CVE-2026-40087

@@ -1,9 +1,12 @@
 """Dict prompt template."""
 
+from __future__ import annotations
+
 import warnings
 from functools import cached_property
 from typing import Any, Literal, Optional
 
+from pydantic import model_validator
 from typing_extensions import override
 
 from langchain_core.load import dumpd
@@ -25,6 +28,12 @@ class DictPromptTemplate(RunnableSerializable[dict, dict]):
     template: dict[str, Any]
     template_format: Literal["f-string", "mustache"]
 
+    @model_validator(mode="after")
+    def validate_template(self) -> DictPromptTemplate:
+        """Validate that the template structure contains only safe variables."""
+        _get_input_variables(self.template, self.template_format)
+        return self
+
     @property
     def input_variables(self) -> list[str]:
         """Template input variables."""

@@ -2,13 +2,15 @@
 
 from typing import Any
 
-from pydantic import Field
+from pydantic import Field, model_validator
+from typing_extensions import Self
 
 from langchain_core.prompt_values import ImagePromptValue, ImageURL, PromptValue
 from langchain_core.prompts.base import BasePromptTemplate
 from langchain_core.prompts.string import (
     DEFAULT_FORMATTER_MAPPING,
     PromptTemplateFormat,
+    get_template_variables,
 )
 from langchain_core.runnables import run_in_executor
 
@@ -40,8 +42,17 @@ def __init__(self, **kwargs: Any) -> None:
                 f" Found: {overlap}"
             )
             raise ValueError(msg)
+
         super().__init__(**kwargs)
 
+    @model_validator(mode="after")
+    def validate_template(self) -> Self:
+        """Validate template string values after Pydantic parsing."""
+        for value in self.template.values():
+            if isinstance(value, str):
+                get_template_variables(value, self.template_format)
+        return self
+
     @property
     def _prompt_type(self) -> str:
         """Return the prompt type key."""

@@ -263,6 +263,46 @@ def _create_model_recursive(name: str, defs: Defs) -> type:
 }
 
 
+def _parse_f_string_fields(template: str) -> list[tuple[str, str | None]]:
+    fields: list[tuple[str, str | None]] = []
+    for _, field_name, format_spec, _ in Formatter().parse(template):
+        if field_name is not None:
+            fields.append((field_name, format_spec))
+    return fields
+
+
+def validate_f_string_template(template: str) -> list[str]:
+    """Validate an f-string template and return its input variables."""
+    input_variables = set()
+    for var, format_spec in _parse_f_string_fields(template):
+        if "." in var or "[" in var or "]" in var:
+            msg = (
+                f"Invalid variable name {var!r} in f-string template. "
+                f"Variable names cannot contain attribute "
+                f"access (.) or indexing ([])."
+            )
+            raise ValueError(msg)
+
+        if var.isdigit():
+            msg = (
+                f"Invalid variable name {var!r} in f-string template. "
+                f"Variable names cannot be all digits as they are interpreted "
+                f"as positional arguments."
+            )
+            raise ValueError(msg)
+
+        if format_spec and ("{" in format_spec or "}" in format_spec):
+            msg = (
+                "Invalid format specifier in f-string template. "
+                "Nested replacement fields are not allowed."
+            )
+            raise ValueError(msg)
+
+        input_variables.add(var)
+
+    return sorted(input_variables)
+
+
 def check_valid_template(
     template: str, template_format: str, input_variables: list[str]
 ) -> None:
@@ -285,6 +325,8 @@ def check_valid_template(
             f" {list(DEFAULT_FORMATTER_MAPPING)}."
         )
         raise ValueError(msg) from exc
+    if template_format == "f-string":
+        validate_f_string_template(template)
     try:
         validator_func(template, input_variables)
     except (KeyError, IndexError) as exc:
@@ -308,43 +350,18 @@ def get_template_variables(template: str, template_format: str) -> list[str]:
     Raises:
         ValueError: If the template format is not supported.
     """
+    input_variables: list[str] | set[str]
     if template_format == "jinja2":
         # Get the variables for the template
-        input_variables = _get_jinja2_variables_from_template(template)
+        input_variables = sorted(_get_jinja2_variables_from_template(template))
     elif template_format == "f-string":
-        input_variables = {
-            v for _, v, _, _ in Formatter().parse(template) if v is not None
-        }
+        input_variables = validate_f_string_template(template)
     elif template_format == "mustache":
         input_variables = mustache_template_vars(template)
     else:
         msg = f"Unsupported template format: {template_format}"
         raise ValueError(msg)
 
