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URL: http://github.com/fastapi/fastapi/pull/15187/files

" media="all" rel="stylesheet" href="https://github.githubassets.com/assets/code-2d31826944fd3be8.css" /> fix: reject newline characters in SSE event and id fields by subhashdasyam · Pull Request #15187 · fastapi/fastapi · GitHub
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fix: reject newline characters in SSE event and id fields #15187

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fix: reject newline characters in SSE event and id fields #15187

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8bfab52
fix: reject newline characters in SSE event and id fields
Mar 21, 2026
06eccbf
fix: add null-byte protection to SSE 'event' field for symmetry with …
Mar 22, 2026
9e51a7d
fix: raise ValueError in format_sse_event instead of stripping invali…
Mar 25, 2026
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31 changes: 27 additions & 4 deletions fastapi/sse.py
View file
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Original file line number Diff line number Diff line change
Expand Up @@ -33,9 +33,21 @@ class EventSourceResponse(StreamingResponse):
media_type = "text/event-stream"


def _check_id_no_null(v: str | None) -> str | None:
if v is not None and "\0" in v:
raise ValueError("SSE 'id' must not contain null characters")
def _check_id_no_null_or_newline(v: str | None) -> str | None:
if v is not None:
if "\0" in v:
raise ValueError("SSE 'id' must not contain null characters")
if "\n" in v or "\r" in v:
raise ValueError("SSE 'id' must not contain newline characters")
return v


def _check_event_no_newline(v: str | None) -> str | None:
if v is not None:
if "\0" in v:
raise ValueError("SSE 'event' must not contain null characters")
if "\n" in v or "\r" in v:
raise ValueError("SSE 'event' must not contain newline characters")
return v


Expand Down Expand Up @@ -86,18 +98,21 @@ class ServerSentEvent(BaseModel):
] = None
event: Annotated[
str | None,
AfterValidator(_check_event_no_newline),
Doc(
"""
Optional event type name.

Maps to `addEventListener(event, ...)` on the browser. When omitted,
the browser dispatches on the generic `message` event.

**Must not contain newline (`\\n`) or carriage return (`\\r`) characters.**
"""
),
] = None
id: Annotated[
str | None,
AfterValidator(_check_id_no_null),
AfterValidator(_check_id_no_null_or_newline),
Doc(
"""
Optional event ID.
Expand Down Expand Up @@ -197,13 +212,21 @@ def format_sse_event(
lines.append(f": {line}")

if event is not None:
if "\n" in event or "\r" in event:
raise ValueError("SSE 'event' must not contain newline characters")
if "\0" in event:
raise ValueError("SSE 'event' must not contain null characters")
lines.append(f"event: {event}")

if data_str is not None:
for line in data_str.splitlines():
lines.append(f"data: {line}")

if id is not None:
if "\n" in id or "\r" in id:
raise ValueError("SSE 'id' must not contain newline characters")
if "\0" in id:
raise ValueError("SSE 'id' must not contain null characters")
lines.append(f"id: {id}")

if retry is not None:
Expand Down
67 changes: 67 additions & 0 deletions tests/test_sse.py
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Original file line number Diff line number Diff line change
Expand Up @@ -221,6 +221,73 @@ def test_server_sent_event_null_id_rejected():
ServerSentEvent(data="test", id="has\0null")


def test_server_sent_event_null_event_rejected():
with pytest.raises(ValueError, match="null"):
ServerSentEvent(data="test", event="has\0null")


def test_server_sent_event_newline_event_rejected():
with pytest.raises(ValueError, match="newline"):
ServerSentEvent(data="test", event="chat\npwned")


def test_server_sent_event_cr_event_rejected():
with pytest.raises(ValueError, match="newline"):
ServerSentEvent(data="test", event="chat\rpwned")


def test_server_sent_event_newline_id_rejected():
with pytest.raises(ValueError, match="newline"):
ServerSentEvent(data="test", id="42\npwned")


def test_server_sent_event_cr_id_rejected():
with pytest.raises(ValueError, match="newline"):
ServerSentEvent(data="test", id="42\rpwned")


def test_format_sse_event_raises_on_newline_in_event():
from fastapi.sse import format_sse_event

with pytest.raises(ValueError, match="newline"):
format_sse_event(event="chat\npwned", data_str="hello")


def test_format_sse_event_raises_on_cr_in_event():
from fastapi.sse import format_sse_event

with pytest.raises(ValueError, match="newline"):
format_sse_event(event="chat\rpwned", data_str="hello")


def test_format_sse_event_raises_on_null_in_event():
from fastapi.sse import format_sse_event

with pytest.raises(ValueError, match="null"):
format_sse_event(event="chat\x00pwned", data_str="hello")


Comment thread
subhashdasyam marked this conversation as resolved.
def test_format_sse_event_raises_on_newline_in_id():
from fastapi.sse import format_sse_event

with pytest.raises(ValueError, match="newline"):
format_sse_event(id="42\npwned", data_str="hello")


def test_format_sse_event_raises_on_cr_in_id():
from fastapi.sse import format_sse_event

with pytest.raises(ValueError, match="newline"):
format_sse_event(id="42\rpwned", data_str="hello")


def test_format_sse_event_raises_on_null_in_id():
from fastapi.sse import format_sse_event

with pytest.raises(ValueError, match="null"):
format_sse_event(id="42\x00pwned", data_str="hello")


def test_server_sent_event_negative_retry_rejected():
with pytest.raises(ValueError):
ServerSentEvent(data="test", retry=-1)
Expand Down
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