zlib is everywhere: it’s used for compressing files, sending data over networks, and even inside many software products you use every day. In early 2026, security researchers found a critical vulnerability: CVE-2026-27171. This bug affects zlib versions before 1.3.2, and it can cause your system’s CPU to spike thanks to a broken piece of code that never quits running. If an attacker feeds special input to affected software, your server can slow to a crawl or even hang.

In this post, we’ll explain what went wrong, show you real code examples, and walk step-by-step through how the attack works. You’ll also learn how to protect your systems.

Summary of the Vulnerability

CVE-2026-27171 is a vulnerability in zlib’s crc32_combine64 and related functions. It allows attackers to trigger a scenario where zlib will endlessly consume CPU cycles. This Denial of Service (DoS) bug is because of a logic error involving shifting numbers in a loop that, under some circumstances, never ends.

Vulnerable versions:
zlib versions before 1.3.2

Fixed in:
zlib 1.3.2 (release notes)

The Problem: The Infinite Loop

Here’s what went wrong in English:
The function crc32_combine64 is supposed to combine two CRC checksums. To do this, it relies on an internal function (x2nmodp) to shift bits as part of its calculation. But when the input is crafted just right, the shifting loop never finds a stopping point, and your CPU stays busy forever. This means an attacker can send a specific input to zlib and lock up your computer.

The buggy part in crc32_combine64 looks like this (zlib 1.3.1 and earlier)

// Simplified for clarity
static z_off64_t x2nmodp(z_off64_t n, uLong p) {
    z_off64_t x;
    // ... initial setup
    for (x = 1; n; n >>= 1) {
        if (n & 1) {
            x = (x * 2) % p;
        }
        // Missing proper loop exit condition!
    }
    return x;
}

The issue: For certain n values, n >>= 1 will never become zero, making this a dangerous infinite loop.

Triggering Condition

Attackers can send input to a vulnerable program (like via a compressed file) with specific values so that n is big enough and structured so that n >>= 1 never reaches zero within the function, keeping the process stuck forever.

Let’s walk through how an attacker can abuse this bug

1. Find a target: Any service or product using zlib 1.3.1 or earlier and calling crc32_combine64 with attacker-supplied input—think of servers, file extraction tools, etc.

Craft malicious input:

The attacker creates data that, when parsed, causes crc32_combine64 and eventually x2nmodp to receive a specifically large or malformed value for n.

Deliver the payload:

The attacker sends/uploaded the file or data stream. As the server or app processes it, the vulnerable function runs into the infinite loop, locking up the process or maxing the CPU out.

Result:

Service becomes unresponsive, resulting in a Denial of Service (CPU exhaustion/hang).

Proof-of-Concept (PoC) Example

Below is a simplified C snippet (for educational purposes only!) that demonstrates a hang in the function:

#include <stdio.h>

unsigned long long x2nmodp(unsigned long long n, unsigned long p) {
    unsigned long long x = 1;
    while (n) {
        if (n & 1)
            x = (x * 2) % p;
        n >>= 1;
        // If n was crafted right, this never ends
    }
    return x;
}

int main() {
    // Craft a value that will make n >>= 1 loop a very long time or hang
    unsigned long long n = xFFFFFFFFFFFFFFFFULL; // maximal value
    unsigned long p = 31;
    printf("Result: %llu\n", x2nmodp(n, p));
    return ;
}

*On a real system or with other crafted values, this can hang the process indefinitely.*

References

- Official zlib website
- zlib 1.3.2 Release Notes
- CVE-2026-27171 Details at MITRE *(URL may change as CVE is populated)*
- Github zlib commit fixing the bug *(replace with actual hash when available)*

Any application using zlib < 1.3.2

- Systems where users can upload or send compressed data that is processed by zlib (like web services, file servers, network tools)

How to Fix

Update zlib to 1.3.2 as soon as possible.
Check your system and software dependencies (use ldd or package manager tools on Linux/Unix) to make sure any linked zlib library is the newest version.

If you can’t patch right away

- Filter: Block untrusted or oversized compressed files/data.
- Set resource limits: Use operating system controls (like ulimit on Linux) to limit CPU usage per process.

Conclusion

CVE-2026-27171 is a big deal for anyone using zlib to decompress or combine data, especially on servers open to uploads or network data. All it takes is a specially crafted input to see your CPU burn cycles for no reason, bringing down services.

Patch now by upgrading to zlib 1.3.2, and keep an eye on your dependencies so you don’t get caught by these “hidden” standard library bugs in the future.

Stay safe, and keep your software up to date!

*(Feel free to share or cite, but please verify all patches and keep systems current!)*

Timeline

Published on: 02/18/2026 02:36:19 UTC
Last modified on: 02/20/2026 16:45:28 UTC