-    # For f-strings, block attribute access and indexing syntax
-    # This prevents template injection attacks via accessing dangerous attributes
-    if template_format == "f-string":
-        for var in input_variables:
-            # Formatter().parse() returns field names with dots/brackets if present
-            # e.g., "obj.attr" or "obj[0]" - we need to block these
-            if "." in var or "[" in var or "]" in var:
-                msg = (
-                    f"Invalid variable name {var!r} in f-string template. "
-                    f"Variable names cannot contain attribute "
-                    f"access (.) or indexing ([])."
-                )
-                raise ValueError(msg)
-
-            # Block variable names that are all digits (e.g., "0", "100")
-            # These are interpreted as positional arguments, not keyword arguments
-            if var.isdigit():
-                msg = (
-                    f"Invalid variable name {var!r} in f-string template. "
-                    f"Variable names cannot be all digits as they are interpreted "
-                    f"as positional arguments."
-                )
-                raise ValueError(msg)
-
     return sorted(input_variables)
 
 

@@ -1300,6 +1300,24 @@ def test_fstring_rejects_invalid_identifier_variable_names() -> None:
         assert result.messages[0].content == expected  # type: ignore[attr-defined]
 
 
+def test_fstring_rejects_nested_replacement_field_in_image_url() -> None:
+    with pytest.raises(ValueError, match="Nested replacement fields are not allowed"):
+        ChatPromptTemplate.from_messages(
+            [
+                (
+                    "human",
+                    [
+                        {
+                            "type": "image_url",
+                            "image_url": {"url": "{img:{img.__class__.__name__}}"},
+                        }
+                    ],
+                )
+            ],
+            template_format="f-string",
+        )
+
+
 def test_mustache_template_attribute_access_vulnerability() -> None:
     """Test that Mustache template injection is blocked.
 

@@ -1,4 +1,9 @@
-from langchain_core.load import load
+import json
+
+import pytest
+
+from langchain_core.load import load, loads
+from langchain_core.prompts import PromptTemplate
 from langchain_core.prompts.dict import DictPromptTemplate
 
