CVE-2023-31137: MaraDNS Integer Underflow Vulnerability in DNS Packet Decompression

Severity: high (CVSS 7.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 1.1% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2023-05-09 · updated 2026-06-17
Weakness: CWE-191

Description

MaraDNS is open-source software that implements the Domain Name System (DNS). In version 3.5.0024 and prior, a remotely exploitable integer underflow vulnerability in the DNS packet decompression function allows an attacker to cause a Denial of Service by triggering an abnormal program termination. The vulnerability exists in the `decomp_get_rddata` function within the `Decompress.c` file. When handling a DNS packet with an Answer RR of qtype 16 (TXT record) and any qclass, if the `rdlength` is smaller than `rdata`, the result of the line `Decompress.c:886` is a negative number `len = rdlength - total;`. This value is then passed to the `decomp_append_bytes` function without proper validation, causing the program to attempt to allocate a massive chunk of memory that is impossible to allocate. Consequently, the program exits with an error code of 64, causing a Denial of Service. One proposed fix for this vulnerability is to patch `Decompress.c:887` by breaking `if(len <= 0)`, which has been incorporated in version 3.5.0036 via commit bab062bde40b2ae8a91eecd522e84d8b993bab58.

Affected products

Debian Linux: 10.0, 11.0
Fedoraproject Fedora: 37, 38
MaraDNS: before 3.4.10; from 3.5.0001 before 3.5.0036 — fixed in 3.4.10, 3.5.0036

How to fix

Upgrade to a fixed version: MaraDNS 3.4.10, 3.5.0036. Check the vendor advisory for mitigations.

References

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