CVE-2025-54121

Summary

When parsing a multi-part form with large files (greater than the default max spool size) starlette will block the main thread to roll the file over to disk. This blocks the event thread which means we can't accept new connections.

Details

Please see this discussion for details: https://github.com/encode/starlette/discussions/2927#discussioncomment-13721403. In summary the following UploadFile code (copied from here) has a minor bug. Instead of just checking for self._in_memory we should also check if the additional bytes will cause a rollover.

    @property
    def _in_memory(self) -> bool:
        # check for SpooledTemporaryFile._rolled
        rolled_to_disk = getattr(self.file, "_rolled", True)
        return not rolled_to_disk

    async def write(self, data: bytes) -> None:
        if self.size is not None:
            self.size += len(data)

        if self._in_memory:
            self.file.write(data)
        else:
            await run_in_threadpool(self.file.write, data)

I have already created a PR which fixes the problem: https://github.com/encode/starlette/pull/2962

PoC

See the discussion here for steps on how to reproduce.

Impact

To be honest, very low and not many users will be impacted. Parsing large forms is already CPU intensive so the additional IO block doesn't slow down starlette that much on systems with modern HDDs/SSDs. If someone is running on tape they might see a greater impact.

Summary

When parsing a multi-part form with large files (greater than the default max spool size) starlette will block the main thread to roll the file over to disk. This blocks the event thread which means we can't accept new connections.

Details

Please see this discussion for details: https://github.com/encode/starlette/discussions/2927#discussioncomment-13721403. In summary the following UploadFile code (copied from here) has a minor bug. Instead of just checking for self._in_memory we should also check if the additional bytes will cause a rollover.

    @property
    def _in_memory(self) -> bool:
        # check for SpooledTemporaryFile._rolled
        rolled_to_disk = getattr(self.file, "_rolled", True)
        return not rolled_to_disk

    async def write(self, data: bytes) -> None:
        if self.size is not None:
            self.size += len(data)

        if self._in_memory:
            self.file.write(data)
        else:
            await run_in_threadpool(self.file.write, data)

I have already created a PR which fixes the problem: https://github.com/encode/starlette/pull/2962

PoC

See the discussion here for steps on how to reproduce.

Impact

To be honest, very low and not many users will be impacted. Parsing large forms is already CPU intensive so the additional IO block doesn't slow down starlette that much on systems with modern HDDs/SSDs. If someone is running on tape they might see a greater impact.

Root has patched CVE-2025-54121 in the starlette package for Root:PyPI. Multiple fixed versions available.

Starlette is a lightweight ASGI (Asynchronous Server Gateway Interface) framework/toolkit, designed for building async web services in Python. In versions 0.47.1 and below, when parsing a multi-part form with large files (greater than the default max spool size) starlette will block the main thread to roll the file over to disk. This blocks the event thread which means the application can't accept new connections. The UploadFile code has a minor bug where instead of just checking for self._in_memory, the logic should also check if the additional bytes will cause a rollover. The vulnerability is fixed in version 0.47.2.

TuxCare rebuilt starlette to address 1 CVE. Fixed in 0.27.0.post2+tuxcare.

TuxCare rebuilt starlette to address 1 CVE. Fixed in 0.13.6.post1+tuxcare.

TuxCare rebuilt starlette to address 8 CVEs. Fixed in 0.25.0.post1+tuxcare.

These are all security issues fixed in the python311-starlette-0.47.2-1.1 package on the GA media of openSUSE Tumbleweed.

This update for python-starlette fixes the following issues

• CVE-2025-54121: denial-of-service when parsing a multi-part form with large files (bsc#1246855).
• CVE-2025-62727: DoS via Range header merging (bsc#1252805).
• CVE-2026-48710: Missing Host header validation poisons request.url.path, bypassing path-based security checks (bsc#1266369).

This update for python-starlette fixes the following issues:

• CVE-2025-54121: Correctly parse multi-part form with large files to avoid DoS. (bsc#1246855)

This update for python-starlette fixes the following issues

• CVE-2025-54121: denial-of-service when parsing a multi-part form with large files (bsc#1246855).
• CVE-2025-62727: DoS via Range header merging (bsc#1252805).
• CVE-2026-48710: Missing Host header validation poisons request.url.path, bypassing path-based security checks (bsc#1266369).