 
@@ -32,3 +37,82 @@ def test_deserialize_legacy() -> None:
         template={"type": "audio", "audio": "{audio_data}"}, template_format="f-string"
     )
     assert load(ser, allowed_objects=[DictPromptTemplate]) == expected
+
+
+def test_dict_prompt_template_rejects_attribute_access_to_rich_objects() -> None:
+    with pytest.raises(ValueError, match="Variable names cannot contain attribute"):
+        DictPromptTemplate(
+            template={"output": "{message.additional_kwargs[secret]}"},
+            template_format="f-string",
+        )
+
+
+def test_dict_prompt_template_loads_payload_rejects_attribute_access() -> None:
+    payload = json.dumps(
+        {
+            "lc": 1,
+            "type": "constructor",
+            "id": ["langchain_core", "prompts", "dict", "DictPromptTemplate"],
+            "kwargs": {
+                "template": {"output": "{message.additional_kwargs[secret]}"},
+                "template_format": "f-string",
+            },
+        }
+    )
+
+    with pytest.raises(ValueError, match="Variable names cannot contain attribute"):
+        loads(payload)
+
+
+def test_dict_prompt_template_dumpd_round_trip_rejects_attribute_access() -> None:
+    payload = {
+        "lc": 1,
+        "type": "constructor",
+        "id": ["langchain_core", "prompts", "dict", "DictPromptTemplate"],
+        "kwargs": {
+            "template": {"output": "{message.additional_kwargs[secret]}"},
+            "template_format": "f-string",
+        },
+    }
+
+    with pytest.raises(ValueError, match="Variable names cannot contain attribute"):
+        load(payload, allowed_objects=[DictPromptTemplate])
+
+
+def test_dict_prompt_template_deserialization_rejects_attribute_access() -> None:
+    payload = json.dumps(
+        {
+            "lc": 1,
+            "type": "constructor",
+            "id": ["langchain_core", "prompts", "dict", "DictPromptTemplate"],
+            "kwargs": {
+                "template": {"output": "{name.__class__.__name__}"},
+                "template_format": "f-string",
+            },
+        }
+    )
+
+    with pytest.raises(ValueError, match="Variable names cannot contain attribute"):
+        loads(payload)
+
+
+def test_dict_prompt_template_legacy_deserialization_rejects_attribute_access() -> None:
+    ser = {
+        "type": "constructor",
+        "lc": 1,
+        "id": ["langchain_core", "prompts", "message", "_DictMessagePromptTemplate"],
+        "kwargs": {
+            "template_format": "f-string",
+            "template": {"output": "{name.__class__.__name__}"},
+        },
+    }
+
+    with pytest.raises(ValueError, match="Variable names cannot contain attribute"):
+        load(ser, allowed_objects=[DictPromptTemplate])
+
+
+def test_prompt_template_blocks_attribute_access() -> None:
+    with pytest.raises(
+        ValueError, match="Variable names cannot contain attribute access"
+    ):
+        PromptTemplate.from_template("{name.__class__}", template_format="f-string")

@@ -1,7 +1,11 @@
 import json
 
+import pytest
+from pydantic import ValidationError
+
 from langchain_core.load import dump, loads
 from langchain_core.prompts import ChatPromptTemplate
+from langchain_core.prompts.image import ImagePromptTemplate
 
 
 def test_image_prompt_template_deserializable() -> None:
@@ -15,6 +19,11 @@ def test_image_prompt_template_deserializable() -> None:
     )
 
 
+def test_image_prompt_template_invalid_template_type() -> None:
+    with pytest.raises(ValidationError):
+        ImagePromptTemplate(template=None)
+
+
 def test_image_prompt_template_deserializable_old() -> None:
     """Test that the image prompt template is serializable."""
     loads(

@@ -1,7 +1,12 @@
 import pytest
 from packaging import version
 
-from langchain_core.prompts.string import mustache_schema
+from langchain_core.prompts.string import (
+    check_valid_template,
+    get_template_variables,
+    mustache_schema,
+)
+from langchain_core.utils.formatting import formatter
 from langchain_core.utils.pydantic import PYDANTIC_VERSION
 
