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High

Excessive Iteration in gRPC

  • Disclosed: 09.08.2023
  • Updated: 11.10.2026 13:19

gRPC contains a vulnerability that allows hpack table accounting errors could lead to unwanted disconnects between clients and servers in exceptional cases/ Three vectors were found that allow the following DOS attacks:

  • Unbounded memory buffering in the HPACK parser
  • Unbounded CPU consumption in the HPACK parser

The unbounded CPU consumption is down to a copy that occurred per-input-block in the parser, and because that could be unbounded due to the memory copy bug we end up with an O(n^2) parsing loop, with n selected by the client.

The unbounded memory buffering bugs:

  • The header size limit check was behind the string reading code, so we needed to first buffer up to a 4 gigabyte string before rejecting it as longer than 8 or 16kb.
  • HPACK varints have an encoding quirk whereby an infinite number of 0’s can be added at the start of an integer. gRPC’s hpack parser needed to read all of them before concluding a parse.
  • gRPC’s metadata overflow check was performed per frame, so that the following sequence of frames could cause infinite buffering: HEADERS: containing a: 1 CONTINUATION: containing a: 2 CONTINUATION: containing a: 3 etc…

Affected software

Click a technology to see all vulnerabilities affecting it.

PlatformPackageVulnerable versionsFixed in
Python (PyPI) grpcio < 1.53.2, >= 1.54.0, < 1.54.3, >= 1.55.0, < 1.55.2, >= 1.56.0, < 1.56.2 1.53.2, 1.54.3, 1.55.2, 1.56.2

References

CVSS score 7.5/10
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Weakness (CWE)
CWE-770, CWE-789
Platform
Python (PyPI)
Package
grpcio
Disclosed
09.08.2023
Updated
11.10.2026 13:19

Sources and licenses

OSV / GitHub Advisory Database - PYSEC-2026-1424 - PyPA Advisory Database; CC BY 4.0

OSV / GitHub Advisory Database - GHSA-496j-2rq6-j6cc - CC BY 4.0

Vulnerability history: grpcio

All published vulnerabilities of this package (Python (PyPI)), newest first. Shows whether this is a one-off slip or a package that needs watching.

Total entries
6
First → latest
04.2017 09.2023
Without a fix
0
Exploited in attacks
0

Severity breakdown

  • High 4
  • Medium 1
  • Unrated 1

Disclosures per year

  1. Denial of Service Vulnerability in gRPC TCP Server (Posix-compatible platforms)
    High · 7.5 CVE-2023-4785 >= 1.55.0, < 1.55.3, >= 1.54.0, < 1.54.3, >= 1.53.0, < 1.53.2 1.55.3, 1.54.3, 1.53.2
  2. Excessive Iteration in gRPC this entry
    High · 7.5 CVE-2023-33953 < 1.53.2, >= 1.54.0, < 1.54.3, >= 1.55.0, < 1.55.2, >= 1.56.0, < 1.56.2 1.53.2, 1.54.3, 1.55.2, 1.56.2
  3. gRPC connection termination issue
    Medium · 5.3 CVE-2023-32732 >= 1.53.0, < 1.53.1, >= 1.54.0, < 1.54.2 1.53.1, 1.54.2
  4. gRPC Reachable Assertion issue
    High · 7.5 CVE-2023-1428 >= 1.51.0, < 1.53.0 1.53.0
  5. Connection confusion in gRPC
    High · 7.4 CVE-2023-32731 >= 1.53.0, < 1.53.1, >= 1.54.0, < 1.54.2 1.53.1, 1.54.2
  6. Google gRPC before 2017-03-29 has an out-of-bounds write caused by a heap-based use-after-free related to the grpc_call_destroy function in core/lib/surface/call.c.
    Unrated CVE-2017-8359 < 1.3.0 1.3.0

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