CivetWeb HTTP POST null byte causes denial of service
CVE-2025-9648 in CivetWeb library allows remote DoS via crafted HTTP POST with null byte, causing infinite loop and CPU exhaustion; fixed in commit 782e189.
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What to check
- Applies to
- The 1 CVE named below. Whether you run an affected version is not assessed: check the versions the source lists.
- Exploitation
- No exploitation in the wild and no public exploit is recorded for the CVEs named.
CERT Polska coordinated disclosure of CVE-2025-9648, a denial-of-service vulnerability in the CivetWeb library. The flaw resides in the mg_handle_form_request function, where processing a specially crafted HTTP POST request containing a null byte in the payload triggers an infinite loop during form data parsing. Multiple exploitation attempts lead to complete CPU exhaustion, rendering the affected service unresponsive.
The vulnerability affects only the CivetWeb library itself; standalone executables pre-built by the vendor are not impacted. The issue has been remediated in commit 782e189. Organizations using the CivetWeb library in custom applications should update to a patched version immediately.
This disclosure follows responsible reporting by Artur Łącki and CERT Polska's coordinated vulnerability disclosure process. The low complexity of exploitation and the availability of technical details increase the likelihood of active scanning or exploitation attempts against unpatched library implementations.
Mentioned in this report
Source reporting: https://cert.pl/en/posts/2025/09/CVE-2025-9648
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