 PYDANTIC_VERSION_AT_LEAST_29 = version.parse("2.9") <= PYDANTIC_VERSION
@@ -30,3 +35,47 @@ def test_mustache_schema_parent_child() -> None:
     }
     actual = mustache_schema(template).model_json_schema()
     assert expected == actual
+
+
+def test_get_template_variables_rejects_nested_replacement_field_in_format_spec() -> (
+    None
+):
+    template = "{name:{name.__class__.__name__}}"
+
+    with pytest.raises(ValueError, match="Nested replacement fields are not allowed"):
+        get_template_variables(template, "f-string")
+
+
+def test_formatter_rejects_nested_replacement_field_in_format_spec() -> None:
+    template = "{name:{name.__class__.__name__}}"
+
+    with pytest.raises(ValueError, match="Invalid format specifier"):
+        formatter.format(template, name="hello")
+
+
+def test_check_valid_template_rejects_nested_replacement_field_in_format_spec() -> None:
+    template = "{name:{name.__class__.__name__}}"
+
+    with pytest.raises(ValueError, match="Nested replacement fields are not allowed"):
+        check_valid_template(template, "f-string", ["name"])
+
+
+@pytest.mark.parametrize(
+    ("template", "kwargs", "expected_variables", "expected_output"),
+    [
+        ("{value:.2f}", {"value": 3.14159}, ["value"], "3.14"),
+        ("{value:>10}", {"value": "cat"}, ["value"], "       cat"),
+        ("{value:*^10}", {"value": "cat"}, ["value"], "***cat****"),
+        ("{value:,}", {"value": 1234567}, ["value"], "1,234,567"),
+        ("{value:%}", {"value": 0.125}, ["value"], "12.500000%"),
+        ("{value!r}", {"value": "cat"}, ["value"], "'cat'"),
+    ],
+)
+def test_f_string_templates_allow_safe_format_specs(
+    template: str,
+    kwargs: dict[str, object],
+    expected_variables: list[str],
+    expected_output: str,
+) -> None:
+    assert get_template_variables(template, "f-string") == expected_variables
+    assert formatter.format(template, **kwargs) == expected_output

@@ -4,6 +4,7 @@
 from functools import cached_property
 from typing import Any, Literal, cast
 
+from pydantic import model_validator
 from typing_extensions import override
 
 from langchain_core.load import dumpd
@@ -21,11 +22,35 @@ class DictPromptTemplate(RunnableSerializable[dict, dict]):
     Recognizes variables in f-string or mustache formatted string dict values.
 
     Does NOT recognize variables in dict keys. Applies recursively.
+
+    Example:
+        ```python
+        prompt = DictPromptTemplate(
+            template={
+                "type": "text",
+                "text": "Hello {name}",
+                "metadata": {"source": "{source}"},
+            },
+            template_format="f-string",
+        )
+        prompt.format(name="Alice", source="docs")
+        # {
+        #     "type": "text",
+        #     "text": "Hello Alice",
+        #     "metadata": {"source": "docs"},
+        # }
+        ```
     """
 
     template: dict[str, Any]
     template_format: Literal["f-string", "mustache"]
 
+    @model_validator(mode="after")
+    def validate_template(self) -> "DictPromptTemplate":
+        """Validate that the template structure contains only safe variables."""
+        _get_input_variables(self.template, self.template_format)
+        return self
+
     @property
     def input_variables(self) -> list[str]:
         """Template input variables."""

@@ -9,12 +9,25 @@
 from langchain_core.prompts.string import (
     DEFAULT_FORMATTER_MAPPING,
     PromptTemplateFormat,
+    get_template_variables,
 )
 from langchain_core.runnables import run_in_executor
 
 
 class ImagePromptTemplate(BasePromptTemplate[ImageURL]):
-    """Image prompt template for a multimodal model."""
+    """Image prompt template for a multimodal model.
+
+    Example:
+        ```python
+        prompt = ImagePromptTemplate(
+            input_variables=["image_id"],
+            template={"url": "https://example.com/{image_id}.png", "detail": "high"},
+            template_format="f-string",
+        )
+        prompt.format(image_id="cat")
+        # {"url": "https://example.com/cat.png", "detail": "high"}
+        ```
+    """
 
     template: dict = Field(default_factory=dict)
     """Template for the prompt."""
@@ -43,6 +56,13 @@ def __init__(self, **kwargs: Any) -> None:
                 f" Found: {overlap}"
             )
             raise ValueError(msg)
+
+        template = kwargs.get("template", {})
+        template_format = kwargs.get("template_format", "f-string")
+        for value in template.values():
+            if isinstance(value, str):
+                get_template_variables(value, template_format)
+
         super().__init__(**kwargs)
 
