CVE-2026-44040: libc rand() Used for VNC Auth Challenge Generation (UltraVNC)

libc rand() Used for VNC Auth Challenge Generation (UltraVNC)

UltraVNC through 1.8.2.2 uses a cryptographically weak pseudo-random number generator to produce VNC authentication challenge bytes. In rfb/vncauth.c:119-129, the vncRandomBytes() function seeds libc rand() with time(0) + getpid() + rand() and generates a 16-byte challenge. The combined seed space is approximately 31 bits (libc rand() internal state) and is entirely determined by publicly-observable values (wall-clock time and process ID). An attacker who can observe the authentication exchange can enumerate the seed space and predict the challenge within seconds, enabling forgery or offline brute-forcing of responses. Note: on Windows, the active code path may use vncEncryptBytes2.cpp which calls CryptGenRandom; reachability on shipped Windows binaries requires compile-graph verification and is under investigation.

CVE IDCVE-2026-44040

CVSS v3.14.8 Medium

VendorUltraVNC

CWECWE-338: Use of Cryptographically Weak PRNG

DisclosedJun 3, 2026

StatusFixed

01/Description

What this actually is.

Technical background, root cause, and affected surface.

vncRandomBytes() in rfb/vncauth.c:119-129 seeds libc rand() with time(0)+getpid()+rand() and generates a 16-byte VNC auth challenge. The combined seed space is ~31 bits (libc rand() internal state). Note: Windows builds may use CryptGenRandom via vncEncryptBytes2.cpp — Windows-path reachability pending compile-graph verification.

Vendor: UltraVNC
Product: UltraVNC Server
Severity: Medium
CVSS Score: 4.8
Status: Fixed
Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N
CWE: CWE-338: Use of Cryptographically Weak PRNG

02/Proof of Concept

From one request
to root shell.

Reproduced in a sandboxed environment. Requires only LAN or WiFi adjacency.

VectorCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N

ScopeUnchanged

ImpactC:L / I:L / A:N

SeverityMedium

predict_challenge.py demonstrates deterministic challenge prediction from time-based seed. Private PoC; not released publicly.

03/Vulnerable Code

The bug, and the fix.

// rfb/vncauth.c:119-129 vncRandomBytes(unsigned char *bytes) { srand((unsigned int) time(0) + getpid() + rand()); for (i=0; i<CHALLENGESIZE; i++) bytes[i] = (unsigned char)(rand() & 0xff); }

Root cause: Legacy cross-platform code uses libc rand() with low-entropy time-based seed. vncEncryptBytes2.cpp with CryptGenRandom was added for Windows but compile-graph verification needed to confirm this file shadows vncauth.c on shipped Windows binaries.

04/Trigger Conditions

When does this fire?

All conditions must be true for the exploit to succeed.

VNC server sends auth challenge generated by vncRandomBytes(). 2. Attacker observes connection timestamp (passive sniff). 3. Enumerates seed space with time(0)+pid+rand(). 4. Predicts 16-byte challenge; forges valid DES-encrypted response.

Required

05/Impact

What an attacker does to you.

Post-exploitation outcomes mapped to CVSS impact metrics.

Predictable VNC auth challenge. With connection timing, attacker brute-forces valid auth responses offline. Impact limited if Windows builds use CryptGenRandom (verification pending).

06/Remediation

Fix it. In this order.

A runbook, not a checklist. Sequence matters — assume compromise before you act.

Use CryptGenRandom (Windows) or getrandom() (Unix) exclusively. Vendor fix: CryptGenRandom as primary, /dev/urandom as fallback, legacy srand path unreachable last resort.

Securin advisory — For coordinated remediation support or threat-actor briefings related to CVE-2026-44040, contact disclose@securin.io

07/Disclosure Timeline

Vendors moved in days.
Attackers in hours.

Reconstructed from vendor advisories, CISA bulletins, and Securin research records.

2026-06-02: Discovered during audit; 2026-06-17: Reported to vendor; 2026-06-17: Fix validated; 2026-09-03: Planned public disclosure

Timeline recorded · Disclosure coordinated by Securin

08/References

Cite, verify, go deeper.

Primary sources — NVD, CISA KEV, and machine-readable IoC feed.

NVD

NVD — CVE-2026-44040

nvd.nist.gov/vuln/detail/CVE-2026-44040 →

SEC

Securin VI — Full Technical Analysis

vi.securin.io →

REF

CVE-2026-44040; rfb/vncauth.c:119-129; phase3_deep_analysis.md FINDING-001

libc rand() Used for VNC Auth Challenge Generation (UltraVNC)

CVE IDCVE-2026-44040

CVSS v3.14.8 Medium

VendorUltraVNC

CWECWE-338: Use of Cryptographically Weak PRNG

DisclosedJun 3, 2026

StatusFixed

01/Description

What this actually is.

Technical background, root cause, and affected surface.

Vendor: UltraVNC
Product: UltraVNC Server
Severity: Medium
CVSS Score: 4.8
Status: Fixed
Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N
CWE: CWE-338: Use of Cryptographically Weak PRNG

02/Proof of Concept

From one request
to root shell.

Reproduced in a sandboxed environment. Requires only LAN or WiFi adjacency.

VectorCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N

ScopeUnchanged

ImpactC:L / I:L / A:N

SeverityMedium

predict_challenge.py demonstrates deterministic challenge prediction from time-based seed. Private PoC; not released publicly.

