4.9

CVSS Score

4.0

-

CVSS Score

Basic Information

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CVE-2026-48487

CVE-2026-48487: zeroconf: Unvalidated rdlength in record payload readers allows LAN-local cache corruption via crafted mDNS packet

Impact

_read_character_string and _read_string in src/zeroconf/_protocol/incoming.py sliced self.data[self.offset : self.offset + length] and advanced self.offset by the declared length without checking it against self._data_len. Python's slice silently returns fewer bytes when the end index runs past the buffer, so a record whose 16-bit RDLENGTH (RFC 1035 §3.2.1) over-advertised by tens of kilobytes was constructed from a truncated payload, appended to DNSIncoming._answers, and committed to the cache before any later parse failure surfaced. The follow-up _read_name for the next record then failed, but the corrupt record had already entered the answer list and propagated to DNSCache and ServiceInfo.

Any unauthenticated host on the local link (UDP/5353, 224.0.0.251 / ff02::fb) can multicast a single mDNS response carrying a TXT, HINFO, or A/AAAA record that advertises rdlength=65535 and only a handful of real payload bytes; consumers calling ServiceInfo.properties then parse the truncated bytes as if they matched the wire, and downstream integrations (Home Assistant and other zeroconf-driven discovery) trust the decoded record. The bug is parser-state desync rather than RCE, but it seeds the cache with attacker-shaped key/value and address records for a TTL window and is a building block for higher-impact chains.

The impact is likely lower than the other recently released advisories as there is no additional risk of OOM so the severity was manually set to low to override the score CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N (6.5, Medium) since that doesn't fully consider the mDNS threat model.

Patches

Fixed in zeroconf 0.149.16 (PR #1756). Note that this change originally intended to ship in 0.149.13 but we ran out of space on PyPI: see https://github.com/python-zeroconf/python-zeroconf/issues/1769

Upgrade to >= 0.149.16.

Workarounds

There is no in-process workaround; upgrading is the fix. Otherwise, restrict mDNS (UDP/5353) to trusted Layer-2 segments via AP client isolation, guest-network separation, or host firewall rules.

Resources

(GitHub Advisory)

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CVE-2026-48487

CVE-2026-48487:

4.9

CVSS Score

4.0

-

CVSS Score

Basic Information

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Technical Details

Package Name	Ecosystem	Vulnerable Versions	First Patched Version
zeroconf	pip	< 0.149.16	0.149.16

Technical Details

Vulnerability Intelligence
Miggo AI

Root Cause Analysis

The vulnerability lies in the zeroconf library's handling of incoming mDNS packets. Specifically, the _read_character_string and _read_string methods within the DNSIncoming class in src/zeroconf/_protocol/incoming.py were found to be vulnerable. These functions are responsible for parsing specific data fields from the mDNS packet payload.

The root cause is an improper handling of length parameters, a vulnerability type known as CWE-130. The functions would read a length value from the packet and use it to slice a portion of the packet data. However, they failed to validate whether this length, when added to the current offset, would exceed the total length of the packet data.

Due to Python's slicing behavior, if the end index of a slice is out of bounds, it doesn't raise an error but instead returns all bytes up to the end of the buffer. The vulnerability arises because even though a truncated payload was read, the internal offset of the parser was still advanced by the full, attacker-controlled length value. This desynchronized the parser's state.

An attacker on the local network could send a specially crafted mDNS response with a record (e.g., TXT, HINFO, A/AAAA) that advertises a very large rdlength (resource data length) but provides only a small payload. The vulnerable functions would read the truncated payload, create a corrupt record, and add it to the application's DNS cache. Subsequent parsing of the packet would fail, but the corrupted data would have already been cached. This allows an attacker to poison the cache with malformed data, which could disrupt services or be a stepping stone for more complex attacks.

The patch addresses this by adding an explicit bounds check in both _read_character_string and _read_string to ensure the declared length does not cause the read to go past the end of the packet data, raising an IncomingDecodeError if the check fails. This prevents the corrupt record from being processed and cached.

The identified vulnerable functions are DNSIncoming._read_character_string and DNSIncoming._read_string because they contain the flawed logic that processes the malicious input without proper validation. These are the functions that would appear in a runtime profile during the exploitation of this vulnerability.

Vulnerable functions

DNSIncoming._read_character_string

src/zeroconf/_protocol/incoming.py

The function reads a "character string" from a DNS packet. It reads a length byte and then slices that many bytes from the packet data. Before the patch, it did not validate if the read length would exceed the packet's boundaries. Python's slicing behavior would return a truncated byte string instead of raising an error, but the offset would still be advanced by the full, incorrect length. This desynchronized the parser state, allowing a crafted packet to inject a corrupt record into the cache.

DNSIncoming._read_string

src/zeroconf/_protocol/incoming.py

The function reads a string of a given length from a DNS packet. Similar to `_read_character_string`, it did not validate the provided length against the remaining packet size. This allowed an attacker to specify a large length, causing the parser to read a truncated payload while advancing its internal offset by the full, attacker-controlled length, leading to cache corruption with malformed data.

Vulnerability Intelligence
Miggo AI

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WAF Rule

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Reasoning

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