     @property

@@ -219,6 +219,46 @@ def _create_model_recursive(name: str, defs: Defs) -> type[BaseModel]:
 }
 
 
+def _parse_f_string_fields(template: str) -> list[tuple[str, str | None]]:
+    fields: list[tuple[str, str | None]] = []
+    for _, field_name, format_spec, _ in Formatter().parse(template):
+        if field_name is not None:
+            fields.append((field_name, format_spec))
+    return fields
+
+
+def validate_f_string_template(template: str) -> list[str]:
+    """Validate an f-string template and return its input variables."""
+    input_variables = set()
+    for var, format_spec in _parse_f_string_fields(template):
+        if "." in var or "[" in var or "]" in var:
+            msg = (
+                f"Invalid variable name {var!r} in f-string template. "
+                f"Variable names cannot contain attribute "
+                f"access (.) or indexing ([])."
+            )
+            raise ValueError(msg)
+
+        if var.isdigit():
+            msg = (
+                f"Invalid variable name {var!r} in f-string template. "
+                f"Variable names cannot be all digits as they are interpreted "
+                f"as positional arguments."
+            )
+            raise ValueError(msg)
+
+        if format_spec and ("{" in format_spec or "}" in format_spec):
+            msg = (
+                "Invalid format specifier in f-string template. "
+                "Nested replacement fields are not allowed."
+            )
+            raise ValueError(msg)
+
+        input_variables.add(var)
+
+    return sorted(input_variables)
+
+
 def check_valid_template(
     template: str, template_format: str, input_variables: list[str]
 ) -> None:
@@ -243,6 +283,8 @@ def check_valid_template(
             f" {list(DEFAULT_FORMATTER_MAPPING)}."
         )
         raise ValueError(msg) from exc
+    if template_format == "f-string":
+        validate_f_string_template(template)
     try:
         validator_func(template, input_variables)
     except (KeyError, IndexError) as exc:
@@ -268,43 +310,18 @@ def get_template_variables(template: str, template_format: str) -> list[str]:
     Raises:
         ValueError: If the template format is not supported.
     """
+    input_variables: list[str] | set[str]
     if template_format == "jinja2":
         # Get the variables for the template
-        input_variables = _get_jinja2_variables_from_template(template)
+        input_variables = sorted(_get_jinja2_variables_from_template(template))
     elif template_format == "f-string":
-        input_variables = {
-            v for _, v, _, _ in Formatter().parse(template) if v is not None
-        }
+        input_variables = validate_f_string_template(template)
     elif template_format == "mustache":
         input_variables = mustache_template_vars(template)
     else:
         msg = f"Unsupported template format: {template_format}"
         raise ValueError(msg)
 
-    # For f-strings, block attribute access and indexing syntax
-    # This prevents template injection attacks via accessing dangerous attributes
-    if template_format == "f-string":
-        for var in input_variables:
-            # Formatter().parse() returns field names with dots/brackets if present
-            # e.g., "obj.attr" or "obj[0]" - we need to block these
-            if "." in var or "[" in var or "]" in var:
-                msg = (
-                    f"Invalid variable name {var!r} in f-string template. "
-                    f"Variable names cannot contain attribute "
-                    f"access (.) or indexing ([])."
-                )
-                raise ValueError(msg)
-
-            # Block variable names that are all digits (e.g., "0", "100")
-            # These are interpreted as positional arguments, not keyword arguments
-            if var.isdigit():
-                msg = (
-                    f"Invalid variable name {var!r} in f-string template. "
-                    f"Variable names cannot be all digits as they are interpreted "
-                    f"as positional arguments."
-                )
-                raise ValueError(msg)
-
     return sorted(input_variables)
 
 

@@ -1951,6 +1951,24 @@ def test_fstring_rejects_invalid_identifier_variable_names() -> None:
         assert result.messages[0].content == expected  # type: ignore[attr-defined]
 
 
+def test_fstring_rejects_nested_replacement_field_in_image_url() -> None:
+    with pytest.raises(ValueError, match="Nested replacement fields are not allowed"):
+        ChatPromptTemplate.from_messages(
+            [
+                (
+                    "human",
+                    [
+                        {
+                            "type": "image_url",
+                            "image_url": {"url": "{img:{img.__class__.__name__}}"},
+                        }
+                    ],
+                )
+            ],
+            template_format="f-string",
+        )
+
+
 def test_mustache_template_attribute_access_vulnerability() -> None:
     """Test that Mustache template injection is blocked.
 