03/Vulnerable Code

The bug, and the fix.

// rfb/vncauth.c:119-129 vncRandomBytes(unsigned char *bytes) { srand((unsigned int) time(0) + getpid() + rand()); for (i=0; i<CHALLENGESIZE; i++) bytes[i] = (unsigned char)(rand() & 0xff); }

04/Trigger Conditions

When does this fire?

All conditions must be true for the exploit to succeed.

VNC server sends auth challenge generated by vncRandomBytes(). 2. Attacker observes connection timestamp (passive sniff). 3. Enumerates seed space with time(0)+pid+rand(). 4. Predicts 16-byte challenge; forges valid DES-encrypted response.

Required

05/Impact

What an attacker does to you.

Post-exploitation outcomes mapped to CVSS impact metrics.

Predictable VNC auth challenge. With connection timing, attacker brute-forces valid auth responses offline. Impact limited if Windows builds use CryptGenRandom (verification pending).

06/Remediation

Fix it. In this order.

A runbook, not a checklist. Sequence matters — assume compromise before you act.

Use CryptGenRandom (Windows) or getrandom() (Unix) exclusively. Vendor fix: CryptGenRandom as primary, /dev/urandom as fallback, legacy srand path unreachable last resort.

Securin advisory — For coordinated remediation support or threat-actor briefings related to CVE-2026-44040, contact disclose@securin.io

07/Disclosure Timeline

Vendors moved in days.
Attackers in hours.

Reconstructed from vendor advisories, CISA bulletins, and Securin research records.

2026-06-02: Discovered during audit; 2026-06-17: Reported to vendor; 2026-06-17: Fix validated; 2026-09-03: Planned public disclosure

Timeline recorded · Disclosure coordinated by Securin

08/References

Cite, verify, go deeper.

Primary sources — NVD, CISA KEV, and machine-readable IoC feed.

NVD

NVD — CVE-2026-44040

nvd.nist.gov/vuln/detail/CVE-2026-44040 →

SEC

Securin VI — Full Technical Analysis

vi.securin.io →

REF

libc rand() Used for VNC Auth Challenge Generation (UltraVNC)

What this actually is.

From one request
to root shell.

The bug, and the fix.

When does this fire?

VNC server sends auth challenge generated by vncRandomBytes(). 2. Attacker observes connection timestamp (passive sniff). 3. Enumerates seed space with time(0)+pid+rand(). 4. Predicts 16-byte challenge; forges valid DES-encrypted response.

What an attacker does to you.

Fix it. In this order.

Vendors moved in days.
Attackers in hours.

Cite, verify, go deeper.

NVD — CVE-2026-44040

Securin VI — Full Technical Analysis

CVE-2026-44040; rfb/vncauth.c:119-129; phase3_deep_analysis.md FINDING-001

Let Securin level up your security posture.

libc rand() Used for VNC Auth Challenge Generation (UltraVNC)

What this actually is.

From one request
to root shell.

The bug, and the fix.

When does this fire?

VNC server sends auth challenge generated by vncRandomBytes(). 2. Attacker observes connection timestamp (passive sniff). 3. Enumerates seed space with time(0)+pid+rand(). 4. Predicts 16-byte challenge; forges valid DES-encrypted response.

What an attacker does to you.

Fix it. In this order.

Vendors moved in days.
Attackers in hours.

Cite, verify, go deeper.

NVD — CVE-2026-44040

Securin VI — Full Technical Analysis

CVE-2026-44040; rfb/vncauth.c:119-129; phase3_deep_analysis.md FINDING-001

Let Securin level up your security posture.

libc rand() Used for VNC Auth Challenge Generation (UltraVNC)

What this actually is.

From one requestto root shell.

The bug, and the fix.

When does this fire?

VNC server sends auth challenge generated by vncRandomBytes(). 2. Attacker observes connection timestamp (passive sniff). 3. Enumerates seed space with time(0)+pid+rand(). 4. Predicts 16-byte challenge; forges valid DES-encrypted response.

What an attacker does to you.

Fix it. In this order.

Vendors moved in days.Attackers in hours.

Cite, verify, go deeper.

NVD — CVE-2026-44040

Securin VI — Full Technical Analysis

CVE-2026-44040; rfb/vncauth.c:119-129; phase3_deep_analysis.md FINDING-001

Let Securin level up your security posture.

libc rand() Used for VNC Auth Challenge Generation (UltraVNC)

What this actually is.

From one requestto root shell.

The bug, and the fix.

When does this fire?

VNC server sends auth challenge generated by vncRandomBytes(). 2. Attacker observes connection timestamp (passive sniff). 3. Enumerates seed space with time(0)+pid+rand(). 4. Predicts 16-byte challenge; forges valid DES-encrypted response.

What an attacker does to you.

Fix it. In this order.

Vendors moved in days.Attackers in hours.

Cite, verify, go deeper.

NVD — CVE-2026-44040

Securin VI — Full Technical Analysis

CVE-2026-44040; rfb/vncauth.c:119-129; phase3_deep_analysis.md FINDING-001

Let Securin level up your security posture.

From one request
to root shell.

Vendors moved in days.
Attackers in hours.

From one request
to root shell.

Vendors moved in days.
Attackers in hours.