becomes
<list of 300 items> and its contents do not appear
test_long_string_input_is_truncated — a 100 KB string input is truncated
test_small_input_is_still_reported_verbatim — the diagnostics a legitimate
client needs are unchanged
test_union_loc_is_cleaned_in_the_message — a union loc renders as
body.input.0.content, not 800 characters of branch names

Test Result

Whole directory passes with the fix:

$ pytest tests/entrypoints/serve/exception_handling/ -q
44 passed, 15 warnings in 11.36s

Reverting only handlers/validation.py to main and running the new class:

$ pytest .../test_validation_exception_handler.py -q -k TestValidationErrorBodyIsBounded
FAILED ...::test_many_errors_are_capped
FAILED ...::test_container_input_is_described_not_echoed
FAILED ...::test_long_string_input_is_truncated
3 failed, 1 passed, 12 deselected, 15 warnings in 5.14s

test_small_input_is_still_reported_verbatim passes both before and after, which
is the point — the bound does not cost legitimate clients their diagnostics.

$ ruff check <touched files>          # All checks passed!
$ ruff format --check <touched files> # 2 files already formatted

`validation_exception_handler` renders every entry of `exc.errors()` into the 400 body. Pydantic puts the whole offending value under `input` in each entry, so a request whose `input` fails per-element echoes that input once per error and the response grows with request_size x error_count. Measured against the real `ResponsesRequest` model and the real handler: a 4,475-byte `/v1/responses` body produces 12,001 errors and a 23.6 MB response, and a 71 KB body produces 378 MB and 7 s of event-loop time. The amplification factor is flat at ~5,300x, so it scales with whatever body limit sits in front of the server. Reported in vllm-project#49239 as 12,047 errors and a ~114 MB body, which stalled the proxy in front of vLLM. Report at most 10 entries followed by "...and N more errors" (the true count still leads the message), replace each entry's `input` with a bounded summary, and cap each rendered entry at 1000 characters since `loc` tuples for union-typed fields are long on their own. `_summarize_error_input` describes containers rather than rendering them: `repr()` of a large list materializes the whole string first, which is the cost being avoided. `str()` on a scalar is wrapped because it can raise -- an integer past `sys.get_int_max_str_digits()` would otherwise turn the error handler into a 500. Small values are still echoed verbatim, so a client debugging a genuine mistake loses nothing. After the fix the response is flat at ~9.5 KB and 0.026 s at 192,001 errors. The handler is registered for `RequestValidationError` app-wide, so this applies to every endpoint. Fixes vllm-project#49239 Signed-off-by: lzhan011 <35493221+lzhan011@users.noreply.github.com>

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github-actions · 2026-09-01T04:05:34Z

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DarkLight1337 · 2026-09-01T04:34:36Z

Could you print out some examples of what the error message looks like now and put them in the PR description?

For a union-typed field pydantic spells every branch out in `loc`. On `/v1/responses` that is ~800 characters of type names per entry, which dominated the bounded message: 8 of the 10 reported entries were padded to the 1000-character cap by their `loc` alone. `clean_loc_for_param` already exists in this module and is already applied to `param` for exactly this reason. Applying it to the rendered `loc` as well takes the example in the PR description from 9,384 to 2,292 characters and turns the entry into something a caller can act on: before: 'loc': ('input', 'list[union[EasyInputMessageParam,Message,...'] after: 'loc': 'input.0.EasyInputMessageParam.content.0.ResponseInputTextParam.text' Signed-off-by: lzhan011 <35493221+lzhan011@users.noreply.github.com>

lzhan011 · 2026-09-01T04:48:53Z

Added, thanks — the PR description now has a "What the error body actually looks like" section with real before/after output for two cases.

Short version. The shape from #49239 (a 4,475-byte /v1/responses body producing 12,001 errors) goes from 23,658,207 bytes to 2,391:

# before
12001 validation errors:
  {'type': 'string_type', 'loc': ('input', 'str'), 'msg': 'Input should be a valid string', 'input': [{'type': 'message', 'role': 'user', ... <4389 more chars on this line: the whole input, again>
  <...11,999 more entries, each carrying another copy...>

# after
12001 validation errors:
  {'type': 'string_type', 'loc': 'input', 'msg': 'Input should be a valid string', 'input': '<list of 50 items>', 'url': 'https://errors.pydantic.dev/2.13/v/string_type'}
  {'type': 'string_type', 'loc': 'input.0.EasyInputMessageParam.content', ...}
  {'type': 'string_type', 'loc': 'input.0.EasyInputMessageParam.content.0.ResponseInputTextParam.text', 'msg': 'Input should be a valid string', 'input': '12345', ...}
  <...7 more entries...>
  ...and 11991 more errors

Writing those out surfaced something I had missed, so I pushed one more commit for it. Bounding the size alone still left the body at 9,384 characters, because for a union-typed field pydantic spells every branch out in loc — ~800 characters of type names per entry, which was padding 8 of the 10 reported entries to the per-entry cap on its own. clean_loc_for_param already exists in this module and is already applied to param for exactly that reason, so the rendered loc now goes through it too. That is the difference between 9,384 and 2,292 characters, and it is what turns the third entry above into something that actually names the field.