@@ -1,4 +1,9 @@
-from langchain_core.load import load
+import json
+
+import pytest
+
+from langchain_core.load import load, loads
+from langchain_core.prompts import PromptTemplate
 from langchain_core.prompts.dict import DictPromptTemplate
 
 
@@ -32,3 +37,82 @@ def test_deserialize_legacy() -> None:
         template={"type": "audio", "audio": "{audio_data}"}, template_format="f-string"
     )
     assert load(ser, allowed_objects=[DictPromptTemplate]) == expected
+
+
+def test_dict_prompt_template_rejects_attribute_access_to_rich_objects() -> None:
+    with pytest.raises(ValueError, match="Variable names cannot contain attribute"):
+        DictPromptTemplate(
+            template={"output": "{message.additional_kwargs[secret]}"},
+            template_format="f-string",
+        )
+
+
+def test_dict_prompt_template_loads_payload_rejects_attribute_access() -> None:
+    payload = json.dumps(
+        {
+            "lc": 1,
+            "type": "constructor",
+            "id": ["langchain_core", "prompts", "dict", "DictPromptTemplate"],
+            "kwargs": {
+                "template": {"output": "{message.additional_kwargs[secret]}"},
+                "template_format": "f-string",
+            },
+        }
+    )
+
+    with pytest.raises(ValueError, match="Variable names cannot contain attribute"):
+        loads(payload)
+
+
+def test_dict_prompt_template_dumpd_round_trip_rejects_attribute_access() -> None:
+    payload = {
+        "lc": 1,
+        "type": "constructor",
+        "id": ["langchain_core", "prompts", "dict", "DictPromptTemplate"],
+        "kwargs": {
+            "template": {"output": "{message.additional_kwargs[secret]}"},
+            "template_format": "f-string",
+        },
+    }
+
+    with pytest.raises(ValueError, match="Variable names cannot contain attribute"):
+        load(payload, allowed_objects=[DictPromptTemplate])
+
+
+def test_dict_prompt_template_deserialization_rejects_attribute_access() -> None:
+    payload = json.dumps(
+        {
+            "lc": 1,
+            "type": "constructor",
+            "id": ["langchain_core", "prompts", "dict", "DictPromptTemplate"],
+            "kwargs": {
+                "template": {"output": "{name.__class__.__name__}"},
+                "template_format": "f-string",
+            },
+        }
+    )
+
+    with pytest.raises(ValueError, match="Variable names cannot contain attribute"):
+        loads(payload)
+
+
+def test_dict_prompt_template_legacy_deserialization_rejects_attribute_access() -> None:
+    ser = {
+        "type": "constructor",
+        "lc": 1,
+        "id": ["langchain_core", "prompts", "message", "_DictMessagePromptTemplate"],
+        "kwargs": {
+            "template_format": "f-string",
+            "template": {"output": "{name.__class__.__name__}"},
+        },
+    }
+
+    with pytest.raises(ValueError, match="Variable names cannot contain attribute"):
+        load(ser, allowed_objects=[DictPromptTemplate])
+
+
+def test_prompt_template_blocks_attribute_access() -> None:
+    with pytest.raises(
+        ValueError, match="Variable names cannot contain attribute access"
+    ):
+        PromptTemplate.from_template("{name.__class__}", template_format="f-string")

@@ -1,7 +1,10 @@
 import json
 
+import pytest
+
 from langchain_core.load import dump, loads
 from langchain_core.prompts import ChatPromptTemplate
+from langchain_core.prompts.image import ImagePromptTemplate
 
 
 def test_image_prompt_template_deserializable() -> None:
@@ -107,3 +110,31 @@ def test_image_prompt_template_deserializable_old() -> None:
             }
         ),
     )
+
+
+def test_image_prompt_template_rejects_attribute_access_in_template_values() -> None:
+    with pytest.raises(ValueError, match="Variable names cannot contain attribute"):
+        ImagePromptTemplate(
+            input_variables=["image"],
+            template={"url": "https://example.com/{image.__class__.__name__}.png"},
+        )
+
+
+def test_image_prompt_template_deserialization_rejects_attribute_access() -> None:
+    payload = json.dumps(
+        {
+            "lc": 1,
+            "type": "constructor",
+            "id": ["langchain", "prompts", "image", "ImagePromptTemplate"],
+            "kwargs": {
+                "template": {
+                    "url": "https://example.com/{image.__class__.__name__}.png"
+                },
+                "input_variables": ["image"],
+                "template_format": "f-string",
+            },
+        }
+    )
+
+    with pytest.raises(ValueError, match="Variable names cannot contain attribute"):
+        loads(payload)

@@ -1,7 +1,12 @@
 import pytest
 from packaging import version
 
-from langchain_core.prompts.string import get_template_variables, mustache_schema
+from langchain_core.prompts.string import (
+    check_valid_template,
+    get_template_variables,
+    mustache_schema,
+)
+from langchain_core.utils.formatting import formatter
 from langchain_core.utils.pydantic import PYDANTIC_VERSION
 
 PYDANTIC_VERSION_AT_LEAST_29 = version.parse("2.9") <= PYDANTIC_VERSION
@@ -39,3 +44,47 @@ def test_get_template_variables_mustache_nested() -> None:
     expected = ["user"]
     actual = get_template_variables(template, template_format)
     assert actual == expected
+
+
+def test_get_template_variables_rejects_nested_replacement_field_in_format_spec() -> (
+    None
+):
+    template = "{name:{name.__class__.__name__}}"
+
+    with pytest.raises(ValueError, match="Nested replacement fields are not allowed"):
+        get_template_variables(template, "f-string")
+
+
+def test_formatter_rejects_nested_replacement_field_in_format_spec() -> None:
+    template = "{name:{name.__class__.__name__}}"
+
+    with pytest.raises(ValueError, match="Invalid format specifier"):
+        formatter.format(template, name="hello")
+
+
+def test_check_valid_template_rejects_nested_replacement_field_in_format_spec() -> None:
+    template = "{name:{name.__class__.__name__}}"
+
+    with pytest.raises(ValueError, match="Nested replacement fields are not allowed"):
+        check_valid_template(template, "f-string", ["name"])
+
+
+@pytest.mark.parametrize(
+    ("template", "kwargs", "expected_variables", "expected_output"),
+    [
+        ("{value:.2f}", {"value": 3.14159}, ["value"], "3.14"),
+        ("{value:>10}", {"value": "cat"}, ["value"], "       cat"),
+        ("{value:*^10}", {"value": "cat"}, ["value"], "***cat****"),
+        ("{value:,}", {"value": 1234567}, ["value"], "1,234,567"),
+        ("{value:%}", {"value": 0.125}, ["value"], "12.500000%"),
+        ("{value!r}", {"value": "cat"}, ["value"], "'cat'"),
+    ],
+)
+def test_f_string_templates_allow_safe_format_specs(
+    template: str,
+    kwargs: dict[str, object],
+    expected_variables: list[str],
+    expected_output: str,
+) -> None:
+    assert get_template_variables(template, "f-string") == expected_variables
+    assert formatter.format(template, **kwargs) == expected_output