An ordinary small mistake ({"model": "m", "input": 123}) goes from 1,769 to 400 bytes and still shows the offending value verbatim — test_small_input_is_still_reported_verbatim pins that.

Happy to drop the loc cleaning back out if you would rather keep this PR to the size bound only.

DarkLight1337

This looks good, thanks!

DarkLight1337 · 2026-09-01T04:52:42Z

/ci run

github-actions · 2026-09-01T04:53:00Z

✅ Triggered Buildkite CI #86502 for commit 63d1a4d47d2a.

lzhan011 · 2026-09-01T05:44:27Z

CI came back red on Entrypoints Integration (API Server OpenAI - Part 1). I looked into it, and it is not this PR:

SUBFAILED entrypoints/openai/test_openai_schema.py::test_openapi_stateless[POST /inference/v1/generate]
  [500] Internal Server Error:
    {"error":{"message":"","type":"InternalServerError","param":null,"code":500}}

Schemathesis fuzzed /inference/v1/generate with a garbage sampling_params object and got a 500 where a 4xx was expected.

What rules this PR out, from the build log:

This PR only changes the handler for RequestValidationError, which returns 400. For a 500 to come out, either the request never reached that handler or the handler raised.
It did not raise. validation.py, _format_error, _summarize_error_input, clean_loc_for_param, sanitize_message and exception_handling appear nowhere in the log — no traceback through the changed code.
The changed handler ran fine in the same job. The same endpoint returned 22 400 Bad Request responses in that run, all through it.

It also matches a known signature. #54116 was filed for the same test and the same endpoint with the byte-identical body ("message":"", InternalServerError), on a PR that "touches only fused_moe/distributed MoonEP files and does not modify entrypoints, so this is a latent main issue surfaced by the property-based fuzz". Same family here, different unvalidated field: that one was ec_transfer_params, this one is sampling_params. #53357 is open for surfacing SamplingParams.verify failures as 400 rather than 500, which looks like the same root cause.

Retrying CI, since the fuzzer picks a different seed each run. Happy to file a separate CI-failure issue for the sampling_params variant if it is not already covered by #53357.

lzhan011 · 2026-09-01T05:44:34Z

/ci retry

github-actions · 2026-09-01T05:44:52Z

✅ Queued 1 failed job(s) for retry in Buildkite CI #86502.

lzhan011 · 2026-09-01T06:19:35Z

CI is green now — the retried job passed (30m 8s) and Buildkite is 68/68 with nothing red.

Auto-merge has not fired though, and the reason is a stale check rather than a real failure, so noting the mechanics here to save you the diagnosis:

pre-run-check ran at 04:48:41 on the pull_request event and failed, correctly at the time: no ready label yet and the author has 0 merged PRs.
The ready label was added at 04:53:00, four minutes later.
.github/workflows/pre-commit.yml does listen for labeled, so this would normally re-run — but the label was added by github-actions[bot], and GitHub does not start workflow runs from events caused by the default GITHUB_TOKEN. There is exactly one run of that workflow on this head SHA, the 04:48 one.

So the failing check is evaluating a state that no longer exists. Re-running it should turn it green; I do not have the rights to (gh run rerun → "Must have admin rights to Repository").

I deliberately have not pushed anything to force a re-run, since a new commit would dismiss your approval and cost another review round for no change in content.

…ject#54684) Signed-off-by: lzhan011 <35493221+lzhan011@users.noreply.github.com>

lzhan011 requested review from AndreasKaratzas, DarkLight1337, NickLucche, aarnphm, njhill and robertgshaw2-redhat as code owners September 1, 2026 03:10

claude Bot reviewed Sep 1, 2026

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mergify Bot added frontend bug Something isn't working labels Sep 1, 2026

DarkLight1337 approved these changes Sep 1, 2026

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DarkLight1337 enabled auto-merge (squash) September 1, 2026 04:52

github-actions Bot added the ready ONLY add when PR is ready to merge/full CI is needed label Sep 1, 2026

DarkLight1337 merged commit fa99a6f into vllm-project:main Sep 1, 2026
72 of 73 checks passed

am-cohere pushed a commit to am-cohere/vllm that referenced this pull request Sep 1, 2026

[Bugfix][Security] Bound the validation-error response body (vllm-pro…

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…ject#54684) Signed-off-by: lzhan011 <35493221+lzhan011@users.noreply.github.com>

mylibrar pushed a commit to tanyuqian/vllm that referenced this pull request Sep 3, 2026

[Bugfix][Security] Bound the validation-error response body (vllm-pro…

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…ject#54684) Signed-off-by: lzhan011 <35493221+lzhan011@users.noreply.github.com>

D-G-Dimitrov pushed a commit to D-G-Dimitrov/vllm that referenced this pull request Sep 9, 2026

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…ject#54684) Signed-off-by: lzhan011 <35493221+lzhan011@users.noreply.github.com>

ksriv001 mentioned this pull request Sep 9, 2026

Sanitize 4xx/5xx bodies from the vLLM API server (Cobalt PT40893_8, tt-cloud-console#1152) tenstorrent/tt-inference-server#5133

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[Bugfix][Security] Bound the validation-error response body - #54684

[Bugfix][Security] Bound the validation-error response body#54684
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Purpose

The fix

What the error body actually looks like

Test Plan

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